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•magazine

September 2013

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Contents

Community

8 Elektor World

• The CAN Man Came

• Get the Picture

• Draught Version 1.0

• The Ghost in the Castle

• From the Pedal to the Saddle

• From DIY Plotter to JVE CNC

• DesignSpark

20 Modular RF Link

using Manchester Code (1)

Flere we explore the use of Lynx
radio modules for building a
reliable medium range (>600 ft)
over-air data link between two
PCs or microcontroller systems.
This month we kick off with a
description of the hardware side of
things.

54 Using Libraries

Neil Gruending continues his get-
u-going advice for the DesignSpark
electronics design software suite.
This month he discloses quick ways
to working with libraries.

Projects

10 Q-Watt Audio Power Amplifier

Audio fans, here's a fully analog,
very powerful, very low distortion
design from the famed Elektor
Audio Labs. It's based on the
LME49881 integrated circuit
from Texas Instruments, and has
complementary bipolar transistors
in the power output stage.

28 Gnublin Extension Boards

Thanks to Linux being used
as the common abstraction

layer, Raspberry Pi, the Elektor
Embedded Linux Board and even
the new BeagleBone Black can use
the extension boards described
in this article: Relay Module,
Temperature Module, Display
Module, Step Module, and I/O
Expander.

34 Android Elektor Cardiyscope

Much of the power and versatility
of the Elektor Cardioscope is due
to the clever software developed
for the project. This month we
describe how the PIC24 controller
does just what the doctor ordered.

44 From Basic to Python (3)

In this concluding part of the series
we describe how Python can be
used to provide a communication
platform for the ElektorBus. As
it turns out, the exercise is less
complex than when using vintage
Basic, with far better results
though within easy reach.

4 | September 2013 | www.elektor-magazine.com

Volume 39

No. 441

September 2013

• Labs

58 .Labs Tips & Tricks

A Cheater's Guide to success
at Elektor.Labs and from there
on, towards publication in the
magazine.

60 90 Degrees and Rising

Components running hot to the
touch can be a real hazard, and in
some cases there's no way to avoid
fitting them to a heatsink as one of
our junior lab workers discovered
the hard way.

• Industry

62 News & New Products

A selection of news items received
from the electronics industry, labs
and organizations.

• Tech the Future

66 Internet @ the Physical Layer

Concerns have been raised
on the expansion rate of data
versus that of the hardware
structure supporting the Internet,
particularly at the IXs. Are these
concerns justified?

• Magazine

70 Retronics:

Radiometer PHM22 /

PHA928a Blood pH / 0 2 / C0 2
Analyzer

In the 1960s a cartload of
equipment and a lot of patience
were required to perform simple
hematology related tests in hospital
labs. Series Editor: Jan Buiting.

74 Hexadoku

Elektor's monthly puzzle with an
electronics touch.

76 Gerard's Columns: ConFused

A column or two from our
columnist Gerard Fonte.

82 Next Month in Elektor

A sneak preview of articles on the
Elektor publication schedule.

www.elektor-magazine.com | September 2013 | 5

Community

Volume 39, No. 441
September 2013

ISSN 1947-3753 (USA /Canada distribution)

ISSN 1757-0875 (UK / ROW distribution)

www.elektor.com

Elektor Magazine is published 10 times a year including
double issues in January/February and July/August,
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E-mail: j.dijk@elektor.com

www.elektor.com/advertising

Advertising rates and terms available on request.

Copyright Notice

The circuits described in this magazine are for domestic and
educational use only. All drawings, photographs, printed circuit
board layouts, programmed integrated circuits, disks, CD-ROMs,
DVDs, software carriers, and article texts published in our
books and magazines (other than third-party advertisements)
are copyright Elektor International Media b.v. and may not
be reproduced or transmitted in any form or by any means,
including photocopying, scanning and recording, in whole or in
part without prior written permission from the Publisher. Such
written permission must also be obtained before any part of this
publication is stored in a retrieval system of any nature. Patent
protection may exist in respect of circuits, devices, components
etc. described in this magazine. The Publisher does not accept
responsibility for failing to identify such patent(s) or other
protection. The Publisher disclaims any responsibility for the safe
and proper function of reader-assembled projects based upon or
from schematics, descriptions or information published in or in
relation with Elektor magazine.

© Elektor International Media b.v. 2013

Printed in the USA Printed in the Netherlands

Red Fruits, Italian Heroes,

Penguins, and Texan Bones

Ever since Steve Wozniak assembled a 6502
based microprocessor system, and Steve Jobs
literally created a market for it, we techno-
inclined folks have delighted in creating and
hearing product names with a disarming, if not
charming, ring: apple, raspberry, acorn, pen-
guin, Captain Zilog, KIM, Junior. I'm convinced
a good number of the names given to micro-
processor systems and platforms from the early
days of computing have helped significantly to
unnerdify the craft of programming and staring
at command lines for hours on a 15-inch CRT
screen propped up by pizza boxes.

The Linux community in particular has set the bar in creative product naming with
every new release of "their" operating system. Where the "men in suits" simply put
the next higher number behind the product name, a letter "b", or a year, the follow-
ers of Tux the Penguin came up with names you'd expect from a Tolkien book.

The main embedded platforms with clearly defined entry levels and educational
aims are Raspberry Pi and Arduino, and both are covered extensively in Elektor.
Magazine and Elektor.POST. However, in good engineering tradition there's more
to choose from in a diversified market. Elektor's Embedded Linux board is linked
this month to a range of extension boards through its "Gnublin" connector (that'll
be a young goblin running GNU's Not Unix). The same boards, we're proud to
say, also connect seamlessly to the Raspberry Pi and— as we've just discovered—
to the BeagleBone Black. Have a look at the article on page 28 to see how our
modules for controlling relays, displays, stepper motors, I/O devices and temper-
ature sensors can be connected to the latest embedded microcontroller systems
running Linux as the abstraction layer. Keeping abstraction and fantasy apart was
never easier though as Penguin, Beagle and Gnome seem to get along very well,
at least in this edition of Elektor. Let's hope no unadapt, badly named creatures
appear on stage, like T-Roll or CE02B.

More creatures and creations in this issue!

Jan Buiting, Editor-in-Chief

The Team

Editor-in-Chief:

Associate Editor:

Publisher / President:
Membership Managers:

International Editorial Staff:

Laboratory Staff:

Graphic Design & Prepress:
Online Manager:

Managing Director:

Jan Buiting
Thijs Beckers
Hugo Van haecke

Shannon Barraclough (USA / Canada),

Raoul Morreau (UK / ROW)

Harry Baggen, Eduardo Corral, Wisse Hettinga,
Denis Meyer, Jens Nickel

Ton Giesberts, Luc Lemmens, Tim Uiterwijk,
Clemens Valens, Jan Visser

Giel Dols, Jeanine Opreij, Mart Schroijen

Danielle Mertens

Don Akkermans

6 September 2013 www.elektor-magazine.com

Our network

USA

Hugo Van haecke
+ 1 860-289-0800
h.vanhaecke@elektor.com

United Kingdom

Wisse Hettinga

+31 46 4389428

w.hettinga@elektor.com

Germany

Ferdinand te Walvaart
+49 241 88 909-17
f.tewalvaart@elektor.de

France

Denis Meyer
+31 46 4389435
d.meyer@elektor.fr

Netherlands

Harry Baggen
+31 46 4389429
h.baggen@elektor.nl

Spain

Eduardo Corral
+34 91 101 93 95
e.corral@elektor.es

Italy

Maurizio del Corso
+39 2.66504755
m.delcorso@inware.it

Sweden

Wisse Hettinga

+31 46 4389428

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Joao Martins

+35 19 14 46 55 77

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Portugal

Joao Martins

+35 19 14 46 55 77

j.martins@elektor.com

India

Sunil D. Malekar
+91 9833168815
ts@elektor.in

Russia

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Elektor. Russia@gmail.com

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zkoksal@beti.com.tr

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Johan Dijk

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China

Cees Baay

+86 21 6445 2811

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CIRCUIT CELLAR

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ExpressPCB

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Contact Peter Wostrel (peter@smmarketing.us, Phone 1 978 281 7708,
to reserve your own space in Elektor Magazine, Elektor«POST or Elektor.com

www.elektor-magazine.com September 2013 7

Community

Compiled by

Wisse Hettinga

Elektor World

Every day, every hour, every minute, at every
given moment designers and enthusiasts are
thinking up, tweaking, reverse-engineering
and developing new electronics. Chiefly for fun,
but occasionally fun turns into serious business. Elek-
tor World connects some of these events and activities —
for fun and business.

I I The CAN Man Came

In preparation of a project on CAN, Jan Visser, member of the Elek-
tor Labs team, invited its developer, Hugo Stiers, onto our prem-
ises to discuss some issues with the design. This visit ended up
as an afternoon dedicated to the World of CAN! Hugo is an expert
in the field of CAN and previous instructor with DAF Trucks. He is
the type of technician who only believes what his hand can touch,
so he brought in a truck \ oad of boxes, PCBs, cables and notebooks
to assist in field testing.

Watching Hugo and Jan work together on this project proved a
great pleasure. In no time they created their own 'world of wires'
where time does not seem to exist. The result of the many hours
of tinkering: all is working as it should! Jan worked out the final
details on the CAN project, which is published elsewhere in this
issue. Thanks Hugo for visiting Elektor Labs.

Get the Picture

Are you still pondering on what you can do with Arduino?
Huib Theunissen, partner of one of our sales team mem-
bers, surprised us with a series of 'single shot' photo-
graphs —all timed and triggered with an Arduino board.
He uses all six outputs of the board to trigger drops of
differently colored liquid, a gun firing a bullet, the flash
light of his camera and the shutter release. After tim-
ing these events meticulously down to a split second a
beautiful photograph is taken.

Huib selected this photograph for us, 'The Speed of
Life', which won him first prize in a Nikon challenge.
Congrats Huib! Find more of his work on www. face-
book. com/druppelfotos. Now it is your turn to think
up something nifty to do with Arduino.

8 September 2013 www.elektor-magazine.com

All Around the World ...

The Ghost in the Castle

We were a bit surprised to find some people from LPI (League of Paranormal Inves-
tigators) running around in Elektor House... on a ghost hunt! They were looking for
unearthly remains of French soldiers who died in the castle and the spirit of Entgen
Luyten, the last 'official' witch in Holland. The story goes that she hung herself in the
castle's cellar. Mart, our in-house photographer, followed the ghost hunters around
and shot some photographs... and they turned out a little unusual. Using the camera
flash he has been able to capture the right hunting spirit... "there, right in front of you"!

Draught Version 1.0

We are at The Kite in Oxford, UK, a local pub
just near the railway station which offers some
rooms upstairs for weary travelers. Chief Cli-
ent Officer Johan Dijk and I had just wrapped
up an enjoyable meeting with RS Components
representatives and we were discussing how the
draft proposal we sketched would end up in a
signed contract. The lady bartender couldn't help
overhearing us and offered to help us out: "I can
fix you both up with a good draught (pointing at
the beers on tap) and then take a picture of you
holding it as proof". ...Happy to oblige!

From the Pedal to the Saddle

The Dutch are proud to be a bicycle loving nation. During the day everything is fine,
but towards nightfall they struggle to get the lights of their beloved bikes going, or
avoid the police and a hefty fine. Most bikes are equipped with small dynamos. That
way pedal power is used to generate the electricity for your lights. But there's a big
chance something is broken, and you are in constant fear of being stopped by the police.
Wouter Eisema from Hanze Highschool for Engineering came up with a completely dif-
ferent solution. The heat of your saddle (or more precisely: your butt) is converted to
electricity by Peltier elements which drive LEDs mounted in the back of the saddle. Now
isn't that clever!? We are planning a publication on the full project in the near future.

^ From DIY Plotter to JVE CNC

In 1987 Elektor published a DIY three color plotter, called Mondriaan.
Among the many people building this project was Jonas Vos, a young
artist. It was his first project with Elektor and a first step in making art
using machines. Now a teacher at the Jan van Eyck Academy in Maas-
tricht he decided to build his own CNC machine— a big one! This machine
mills all sorts of materials in an XYZ area of 70x94x31 inch (180x240x80
cm). Students are allowed to use this machine to mill huge objects. In
the picture you can see Jonas with his machine, as he is operating it.

More info on the Jan van Eyck academy: www.janvaneyck.nl

www.elektor-magazine.com September 2013 9

•Projects

Q-Watt

Audio Power Amplifier

Lots of power with low distortion

Good news for all audio enthusiasts: we are proud to present yet another fully
analog circuit developed entirely in house. Despite the simple design of this audio
power amp with just one pair of transistors in the output stage, Q-Watt can deliver
over 200 quality watts into 4 ohms with exceptionally low distortion thanks to the
use of a special audio driver IC.

By Ton Giesberts

(Elektor Labs)

Elektor has a long history with audio power ampli-
fiers. A couple of examples of our "golden old-
ies" are the Edwin, Ekwin and Crescendo ampli-
fiers from the 1970s, which thousands of audio
enthusiasts cut their teeth on. In recent years
things have been quieter in this area, but that
shouldn't be taken to indicate a lack of interest.
Quite the opposite— many people are rediscover-

ing the pleasure of soldering circuits themselves
and putting together top-notch amplifiers with
outstanding sound quality.

Since it's hardly possible to come up with some-
thing original in the realm of audio power ampli-
fiers built with discrete components (except par-
alleling a few dozen NE5532 opamps...), this time
we decided to take the semi-discrete route. This

Q-Watt Measured Performance

(Measured with a power supply consisting of a 500 VA power transformer with two 40 V

secondaries (Nuvotem type 0500P1-

-2-040) and four external 10,000 pF, 100 V buffer capacitors)

• Input sensitivity:

0.88 V (137 W/8Q, THD+N = 0.1%)

0.91 V (145 W / 8 ft, THD+N = 1%)

• Input impedance:

15 kft

• Continuous output power:

137 W into 8 ft (THD+N = 0.1%)

145 W into 8 ft (THD+N = 1%)

220 W into 4 ft (THD+N = 0.1%)

233 W into 4 ft (THD+N = 1%)

• Peak/music power:

218 W into 8 ft (THD+N = 10%)

(DC supply voltage ±56.8 V)

175 W (8 ft, THD + N = 1%)

165 W (8 ft, THD + N = 0.1%)

395 W (4 ft, THD + N = 10%)

316 W (4 ft, THD + N = 1%)

299 W (4 ft, THD + N = 0.1%)

• Power bandwidth:

2.1 Hz to 125 kHz (50 W / 8 ft)

• Slew rate:

26.7 V/ps

• Risetime:

2.4 ps

• Signal to noise ratio:

> 94 dB (linear, B = 22 Hz to 22 kHz)

(reference 1 W / 8 ft)

> 97 dBA

• Harmonic distortion plus noise:

0.0033% (1 kHz, 1 W / 8 ft)

(B = 80 kHz)

0.0006% (1 kHz, 50 W / 8 ft)

10 September 2013 www.elektor-magazine.com

Q-Watt Audio Power Amp

• Intermodulation distortion:
(50 Hz : 7 kHz = 4 : 1)

• Dynamic IM distortion:
((3.15 kHz square wave) +
15 kHz sine wave:)

• Damping factor:

• Efficiency:
(DC supply)

• DC protection:

• DC output offset:

• Switch-on delay:

0.006% (20 kHz, 50W/8Q)
0.0047% (1 kHz, 1W/4Q)
0.0009% (1 kHz, 100 W / 4 ft)
0.009% (20 kHz, 100 W / 4 ft)
0.002% (1 W / 8 ft)

0.0009% (50 W / 8 ft)

0.003% (1 W / 4 ft)

0.0026% (100 W / 4 ft)
0.0033% (1 W / 8 ft)

0.0022% (50 W / 8 ft)
0.0045% (1 W / 4 ft)

0.0027% (100 W / 4 ft)

560 (1 kHz / 8 ft)

311 (20 kHz /8ft)

70.6% (8 ft, THD+N = 0.1%)
72.5% (8 ft, THD+N = 1%)
68.5% (4 ft, THD+N = 0.1%)
70.5% (4 ft, THD+N = 1%)

+ 0.55 V/ -0.86 V
0.2 mV (max. 0.6 mV)

6 s

www.elektor-magazine.com September 2013 11

•Projects

has the advantage of easy DIY construction, and
it results in a very compact design. With a care-
ful choice of components, it's possible to create
a power amplifier with outstanding specs and
sound quality with this approach.

Background

It all started with the Measurement Filter for
Class-D (amplifiers) we published in our July/
August edition in 2011 [1]. We developed this
filter at Elektor Labs so that we could measure

Figure 1.

Schematic of the Elektor
Q-Watt audio power
amplifier. Despite the
simplicity of the design, the
specs of this amplifier are
truly excellent.

K1

O0HHS

R4_
15k

+v

Q

MG6330-R

T6 ... Tio = 5 x 2N5550

110656 - 11

12 September 2013 www.elektor-magazine.com

Q-Watt Audio Power Amp

the output voltages of class-D amplifiers up to
70 V rms . However, we never managed to test the
filter with voltages at this level due to the lack of
a suitable power amplifier. When there's a prob-
lem, you can always trust Elektor designers to
come up with a solution, so they started work-
ing on the design of a fully discrete high-volt-
age amplifier with 23 high-voltage transistors
(MJE340, MJE350, MPSA42 and MPSA92), which
was intended to operate from a balanced ±110 V
supply. The design turned out to be extremely
complicated, and things got a bit out of hand.
Although a PCB was designed for an initial pro-
totype, we had to ask ourselves whether it was
worth spending so much effort just to test a filter.
The design specifications for the amplifier were
truly impressive. It had to be able to deliver
an output signal of 70 V rms up to 20 kHz with
extremely low distortion. The minimum imped-
ance of the measurement filter is 1 kft, resulting
in peak output current requirement of 100 mA
(preferably even more).

We accordingly decided to look for a simpler alter-
native, such as an IC that could deliver such a
high output voltage with sufficient power. Our
search turned up the LME49811 from Texas
Instruments. The title of the datasheet, "Audio
Power Amplifier Series— High Fidelity 200 Volt
Power Amplifier Input Stage with Shutdown",
sounded very promising. The stated specifications
were excellent, but it wasn't clear to us whether
the measured performance figures on the data-
sheet were obtained with or without an exter-
nal power stage. However, it certainly appeared
to be worthwhile to develop an amplifier based
on this IC.

The right transistors

The next step was to select the power transis-
tors (T4 and T5) for the power amplifier. One
of the key characteristics of power transistors
for use in audio amplifiers is a large safe oper-
ating area (SOA). We ultimately found a couple
of very nice devices at Semelab: the MG6330-R
(NPN) and the complementary MG9410-R. These
devices can handle more than 600 mA collector
current at a collector-emitter voltage of 200 V.
This condition occurs when the amplifier is driven
to maximum output amplitude with no load. This
allows the amplifier to be configured for class-AB
operation with a relatively large class-A region.
The DC gain of these power transistors is fairly

linear up to several amperes (slightly less with
the PNP version), which is a good starting point
for a linear output stage. Similar requirements
apply to the driver transistors (T2 and T3). The
selected types-MJE15032 (NPN) and MJE15033
(PNP)— are suitable for voltages up to 250 V, and
here as well the DC gain characteristic is fairly lin-
ear. The driver and output transistors have fairly
high transition frequencies: 30 MHz for the MJE
devices, 60 MHz for the MG6330-R and 35 MHz
for the MG9410-R. The quiescent current setting
is handled by an ordinary BD139.

Audio version

When one of our foreign editors saw the design,
his first question was whether it could be adapted
for use as a 'normal' audio amplifier. That would
attract a much larger audience than a measure-
ment amplifier for high output voltages. The
answer was that it was certainly possible, and
in fact it wouldn't require many changes to the
original design. Some of the component values
would have to be adjusted, and the supply volt-
age would have to be reduced. The end result is
the schematic diagram shown in Figure 1 . With
a lower supply voltage (±56 V, provided by a
transformer with two 40 VAC secondaries), the
power amplifier can deliver a lot of power with
just one pair of complementary output transis-
tors— more than 300 watts of music power into
4 ohms.

In addition to the LME49811 (IC1), the power
amplifier consists of four transistors (T2-T5), a
quiescent current control network with one tran-
sistor (Tl), and a few glue components.

The negative feedback network R4/R3 is dimen-
sioned to provide an input sensitivity of approxi-
mately 1 V rms for a maximum output amplitude of
±55 V with a supply voltage of ±60 V. This input
voltage can easily be provided by any modern
preamplifier. The resistor values are chosen to
ensure that the dissipation of R4 remains just
below 0.25 W at maximum output power. The
values of resistors R1 and R2 are the same as
those of R3 and R4 to maintain the best pos-
sible common-mode rejection at the input of
the LME49811. The resulting input impedance
is approximately 15 kft. The bandwidth of the
input signal is limited at the low end by capac-
itor Cl (with a theoretical corner frequency of
2.2 Hz) and at the high end by C2. In addition to

www.elektor-magazine.com September 2013 13

•Projects

suppressing any HF noise that may be present,
this limits the slew rate to prevent the ampli-
fier from experiencing problems with excessively
steep input signals. Only one capacitor (C3) is
needed for the frequency compensation of the IC.
To make it easy for users to experiment with the
amplifier, we use a trimmer with PTFE (Teflon™)
dielectric for this purpose (Teflon is an excellent
choice for audio circuits). The PCB is also suit-
able for silver mica capacitors with a lead pitch of
5.9 mm. During testing a trimmer setting of one-
third of the rated value (approximately 18 pF)
yielded the best measurement results.

A feedback loop built around IC2 stabilizes the
DC output voltage of the amplifier. It compares
the output voltage to the ground reference and
corrects it by injecting a very low current into
the non-inverting input of the LME49811 (pin 4).
The non-inverting input is used for this correction
because the impedance at this input is higher
than at the inverting input, whose impedance is
largely dependent on the value of R3 (which is
only 390 ft). The response time is a few hundred
milliseconds. We choose an OPA177 for the con-
trol amplifier on account of its outstanding DC
specs (maximum bias current 1.8 nA, maximum
offset 60 pV). The resulting maximum theoretical
offset voltage at the output of the power amplifier
is 0.6 mV, which is negligible for the connected
loudspeakers. The output offset voltage of our
prototype was just 0.2 mV.

The opamp in the DC correction circuit has its
own ±15 V supply voltages tapped off from the
main supply rails with the aid of a few resistors
and Zener diodes (R17, R18, D1 and D2). The
values of R17 and R18 must be adjusted if a
lower supply voltage is used. In this connection
an additional current of 1.5 mA drawn from the
+ 15 V rail by pin 2 of IC1 must also be taken
into account.

A Zobel network (R13-C5) is included at the out-
put of the amplifier. It ensures that the ampli-
fier remains stable with an inductive load or no
load. Coil LI provides additional protection against
capacitive loads, and resistor R12 attenuates any
oscillations or overshoots. On the PCB, R12 is
fitted inside LI to save space.

Two large buffer capacitors (4700 pF each) are
also fitted on the circuit board. The selected types
have low equivalent series resistance (ESR). The
circuit additionally requires an external power
transformer, bridge rectifier and four power sup-
ply capacitors rated at 10,000 pF, 100 V each.
We chose a transformer with two 40-V second-
ary windings. For the prototype at Elektor Labs
we used a low-cost 500 W transformer, with the
result that the output voltage drops a fair amount
with a relatively large load. Somewhat higher
power output than stated in the specifications
would be possible if you use a transformer with
better voltage stability.

Supply lines

Very high peak currents occur in power amplifiers. To buffer the supply voltage, two electrolytic capacitors with low ESR are
mounted on the PCB adjacent to the output transistors, in addition to the external capacitors in the power supply.

With an audio amplifier, it is essential that the supply lines to and on the board do not cause magnetic field interference,
which can increase distortion by inducing currents in the negative feedback loop and other parts of the amplifier. One way
to suppress this undesirable effect is to route the supply lines as close together as possible and to decouple them as close
as possible to the output stage. Due to the class AB configuration of this amplifier, only unidirectional currents flow through
the supply tracks on the board. Routing the positive and negative supply tracks as close together as possible causes the
resulting magnetic field to be nearly sinusoidal, so it causes less distortion. With a double-sided board, these two tracks can
be placed on opposite sides of the board exactly aligned to each other.

These design considerations are very important for power amplifiers with very low distortion figures. Single-point grounding
is also very important in this regard. Here the ground point is located next to C5. The ground lines of the input, negative
feedback, Zobel network, loudspeaker output and power supply all meet at this point.

The PCB is specifically designed for use as a monaural amplifier (monoblock). For a stereo amplifier you can simply build
two of the boards and mount them in an enclosure together with the power supply. You should preferably use two separate
power supplies (one for each channel).

14 September 2013 www.elektor-magazine.com

Q-Watt Audio Power Amp

Protection

We naturally hope that the amplifier will always
work properly, but any electronic circuit can fail
(especially audio power amplifiers, as we know
from experience). Especially at full output power,
the temperature of the output transistors can
rise sharply (to above 70 °C), which can dramat-
ically shorten the lifetime of these semiconduc-
tor devices. Our experience is that when tran-
sistors fail, they usually fail shorted. If a fuse
doesn't blow somewhere in this case, a hefty
DC voltage will be present at the amplifier out-
put, which is naturally not the right way to treat
your precious loudspeakers. For this reason, DC
protection is actually indispensable in any audio
power amplifier.

After the amplifier is switched on, it takes a few
seconds for the DC voltage at the output to sta-
bilize. As usual, the loudspeaker is connected to
the output through a relay. This relay may close
only when the supply voltage for the amplifier is
present, and there is no DC voltage at the output
of the amplifier. In this design, only the positive
supply voltage is monitored by using it as the
supply voltage for the protection circuitry built
around T6 to T10. If there is no supply voltage,
it is simply impossible to energize the relay coil.
DC protection is provided by a pair of transistors
and a low-pass filter (R23/C15) with a time con-
stant of 3.3 s. That may seem like a fairly long
time, but the time required for T7 or T8 to start

conducting and discharge C16 decreases with a
decreasing DC voltage at the output. If there is
a positive DC offset of more than 0.55 V at the
output, T8 will conduct and disengage the relay
via T9/T10. Transistor T7 responds similarly if
there is a negative DC offset greater than 0.85 V.
In addition, both transformer secondary voltages
are monitored so that the relay can be disen-
gaged immediately when the power transformer is
switched off, or a fuse blows. To avoid creating a
ground loop, the secondary transformer voltages
are monitored using optocoupler IC3, which feeds
its output signal to T6 in the protection circuit.
Diodes D3 and D4 in combination with capacitor
C14 act as a full-wave rectifier for the LED in the
optocoupler. The voltage divider R4/R3 is dimen-
sioned so that the LED goes dark immediately
if either of the transformer voltages drops out.
Capacitor C16 in combination with resistors R25
and R26 determines the time delay for engaging
the relay after the supply voltage is switched on
(approximately 6 seconds).

The relay used here has a rated coil voltage of
48 V. It is connected to the 56 V supply rail via
a 1 kft series resistor (R29). If you have trouble
finding a 48 V relay, you can use a 24 V relay
instead. In that case R29 must be a 1 W type
with a value of 2.2 kft.

The protection circuit is dimensioned for a supply
voltage of ±56 V. If you use a lower supply volt-
age, some of the resistor values will have to be

Cooling

Adequate cooling must be provided for the driver transistors, output transistors and IC1. For the IC this consists of a piece
of 2-mm aluminum sheet metal measuring 2.5 x 8 mm, which is mounted on the IC with a pair of screws and nuts. This
heatsink is sufficient to handle the 2 W or so dissipated by the IC with a supply voltage of approximately ±56 V.

Choosing the heatsink for the output transistors involves a tradeoff between the size of the heatsink and the estimated
average output power of the amplifier. A very large heatsink or forced air cooling would be necessary to handle continuous
full output power, but this occurs very rarely in practice. We therefore decided to look for a heatsink that is big enough
to handle the full output power for a short while (several minutes). We found a good match in a heatsink from Fischer
Elektronik, Germany. It's not exactly small, but there's no getting around a low thermal resistance if you want to avoid
overheating with high output power. The selected heatsink has a height of 10 cm and a thermal resistance of 0.7 K/W. To
give you an idea of what this means, with a regulated supply voltage of ±56.8 V the amplifier can deliver nearly 300 W
into a4Q load with 0.1% distortion. With an efficiency of 68.5%, this means that it must dissipate about 137 W. With a
continuous sine-wave signal, at full output power the temperature would rise to more than 90 degrees above the ambient
temperature. The emitter resistors R10 and Rll (5 W types) would also be on the ragged edge at this point. However, as
already mentioned this will never happen in normal use with music signals. By the way, there is virtually no manufacturer of
audio amplifiers that dimensions their heatsinks for continuous full power.

www.elektor-magazine.com September 2013 15

•Projects

Figure 2.

The PCB holds the entire
power amplifier, including
buffer capacitors and
protection circuitry.

COMPONENT LIST

Resistors

(5%, 0.25W, unless otherwise stated)

R1,R3 = 390ft

R2,R4,R17,R18,R22,R23,R30 = 15kft

R5 = 8.2kft

R6,R20,R28 = 1.2kft

R7 = 220ft

R8,R9 = 100ft

R10,R11 = 0.2ft 1% 5W, low inductance (Vishay Dale
LVR05R2000FE73)

R12,R13 = 3.9ft 5% 5W
R14 = 220kft
R15,R16 = lOMft
R19 = 27kft
R21 = 470kft
R24 = lMft
R25,R26 = 820kft
R27 = 68kft
R29 = lkft

PI = 470ft trimpot, horizontal

Capacitors

Cl = 4.7pF 63V, MKT (metal/polyester), 5mm or
7.5mm pitch

C2 = 1 nF/400 V, MKT (metal/polyester), 5mm or
7.5mm pitch

C3 = trimmer 5-57pF 250V, horizontal (Vishay BC-
components BFC280908003)

C4,C6,C7 = lOOnF 100V, 5mm or 7.5mm pitch
C5 = 47nF 400V, 5mm or 7.5mm pitch
C8,C9 = 4700pF 100V, 10mm pitch, snap-in, 30mm
diam. (Panasonic ECOS2AP472DA)

CIO = 2.2pF 63V, 5mm or 7.5mm pitch
Cll = 33nF 63V, 5mm or 7.5mm pitch
C12,C13,C16 = lOpF 100V, 2.5mm pitch, 6.3mm
diam.

C14 = lpF 250V, 2.5mm pitch, 6.3mm diam.

C15 = 220pF 16V bipolar, 5mm pitch, 10mm diam.

Inductor

LI = 450nH: 13 turns 14AWG (1.5 mm) enameled
copper wire, 7mm inside diam.

Semiconductors

D1,D2 = 15V 0.5W zener diode

D3,D4 = 1N4004

D5 = 1N4148

D6 = LED, red, 3mm

T1 = BD139

T2 = MJE15032

T3 = MJE15033

T4 = MG6330-R

T5 = MG9410-R

T6-T10 = 2N5550

IC1 = LME49811TB/NOPB

IC2 = OPA177GPG4

IC3 = 4N25

Miscellaneous

K1 = 2-pin pinheader, 0.1" pitch
K2-K6 = Faston (blade) plug, PCB mount, 0.2" pitch
K7 = 3-way PCB screw terminal block, 5mm pitch
RE1 = relay, PCB mount, SPCO, 16A, 48V coil, 5.52kft
(TE Connectivity/Schrack type RT3 14048)

TO-220 isolating washer for Tl, T2, T3, Kapton MT
film, 0.15mm, 6kV

TO-3P isolating washer for T4,T5, Kapton MT film,
0.15mm, 6kV

TO-220 3-mm isolating bush forT2,T3
Heatsink, 0.7K/W (e.g. Fischer type SK 47/100 SA)
Heatsink for IC1, dim. 25x 80 mm, 2 mm thick
aluminum

PCB # 110656-1, see www.elektor.com/1 10656

Power Supply (for one amplifier)

Power transformer: sec. 2x40V, 500VA (e.g. Nuvotem
0500P1-2-040 for 230 VAC mains)

Bridge rectifier: 200V, 35A (e.g. GBPC3502)

(Fairchild)

Four 10,000pF, 100V electrolytic capacitors (2 in par-
allel on each supply rail)

16 September 2013 www.elektor-magazine.com

Q-Watt Audio Power Amp

adjusted. This also applies to the resistors in the
negative feedback network if you want to main-
tain an input sensitivity of around 1 V. Bear in
mind that the gain factor of the LME49811 must
be at least 20 (26 dB).

Construction

Figure 2 shows the circuit board layout designed
for this amplifier. As promised by the title of
this article, everything has been kept nice and
compact.

Building the board is certainly not difficult, but
there are a few points that require attention. Most
of the components can be soldered directly on
the board, with the exception of T1-T5, IC1 and
the supply capacitors C8 and C9. Blade connec-
tors (Faston 6.3 x 0.8 mm) are soldered to the
PCB for connecting the supply voltage and the
loudspeaker.

Coil LI consists of 13 turns of AWG #14 (approx.
1.5 mm) enameled copper wire wound on a 7-mm
drill bit. Leave the ends long enough to allow
the coil to be mounted a small distance above
the board. The coil leads must be bent to point
directly away from the middle of the coil. Place
resistor R12 inside coil LI and bend its leads so
they line up with the corresponding holes in the
PCB. Fit these two components on the board at
the same time, and when soldering the leads
ensure that the coil is raised a bit above the board
surface and the resistor is properly located in the
middle of the coil (see Figure 3).

Before going any further, you need to know what
enclosure you will be using, so that you can deter-
mine how to secure the heatsink and the circuit
board in the enclosure. The most convenient
solution is to attach two aluminum brackets to
the heatsink and use them to secure the circuit
board. This way you can still perform tasks on
the PCB after the transistors have been mounted
on the heatsink.

The circuit board must be mounted on the heat-
sink so that the leads of the transistors are as
close as possible to the corresponding pads on
the board. Using needle-nose pliers, form the
leads of T1-T5 into shallow S shapes so that the
leads project slightly and fit into the holes in the
PCB without any mechanical stress. Make the first
bend as close as possible to the package. Never
bend the leads directly; always place a small
metal plate against the pins next to the pack-
age to prevent the formation of microcracks in

Figure 3.

Detail of output coil LI with
power resistor R12 fitted
co-axially.

the package. Make the second bend at the level
of the holes in the PCB. Figure 4 shows what it
should look like. The insulator pads for the tran-
sistors can be temporarily placed between the
transistors and the heatsink to determine the
exact position of the second bend. Actually this
is not critical unless you use ceramic insulator
pads. Secure the transistors firmly to the heat-
sink (with the insulator pads in place) before
soldering the leads to the board.

Next comes IC1. Start by attaching a heatsink
plate, consisting of a piece of 2-mm aluminum
sheet metal measuring 2.5 x 8 mm, to the IC with
a pair of screws and nuts. Mount the heatsink so
that it is a bit above the board when the IC is fit-
ted, to avoid contact with Rl, R4 and R5. Caution:

Figure 4.

The leads of all transistors
mounted on the heatsink
are formed with two bends
so they fit precisely in the
corresponding holes in the
PCB without mechanical
stress.

www.elektor-magazine.com September 2013 17

•Projects

Figure 5.

There's just enough space
on the circuit board for
the heatsink that has to be
mounted on IC1.

the metallic rear surface of the IC is connected
to the negative supply voltage. This means that
if you do not use insulated mounting hardware,
the heatsink will be at the negative supply volt-
age. For safety, we recommend using insulating
mounting hardware here. Then solder the IC to
the PCB. Just enough space for the heatsink has
been kept free on the board (see Figure 5). Bend
LI slightly away from the heatsink.

The final task is to mount the two large buffer
capacitors C8 and C9. That way they don't get
in the way of earlier activities.

Q-Watt on test run

Before you connect your Q-Watt amplifier directly
to the power supply, you have to set the quies-
cent current of the output stage. To do this, first
connect two 47 ft, 5 W power resistors in series
with the positive and negative supply voltage
terminals. This prevents damage to the ampli-
fier circuit if something is wrong, such as a short
somewhere. The worst that can happen is that
the two power resistors go up in smoke. Another
option is to use a regulated power supply with
current limiting, but most of us don't have this
sort of supply available for ±56 V. Connect an
ammeter in series with the positive supply line.
Before switching on the supply voltage, turn PI
fully counterclockwise, and remember to con-
nect the secondary windings of the transformer
to terminal block K7 on the circuit board. After
the power is switched on, the current through
the positive supply line should be approximately

30 mA when the output relay is engaged. Slowly
turn PI to the right (clockwise) until the current
increases by 30 mA (60 mA total). This relatively
low quiescent current is more than adequate. The
quiescent current will rise slightly as the heat-
sink temperature increases. However, it will nor-
mally remain below 90 mA. At very high output
power levels, the junction temperatures of the
two output transistors will rise much faster than
the temperature of the heatsink, so the quiescent
current transistor will not be able to fully track
the change. This causes the quiescent current
to rise briefly to several hundred milli-amps, but
it declines quickly when the temperature drops
again. That's actually a nice extra feature with
this amplifier, since the class A range of the power
amplifier effectively increases with the output
power level.

We hope you have a lot of fun building this com-
pact power amplifier, and a lot of listening plea-
sure afterwards.

( 110656 - 1 )

More details about this power
amplifier are available at

www.elektor-projects.com/project/110656-

simple-audio-power-amplifier.l3247.html

Internet Reference

[1] Measurement Filter for Class D, Elektor July &
August 2011, www. elektor. com/100540

18 September 2013 www.elektor-magazine.com

Q-Watt Audio Power Amp

Q-Watt Measured Characteristic Curves

Test equipment: Audio Precision System Two Cascade Plus 2722 Dual Domain

Plot A

THD+N at output power levels of 1 W / 8 Q and 50 W
/ 8 ft, B = 80 kHz. The 1 W curve consists primarily
of noise (THD+N = 0.0034%). The distortion does
not rise above the noise until just before 20 kHz
(THD+N = 0.0052%). At 50 W (which is exactly
20 V, so the results can be compared readily to the
performance figures on the LME49811 datasheet)
the noise floor is much lower relative to the output
voltage. Here you can see that the distortion
increases earlier at higher frequencies. At 1 W the
distortion is still under the noise. The distortion
above 10 kHz is nearly the same as the 1 W curve.

The curve for 100 W is not shown here because it is
virtually the same as the 50 W curve. The distortion
is very low at all output power levels until just before
the clipping level.

Plot B

THD+N versus output power (1 kHz / 8 Q,

B = 22 kHz). The measurement bandwidth was
reduced here to improve the visibility of the rise in
distortion. Here again you can see that the distortion
remains very low, while the noise floor drops as the
output voltage rises. The clipping point is reached
at 127 W, and the distortion rises rapidly above this
point. At 137 W the THD+N reaches 0.1%, which is
still a usable level for good sound quality. If you really
overdrive the amplifier, it can deliver as much as
174 W with 10% distortion. Here it should be noted
that with the low-cost power transformer used for
the prototype, the supply voltage drops significantly
at full output power (at 10% THD it falls to ±51.5 V).
Even more output power would be possible with
a transformer that provides a more stable supply
voltage.

Plot C

FFT of a 1 kHz signal at 50 W / 8 ft (20 V rms).

The levels of the residual harmonics in the supply
voltage and the harmonics of the 1 kHz signal are
very low, and in practice they would be inaudible.

The third harmonic is at -113 dB, equivalent to just
0.0002%. The THD+N at this power level is 0.0006%
(B = 80 kHz).

1

0.5

0.2

0.1

0.05

%

0.02

0.01

0.005

0.002

0.001

0.0006

J4P)

20 50 100 200 500

Hz

A

k 2k 5k 10k 20k

110656-12

1m 2m 5m 10m 20m 50m 100m 200m 500m 1 2 5 10 20 50 100 300

W 110656 - 13

B

Hz 110656-14

www.elektor-magazine.com September 2013 19

r— i 1

r } DESIGNSPARK PCB

i 1

i

Clock

Data

Differential

Manchester

By

Marcelo Maggi (USA)

Modular RF Link using
Manchester Code (1)

When there is a need to send data to a distant point without using a wired con-
nection, either because the location's infrastructure does not allow a twisted pair
to link both extremes, or just because one or both of the connected circuits are
intended to be portable, or simply wireless, there are several ways to solve this,
each of them with its own pros and cons. Let's investigate, solve and solder.

Part 1: Hardware

In this article we will describe a method to send
data via Radio Frequency (RF), in the 315 MHz
or 433 MHz ISM bands, at a maximum bit rate
of 5000 bps, using low cost but highly reliable
components, and implement an RF-friendly pro-
tocol, the Manchester Code, reaching distances
in excess of about 600 feet (200 m).

Two general purpose units have been designed,
one for transmission (TX) and one for reception
(RX), ready to be used in any application, just by
adapting the code of the included microcontroller.

Introducing TX, RX and antennas

Creating an RF link involves hardware and soft-
ware (or firmware), both at the TX and RX end.
This first installment of the article will describe
the hardware.

Designing a reliable RF circuit with discrete com-
ponents is not an easy job, and the results are
usually far from the expectations— much worse,
that is. Fortunately, Linx Technologies have
already taken care of this difficult task by offer-
ing complete RF modules encapsulated in a hybrid
package. A wide range of used frequencies is
available, but we will focus on the 315, 418 and
433 MHz bands for the purpose of this article, as
these are the main free bands available, depend-
ing on where you live.

Figure 1 shows the transmitter module and its
pin assignments, as shown in the part's data-
sheet (available from [1]). Figure 2 is a mimic
of the previous, but now depicting the receiver
module. There are few active pins. The receiver
has more pins, though most are not connected

20 September 2013 www.elektor-magazine.com

Manchester-Code RF Link

Transmit 5000 bps across >600 ft

(NC). According to the datasheet you only need
an antenna for basic operation, besides the obvi-
ous regulated power supply.

Speaking of antennas, this is another very
important, but usually forgotten, element of a
successful RF link. Once again Linx Technol-
ogies provide a solution through its antenna
division, Antenna Factor. We will use the V4
wavelength monopole reduced height
antenna. Figure 3 shows the 315 MHz
band antenna on top of its datasheet.

Frequency optimi-
zation is indicated by a colored band
on the body of the antenna; a green band indi-
cates 315 MHz, while blue and red represent
418 MHz and 433 MHz respectively.

Designing the transmitter (TX)

The TX module simply transmits whatever sig-
nal is put onto the DATA pin, with a couple of
restrictions of course, but there is no intelligence
included, i.e. no data synchronization, no code
protection, etc. The user needs to provide/imple-
ment this.

Have a look at the TX circuit's schematics, shown
in Figure 4. The circuit is fairly simple; there's
only one connection between the two main com-
ponents, the microcontroller and the TX module:
pin B0 (RB0/INT, pin 6) of the microcontroller
connects to the DATA input of the TX module. The
remainder of components is required for proper
operation of the two main ones, but they play
no active role in the actual data transmission.
Components Cl, C2, C3, C7 and C8 for instance
are ceramic 0.1 pF decoupling capacitors. R5
is intended to keep the microcontroller running
(pin 4 is the reset pin). The oscillator is formed

by a standard 20 MHz quartz crystal.

The TX module works at 3 V, so the output of
our (external) 5-V power supply is first applied
to a fixed-voltage regulator type LP2950-30LPR,
which provides a steady 3.0 volts to the module.
Cl, C5, C6 and R4 are added for additional stabil-
ity, as recommended in the module's datasheet.
Two additional resistors are connected to the TX
module. R3 connects the PD pin to the 3 V supply
rail, keeping it High. If Low, this pin will put the
TX in a low-current state, in which it is unable
to transmit. R1 is a simple wire jumper (0 ft).
However, when the transmit power is higher than
you are allowed to use according to local regu-
lations for ISM-band approved transmitters, it is
possible to lower the transmit power by adjusting
this resistor. With 0 ft the maximum transmit
power is selected, but increasing the resistance

GND

PDN

DATA

VCC

GND

GND

LADJ/VCC ANT

8

7

6

5

Figure 1.

The Linx TX Module
with its pinout from the
datasheet. There's just a few
connections needed to get
this module to work.

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VCC

NC

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PDN

NC

7 y

RSSI

NC

8 F

DATA

NC

16

15

14

13

12

11

10

9

Figure 2.

The Linx RX Module with its
pinout. A lot of pins aren't
connected

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Figure 3.

The antenna is a V4X type
from Antenna Factor.

www.elektor-magazine.com | September 2013 21

DESIGNSPARK PCB

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RA1/AN1

RA2/AN2/VREF

RB6/T10S0/T1CK1/PGC RA3/AN3/CMP1
RB7/T10SI/PGD RA4/T0CK1/CMP2

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Figure 4.

The transmitter schematic,
with a PIC controlling the
LCD and data connection.

Figure 5.

Transmit Output Power can
be trimmed using R3.

will reduce the power as shown in Figure 5.
Since the transmitter operates at 3 V, the voltage
on its data input pin must not exceed that level.
Hence R8 and R9 are added, effectively forming
a voltage divider.

For the microcontroller, plenty of options are

available. The selected Microchip PIC microcon-
troller is a member of the subfamily of 18-pin
PICs. The PIC16F628A is a 3.5 K version, but
you may just as well use the PIC16F648A, a 7 K
version, with larger SRAM and EEPROM, which
can hold a larger program. Even the 'old faithful'
PIC16F84A can be used— a real advantage when
you happen to be familiar with it.

As can be seen in the schematic, access to most of
the important pins of the microcontroller and the
TX module is by way of K2 and K3, enabling full
control over the operation if required, e.g. reset
the microcontroller, input external data to be sent
by the transmitter, etc. On the TX module, the
transmit data and -power and the low-current state
can be user controlled. K3 even allows a totally
different microcontroller to be used, like the ones

22 September 2013 www.elektor-magazine.com

Manchester-Code RF Link

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120049 - 12

from Atmel. Just take the PIC out of its socket and
drive the TX module directly using K3 (remember
to use 3 V swing). Be sure to provide the 5 V sup-
ply needed for operation and everything has been
taken care of to achieve reliable TX operation.

A final note: anyone with a bare knowledge of
RF circuitry avoids testing any 10+ MHz RF cir-
cuit on a breadboard. The results— if any— will
be wayward at best. However, this unit mounted
on its PCB will be perfectly breadboard friendly,
since all the RF related stuff is taken care of by
the TX module.

Designing the receiver (RX)

Take a look at the schematic in Figure 6. Many
already 'familiar' configurations can be seen;

plenty of decoupling capacitors (C2, C3, C7, C8
and C9), a 3 V power supply similar to the one
presented in the TX diagram (the RX module
also works on 3 V), a noise canceling capacitor
wired to the microcontroller's power pins (C6),
RIO preventing the microcontroller from reset-
ting, and a quartz crystal oscillating at 20 MHz.
When we discuss the data handling program in
the second installment, we will show the impor-
tance of having the microcontrollers run at the
same frequency on both the TX- and RX side. The
connectors for accessing the circuitry from the
outside have a similar layout. The only difference
is that in the RX module an RSSI (received signal
strength indicator) output is available. The ana-
log RSSI signal (useful to implement a squelch
circuit) is routed to the middle pin of K3 instead

Figure 6.

The receiver schematic
resembles that of the
transmitter a great deal.

www.elektor-magazine.com | September 2013 23

DESIGNSPARK PCB

Figure 7. Top copper layout of the TX PCB.

Figure 8. Bottom copper layout of the TX PCB.

Figure 9. Component layout of the TX PCB.

of the transmit power adjustment pin (logically
not available on the receiver module).

Parts R9 and Dl, connected to pin 7 (RBI) of
IC2, allow a quick check to be run whether the
link is working or not. A very simple program lets
the transmitter send the command to activate
pin RB7 at the receiver side. If the command is
transmitted and received correctly, the LED will
light up. We will review this in detail when dis-
cussing the software.

Transistors T1 and T2 effectively form a non-in-
verting level converter from the 3 V output of
the RX module to the 5 V input of the microcon-
troller. Two notes:

• Yes, the microcontroller could have been
operated at 3 V. But in order to keep the
design universal, it is designed for 5 V
microcontrollers, to enable the use of older
PICs that run on 5 V only.

• One transistor and a couple of resistors
could have been saved had an inverting level
converter been implemented, but then the
signal inverting had to be dealt with in soft-
ware. We didn't want to complicate things.

COMPONENT LIST

Transmitter

Resistors

R1 = Oft
R2,R6 = 180ft
R3 = 470ft
R4 = 10ft
R5, R9 = lkft
R7 = 330ft
R8 = 680ft

PI = lOkft multiturn preset

Capacitors

C1,C2,C3,C7,C8 = lOOnF
C4,C6 = lOpF 25V
C5 = 3.3pF 50V

Semiconductors

Dl = LED red, 5 mm pitch
IC1 = LP2950-30LPR
IC2 = PIC16F628A-I/P

IC3 = TXM-315-LR, Linx Technologies (418 or 433
MHz version as appropriate)

Miscellaneous

ANT = ANT-3 15-PW-LP, Linx Technologies
K1 = 2-pin PCB screw terminal block, 5mm pitch
K2 = 16-pin SIL connector, 0.1" pitch
K3 = 3-pin SIL connector, 0.1" pitch
LCD1 = 2x16 characters, DEM16217, Elektor Store
#120061-71

XI = 20 MHz quartz crystal
PCB #120049-3

24 ! September 2013 www.elektor-magazine.com

Admittedly, if one were to design for mass
production, the CFO would baulk at our
solution.

Like with the TX module, you may use whatever
microcontroller you like, just by removing the
original PIC from its socket and using the three
RX module lines. The PCB is 100% breadboard
friendly and connects to your other designs in
a snap.

Building the transmitter (TX)

Linx Technologies manufactures the TX modules
in three frequencies, which are pin-compatible, so
they can easily be interchanged. As stated before,
RF design requires special precautions in order
to obtain the desired performance. Although Linx
Technologies have made a big effort to provide
reliable and very stable modules, we must follow
their recommendations regarding the PCB layout
in order to achieve maximum performance. There
are three key instructions to follow:

1. A ground plane on the layer opposite to the
module must be implemented.

COMPONENT LIST

Receiver

Resistors

R1 = not fitted

R2 = 10ft

R3,R5,R10 = lkft

R4,R6 = lOkft

R7 = 330ft

R8, R9 = 180ft

PI = lOkft multiturn preset

Capacitors

C1,C2,C3,C7,C8,C9 = 100 nF
C4 = 3.3uF 50V
C5,C6 = lOuF 25V

Semiconductors

D1 = LED red, 5 mm pitch
IC1 = LP2950-30LPR
IC2 = PIC16F628A-I/P

IC3 = RXM-315-LR, Linx Technologies (418 or 433
MHz version as appropriate)

T1,T2 = BC547B

Miscellaneous

ANT = ANT-31 5- PW-LP, Linx Technologies
K1 = 2-pin PCB screw terminal block
K2 = 16-pin SIL connector
K3 = 3-pin SIL connector
LCD1 = 2x16 character, DEM16217, Elektor
#120061-71

XI = quartz crystal, 20 MHz
PCB #120049-4

fl

Manchester-Code RF Link

Figure 10. Our first prototype still needs some changes.

Figure 11. Component layout of the RX PCB.

Figure 12. The LCD fits right onto the back of the Receiver Module prototype.

www.elektor-magazine.com | September 2013 25

DESIGNSPARK PCB

2. No traces should run directly underneath
the module, and no conductive items should
be placed within a 0.15 inch (3.8 mm)
radius of the module's top and sides.

3. The antenna path should be as short as
possible.

ponents, and an antenna path that's as short as
possible (antenna screwed down and soldered).
The PCB artwork of the complete RX circuitry as
well as that of the TX module can be downloaded
from the article web page [2]. The component
layout of the receiver module is pictured in Fig-
ure 11. The board size is identical to that of the
transmitter.

At 0.1 inch the
lead pitch of
the connectors
is exactly the
same as in the
TX module and
100% bread-
board compat-
ible. As with the
transmitters,
Linx Technolo-
gies offers the
receivers geared to three different UHF-band
ISM frequencies.

In Figure 12 our first receiver prototype is
shown, again with the microcontroller socketed
for easy reprogramming.

With these guidelines in mind, we started the PCB
design around the TX module area, and the rest of
the components got accommodated accordingly.
Figure 7 shows the PCB layout for the complete
TX circuit viewed from the component side, while
Figure 8 shows the copper (solder) side view.
Note the isolated position of the TX module, the
ground plane on the copper side and only a few
traces on the component side. As recommended,
the antenna path is very short. The antenna itself
is fixed onto the PCB with a screw. A touch of
solder is a good practice to secure electrical and
mechanical robustness. The lead pitch of K2 and
K3 is 0.1 inch (2.54 mm), so the circuit is easily
plugged onto a bread board.

The component layout appears in Figure 9. The
board measures 3.35 x 1.80 inch (85 x 46 mm).
Figure 10 shows a first prototype of the circuit.
The microcontroller is mounted in a socket for
easy removal in order to be able to easily repro-
gram it with updated firmware.

Building the receiver (RX)

All of the caveats stated during the assembling of
the TX module apply to the RX module as well: a
large ground plane at the copper side, reasonable
separation of the module from the other com-

Although it may seem obvious, it will do no harm
to emphasize this: for an RF link to work, all RF
components: TX module, TX antenna, RX mod-
ule and RX antenna, MUST be tuned to the same
frequency. The antenna obviously does not dis-
criminate between transmission and reception,
so the same model is used for the TX and RX
modules. A note here: while these antennas are
pretty good, they are not perfect. At the trans-
mitting side you may want to limit the power
(and sometimes the efficiency of the antenna)
to stay at or below the level permitted by local
regulations. At the receiver end you may want to
boost efficiency as much as possible. So feel free
to experiment with straight V4 wavelength mono-
poles (i.e. rods) in case you need a larger range.

This concludes the first part of this article. In
next month's second and concluding part we will
discuss the software, which has been developed
as a true general-purpose solution.

(120049)

Internet Links

[1] www.linxtechnologies.com

[2] www.elektor.com/120049

26 September 2013 www.elektor-magazine.com

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•Projects

Gnublin

Extension Boards

Plus command-line tools
for the Elektor Linux board
and Raspberry Pi

By

Benedikt Sauter [l]

Previously we published details of a relay board that can be connected to the
Elektor Linux board, the Raspberry Pi, and other microcontroller boards.

That was but one example of a wide range of expansion boards developed by
the Embedded Projects team. The family is complemented by handy command-
line tools and a C/C++ API to help you develop your own applications.

Figure 1.

The Gnublin API spares
you the need to deal with
SPI, I2C and other device
drivers.

Once a connector specification has been agreed
upon, it becomes possible to put together
microcontroller boards and expansion boards
in arbitrary combinations. We have previously
described [2] the 14-way Gnublin connector on
the Elektor Linux board, which will also feature
as the 'Embedded Extension Connector' on the
Xmega web server board that we will describe in
the next issue. In this article we will describe a
range of other expansion boards and in particular
show how easy it is to control them under Linux.
To allow for rapid testing the development team
has written a small command-line program to
accompany each module. A complete C/C++ API
is also available to aid in application develop-

GNUBLIN Tools

GNUBLIN API

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ment. The API sits on top of the standard device
drivers for I2C, SPI, GPIO, ADC and so on, and
the application developer is spared having to
deal with these drivers directly. Instead, he can
control a device (such as a stepper motor) using
straightforward function calls (see Figure 1). The
team is currently also working on a Python API:
more information on this at [4].

The idea

Linux provides a very good abstraction layer that
allows applications to be developed in a way that
is independent of the target processor: one simply
writes an application 'for Linux'. The new expan-
sion boards, which are available via www. elektor.
com/gnublin, extend this idea to cover projects
that use motors, displays, temperature sensors,
relays and much more besides. The boards are
connected to the Elektor Linux board using a
simple flatcable.

• Relay Module (controls eight relays)
(130212-91, Figure 2)

• Temperature Module (temperature sensor)
(130212-95, Figure 3)

• Display Module (4x20) (four-line text dis-
play) (130212-92, Figure 4)

• Step Module (stepper motor driver)
(130212-93, Figure 5)

• I/O Expander Module (16 digital inputs and

28 September 2013 www.elektor-magazine.com

Gnublin Extension Boards

outputs) (130212-94, Figure 6)

• - Extension Module (display, buttons, real-
time clock, buzzer and port expander)
(120596-91, Figure 7)

If you need to connect more than one expansion
board you can use the Bridge Board (130212-
71, Figure 8).

As we mentioned previously, an adapter board is
also available to interface the expansion boards to
the popular Raspberry Pi mini-computer (120212-
72). A new development is an adapter board for
the BeagleBone Black (130212-74).

Command-line tools

The command line is the sine qua non of working
with the Linux board. From the command line you
can launch and stop applications, administer the
Linux system, read system messages and much
more besides.

The development team has written a number
of small command-line tools to help control the
Gnublin expansion boards and some of the inter-
nal functions of the Linux board. Typing gnublin-
at the command line and then pressing 'Tab' will
produce a list of these mini-programs. Table 1
shows a sample.

The tools are very convenient for initial testing:
you can easily determine whether the hardware
is connected correctly. As many will know to their
cost, it is easy to spend a long time fruitlessly
hunting for bugs in software only to discover that
the power supply is not connected!

The C/C++ API

After the module has been connected and tested
we can get down to writing a real application. The
C/C++ API mentioned above lets you use a range
of easy-to-understand function calls, avoiding,
for example, the use of pointers and structures.
A separate software module is provided for each
interface and expansion board (see Table 2). To
use these functions with the Gnublin Elektor Linux
board you simply need to include the header

file gnublin. h: Listing 1
shows an example.
Many more code
examples can
be found

on the
wiki [ 3 ] .

The most
recent version of
the source code for
the complete API can be
inspected at [5].

The wiki also shows how a devel-
opment environment can be cre-
ated to simplify working with the
API.

Installing the tools
and API

In principle the API can be used
with any embedded Linux board
that has drivers for I2C and SPI. Most
processors have these interfaces built in,
and access to them is almost always imple-
mented via a device driver. Below we will
look at how the tools and API can be
used in conjunction with the
Elektor Linux board and
the Raspberry Pi.

Table 1. Gnublin command-line tools (sample)

Tool

Example invocation

Description

gnublin-lm75

(command takes no arguments)

Display temperature

gnublin-relay

gnublin-relay -p 1- o 1

Switch on relay 1

gnublin-adcint

gnublin-adcint -c 1

Read internal ADC, channel 1

gnublin-step

gnublin-step -p 3000

Move stepper motor to position 3000

www.elektor-magazine.com September 2013 29

•Projects

Listing 1. Controlling an I 2 C device

#defi ne BOARD_GNUBLIN

//#def i ne BOARD_RASPBERRYPI

#include “gnubli n.h”

int main()

{

gnubli n_i 2c i2c;

i 2c . setAddress (0x42) ;

//i2c slave address

char buffer [8] ;

char RxBuf [8] ;

buffer [0] =0x22 ;

i 2c . send (buffer , 5) ;
i 2c . send (0x12 , buffer, 2);

//send 2 bytes register 0x12

i2c. recei ve(RxBuf , 3);
i 2c . recei ve (0x23 , RxBuf, 3);

// read 3 bytes
// read from register

}

The Elektor
Linux board

Since its first version the
Elektor Linux board has had
a suitable connector fitted. The
red marking on the flat cable must
be oriented towards GPAO (the key on
its connector pointing towards the middle of
the board).

The first version of the board was shipped with
an ELDK filesystem, but this was subsequently
replaced by a complete Debian image. The
procedure for updating older memory cards is
described at [6].

Compiling the expansion board tools on the Linux
board itself takes a good five minutes. We have
therefore built a Debian package that makes
installing the tools much simpler.

First download the Debian package file on the
PC. Then transfer the file to the SD card using
the PC's card reader.

If the board itself is connected to the Internet
you can download the package file directly using
the command line:

wget https : / / g i thub . com/ embedded pro j ects/
gnubli n-api / r aw/master /gnu bli n- tools . deb

The tools are installed as follows:

root@gnubli n : ~# dpkg -i gnubli n-tools . deb

To remove the package at a later date, use the
following command:

root@gnubli n : ~# dpkg -r gnubli n-tools

Raspberry Pi

The easiest way to use the software module on
the Raspberry Pi is to connect directly to the
source code repository. Boot up the Raspberry Pi
and make sure it has an Internet connection. The
repository is cloned using the 'git' command: if
this is not already installed on your Raspberry Pi,
install it as follows:

30 September 2013 www.elektor-magazine.com

Gnublin Extension Boards

pi@raspberrypi ~ $ sudo modprobe
spi -bcm2708

pi@raspberrypi ~ $ sudo modprobe
i 2c-bcm2708
pi@raspberrypi ~ $ sudo modprobe i2c-dev

These drivers are already present in the most
recent version of the Raspberry Pi distribution.
As an alternative to the above commands, the
module names can be entered permanently in
the file '/etc/modules', one module per line:

The API requires the following drivers to be
activated:

pi@raspberrypi ~ $ sudo apt-get install

git

Now fetch a copy of the repository

pi@raspberrypi ~ $ git clone https://
gi thub . com/embeddedproj ects/gnubli n-api .
git

switch to the source directory

pi@raspberrypi ~ $ cd gnublin-api

and compile and install the code, examples and
the API:

pi@raspberrypi ~ $ make && sudo make
i nstall

spi -bcm2708
i 2c-bcm2708
i 2c-dev

The tiny command-line tools can now be used
to test any expansion board connected to the
Raspberry Pi.

( 130212 )

Internet Links

[1] sauter@embedded-projects.net

[2] www.elektor.com/130157

[3] http://wiki.gnublin.org/index.php/API

[4] http://en.gnublin.org/index.php/API_Python

[5] https://github.com/embeddedprojects/
gnublin-api

[6] http://en.gnublin.org/index.php/
GNUBLIN-Elektor

Table 2. Software API objects (sample)

Modul

Interface

Remarks

gnublin_gpio

Internal

gnublin_adc

Internal

Currently only available on Elektor Linux board
(not Raspberry Pi)

gnublin_i2c

i — i

NJ

n

Standard I 2 C bus

gnublin_spi

SPI

Standard SPI devices

gnublin_pwm

Internal

Currently only available on Elektor Linux board

gnublin_module_lm75

i — i

NJ

n

Temperature sensor

gnublin_module_relay

i — i

NJ

n

Relay board

gnublin_module_pca9555

hH

NJ

n

Port expander with 16 digital inputs and outputs

gnublin_module_step

u

rsl

i — i

Stepper motor

gnublin_module_lcd

l-H

NJ

O

Display, 4x20 characters

www.elektor-magazine.com September 2013 31

Flowcode 5 is one of the world’s most
advanced graphical programming
languages for microcontrollers (PIC,
AVR, ARM and dsPIC/PIC24). The great
advantage of Flowcode is that it allows
those with little to no programming
experience to create complex electronic
systems in minutes.

www.elektor.com/flowcode

...for electronics

E-Blocks are small circuit boards each of which contains
a block of electronics that you would typically find in an
electronic or embedded system. There are more than
40 separate circuit boards in the range; from simple LED
boards to more complex boards like device program-
mers, Bluetooth and TCP/IP. E-blocks can be snapped
together to form a wide variety of systems that can be
used for teaching/learning electronics and for the rapid
prototyping of complex electronic systems. Separate
ranges of complementary software, curriculum, sensors
and applications information are available.

MIAC (Matrix Industrial Automotive Controller) is an industrial grade control unit which
can be used to control a wide range of different electronic systems including sensing,
monitoring and automotive. Internally the MIAC is powered by a powerful 18 series
PICmicro device which connects directly to the USB port and can be programmed with
Flowcode, C or assembly. Flowcode is supplied with the unit. MIAC is supplied with an
industrial standard CAN bus interface which allows MIACs to be networked together.

...for industrial control

Flowkit provides In Circuit Debugging for a range of Flowcode applications for PIC and
AVR projects:

• Start, stop, pause and step your Flowcode programs in real time

• Monitor state of variables in your program

• Alter variable values

• In circuit debug your Formula Flowcode, ECIO and MIAC projects

New features in Flowcode 5

Flowcode 5 is packed with new features that make development
easier including:

• New C code views and customization

• Simulation improvements

• Search and replace function

• New variable types and features, constants
and port variables

• Automatic project documentation

• New project explorer makes coding easier

• Implementation of code bookmarks for
program navigation

• Complete redesign of interrupts system allows
developers access to more chip features

• Compilation errors and warnings navigate
to icons

• Disable icons feature

• Improved annotations

• Improved links to support media

• Support for MIAC expansion modules and
MIACbus

. . . f or USB projects

Lf ■< I .jv ■ ‘I~l

- t

...for robotics

■9 -u rsi

■* n e 1

D-

Formula Flowcode is a low cost robot vehicle which is
used to teach and learn robotics, and to provide a platform
for competing in robotics events. The specification of the
Formula Flowcode buggy is high with direct USB program-
ming, line following sensors, distance sensors, 8 onboard
LEDs, sound sensor, speaker and an E-blocks expansion
port. The buggy is suitable for a wide range of robotics
exercises from simple line following through to complete
maze solving. E-blocks expansion allows you to add displays,
connection with Bluetooth orZigbee, and GPS.

ECIO devices are powerful USB programmable microcontrollers with either 28 or 40 pin
standard DIL (0.6”) footprints. They are based on the PIC 18 series and ARM 7 series
microcontrollers. ECIO is perfect for student use at home, project work and building fully
integrated embedded systems. ECIO can be programmed with Flowcode, C or Assembly
and new USB routines in Flowcode allow ultra rapid development of USB projects inclu-
ding USB HID, USB slave, and USB serial bus (PIC only). ECIO can be incorporated into
your own circuit boards to give your projects USB reprogrammability.

More information and products at:

www.elektor.com/eblocks

•Projects

Android Elektor

(Part 2) Cardivscope

Wireless, button-free:
Bluetooth & touch screen

Marcel Cremmel

(France)

in co-operation with

Raymond Vermeulen

(Elektor Labs)

Following on from the description of the hardware for our new ECG interface on
tablets or Android smartphones in the July & August 2013 double edition, we're
coming back now to the PIC functions and how the program runs, before looking
at the Android application. Without going into too much detail though— we're just
going to say enough about this to encourage readers to get hold of the code and
have a go at development under Android.

What does the PIC24 do?

Acquiring and transmitting the samples
(Figure 5)

Three hardware modules included within the
microcontroller are used:

• the 10-bit ADC and its analog multiplexer,

• the UART ( Universal Asynchronous Recei-
ver Transmitter) for communicating with the
Bluetooth module (= BT),

• Timerl for producing the P2HZ and CAL
signals.

34 September 2013 www.elektor-magazine.com

The ADC's analog multiplexer allows us to convert
the three analog inputs DI, DII, and BATT_LEV.

The latter signal is produced
by a resistive divider (R16/
R17) that yields V 2 of the
battery voltage.

The ADC is configured in
autoconversion and autoscan
mode: it takes care of select-
ing, sampling, and converting
the three inputs without involving
the processor.

The 2 kHz sampling frequency is
more than enough for an ECG signal.
The results of the conversions are
stored in three 16-bit variables:
Channel_DI, Channel_DII and
Vbatt.

At the end of each of the three
conversions, i.e. at a frequency
of 2 kHz, an interrupt
(_ADClInterrupt) performs
the following processing :

Cardiyscope

Figure 5.

Acquiring and transmitting
DI and DII samples.

Hearts are trumps - that's just the PIC

AvgSampleDI

AvgSampleDII

AvgVbatt

OxAA

0x55

Natural binary

Natural binary

Natural binary

12 (

3107-16

Figure 6.

UART data sequence format.

• Every 8 samples, i.e. at a rate of 250 Hz:
calculates the average values AvgSam-
pleDI, AvgSampleDII, and AvgVbatt. This
processing makes it possible to reduce the
effect of occasional interference.

• Construction and transmission of the asyn-
chronous serial data sequence to the BT
module.

Figure 6 shows the format adopted for this
8-byte sequence. The data are framed by the
bytes OxAA and 0x55. These will be used in the
Android terminal to perform sequence synchro-
nization and hence to identify and read out the

samples. The sample values lie between 0x0000
and 0x03FF (10-bit conversion in natural binary),
so mis-synchronization is impossible.

Selecting the auto-zero time-constants
(Figure 7)

This software function constantly adapts the DI
and DII signal alignment speed (see "Open-heart
diagrams" paragraph in 1 st article) so as to stabi-
lize each ECG on the screen as rapidly as possible.
To achieve this, the MovingAverageCalc()
function calculates the moving average of the
AvgSampleDI and AvgSampleDII digital sig-
nals over a period of 4 s. The DI_Average and

Figure 7.

Selecting the auto-zero
time-constants.

NB: The numbering for the illustrations and links follows on from that in the first part of this article.

www.elektor-magazine.com September 2013 35

•Projects

Figure 8.

Receiving orders from the
Android smartphone or
tablet.

DII_Average results are compared with the
expected quiescent values in order to select, via
AI and BI or All and BII, an auto-zero time-con-
stant that is faster when the difference is greater.
Let's remind ourselves what is meant by the
expression "moving average". The AvgSampleDI
and AvgSampleDII samples are stored in a 4 s
circular buffer, i.e. here 4x250 = 1,000 words
of 16 bits. The MovingAverageCalc() function
then calculates the arithmetic mean of the last
1,000 samples from the buffer sufficiently fast.
The last sample corresponds to the moment of
the calculation and hence moves overtime.

Receiving orders from the terminal
(Figure 8)

There are not many orders that come to the user
from the terminal:

• a Run/Stop command to enable or block
transmission of the data sequences.

• interface power-down Note that the interface
can only be powered on via its On/Off button

• the CALO and CALI commands to control
production of the calibration signals.

The UART module takes care of serial/parallel
conversion for each byte of the message received.
The byte reception functions provided in the

Microchip libraries do not use interrupts. To avoid
wait loops for these functions, which occupy the
processor to no useful purpose, we are using the
UART receive interrupt. The associated _U2RX-
Interrupt function stocks received characters
in a buffer of sufficient size (256 bytes). These
characters are promptly read by the ReadMs-
gRXD2() function.

The character string variable AnswerRN42
is assigned as soon as a complete message is
received (end sequence = CR-LF). The TestMes-
sageRX_BT() function then compares this with
one of the commands expected.

It affects accordingly the ECG_Run indicator val-
idating the transmission of the data sequences
(Figure 5), the PowerOff signal, and the Calib
indicator confirming the production of the cali-
bration signal.

Producing the calibration signals (Figure 9)

The P2HZ and CAL signals act on the analog mul-
tiplexer IC9 (Figure 3, F2) to periodically replace
the voltages picked up by the electrodes with
a calibration signal with an amplitude of 1 mV.
The frequency of the P2HZ signal is 2 Hz with
a duty cycle of 20 %, both close to those of an
ECG signal. The signal produced by the micro-
controller is attenuated by the network R21/R22/

Figure 9.

Producing the calibration
signals.

_T1 Interrupt) )

Calib

fc

producing the

> P2HZ

ECG calibration

4 MHz

> * 4000

messages

> CAL

CPU clock

t

1 kHz

120107 - 19

36 September 2013 www.elektor-magazine.com

Cardiyscope

Record and view your own electrocardiograms

on your smartphone or tablet!

R65 to obtain an amplitude of 1 mV and a mean
value of zero.

The CAL signal goes high for 10 s every min-
ute if the user has enabled it from the tablet or
smartphone.

These signals are produced by a sequencer built
in to the microcontroller. This comprises:

• a -f 4,000 frequency divider achieved using a
hardware structure ( Timerl module)

• a software interrupt function _TlInter-
rupt activated 1000 times per second. If the
Calib indicator is set, counting variables are
incremented and compared with constants to
produce the P2HZ and CAL signals.

Bluetooth link status (Figure 10)

If there is no BT link, there is no point convert-
ing and transmitting the ECG signals. The STA-
TUS signal produced by the BT module gives this
information: link connected (1) or broken (0).
The _CNInterrupt interrupt function is acti-
vated whenever the STATUS state changes and
accordingly affects the ECG_Run indicator and
the ADON A/D converter validation bit (Figure 5).
The choice made to use an interrupt function
obviates the need to interrogate the STATUS sig-
nal periodically and hence saves processor time.

PIC program sequence

The program architecture is conventional (unlike
the Android application, as we'll be seeing).
After initialization, the following operations are
performed:

• initialization of the variables, the input/out-
put ports, Timerl for producing the calibra-
tion signals (see above) and of the UART2
coupler for communicating with the BT
module.

• configuration of the BT module to
38,400 baud

• initialization of the 10-bit ADC: sampling fre-
quency 2 kHz, autoconversion and autoscan
for the three analog inputs

• validation of the CN interrupt

the program then goes into an endless loop:

° call TestMessageRX_BT(): read and pro-
cess any order received from the terminal
° call MovingAverageCalc(): calculate the
DI_Average moving average
° call SetTimeAZ_DI(): selecting the auto-
zero time-constants for the DI channel
° call MovingAverageCalc() and SetTi-
meAZ_DII() for the DII channel.

The average value calculation functions are placed
in the endless loop, as the time to perform them
is quite long (26,800 CPU cycles, i.e. 6.7 ms). In
accordance with programming rules, one avoids
using interrupt functions to handle lengthy pro-
cessing. In point of fact, the other, lower-priority
interrupt functions would not be executed during
this time, which could lead to an error.

The execution frequency of the endless loop is
around 75 Hz, frequent enough for calculating
the moving averages and selecting the auto-zero
time-constants.

Man/machine interface

It would be hard to find a more user-friendly
(and cheaper) MMI than an Android terminal
(or an iPhone). Elektor has already published a
great many articles on this subject and even a
book, the success of which confirms the great
demand: AndroidApps Programming Step by
Step by Stephan Schwark [4]. We invite read-
ers interested in this subject to explore, extend,
and even critique the Elektorcardioscope code
available on the Elektor website [3]. It's impos-
sible to describe the 1,900 lines of code in just a

_CNInterrupt

Bluetooth

> ECG_Run

STATUS >

link status

detection

> ADON

120107-20

Figure 10.

Detecting the Bluetooth link
status.

www.elektor-magazine.com September 2013 37

•Projects

Applications

Home Contacts Phone Browser

Application Framework

Activity Window Content View Notification

Manager Manager Providers System Manager

Package

Telephony

Resource

Location

XMPP

Manager

Manager

Manager

Manager

Service

Libraries

Android Runtime

Surface Media SOLite

Manager Framework

Core

Libraries

OpenGL|ES FreeType WebKit

Dalvik Virtual

Machine

SGL SSL libc

Linux Kernel

Display

Camera

Bluetooth

Flash Memory

Binder (IPC)

Driver

Driver

Driver

Driver

Driver

USB

Keypad

WiFi

Audio

Power

Driver

Driver

Driver

Drivers

Management

few pages, we're going to give here just enough
information to encourage our readers to delve into
the source code to find the functions described.
Experienced programmers will be able to make
any modifications or improvements to it they
like. And maybe it will motivate others to start
developing Android applications too.

As the dynamic scrolling graph demands speed,
but the graphic performance of applications devel-
oped using Appln ven tor are only mediocre, I had
to give up the idea of using this free environ-
ment. But I would recommend it for other sim-
pler applications, e.g. controlling a Mindstorms
robot in BT, or for any of our readers who want
to get a taste of programming.

Here, I've used Android SDK from Google, also
free. The SDK tools (on PC, Mac, or Linux) are
included in a popular, free IDE: Eclipse. It takes
a long time to install it complete, but it's easy
enough if you follow the procedure described
by Google.

Figure 11.

The Android system architecture (this image, reproduced
here at small scale just for information, can be
downloaded as a high resolution file).

Figure 12.

All you have to do is drag-and-drop selected elements
from the graphic components library in the palette (on
left) to the screen (on the right).

You need to be pretty familiar with Java and
object-oriented language (like C++). Anyone who
already knows how to write programs in C will
be able to learn it if they are curious and enjoy
making a little effort. There are some excellent
available [5] as well as my own document on
my website [6].

Developing for Android

Developing an application for an embedded oper-
ating system like Android requires good knowl-
edge of its architecture (Figure 11). End users
only directly access the applications installed on
their terminal (the top layer in the illustration).

{h Mam Activity .java
< Palette

iSsi Palette

Fern WViqets

[Tj BluetoethSemcejava £ BtLtrtActr.it, .java Qj GraphtYIgava

<f » □ 7m WSVCA (Tablet) " G3 " NoTitltBer

am IE3IBD

Q mainjrml ,'i
■e © ActivitcPnncipak

"I • ”1 'i '8

e Ldiee Medium «n

Cj Text Field*
C Layouts

Developers can use these applications for their
own application, but above all has access to a
rich collection of APIs (Application Programming
Interface) written in Java in order to exploit the
tablet's resources. These are grouped together
in the Application Framework. These APIs make
use of libraries (in C and C++) which themselves
rely on a Linux core.

The originality of Android is its execution engine,
based on a Dalvik VM virtual machine (= VM).
The principle is close to the Java virtual machine
(JVM) used in PC and Mac: the Java compiler pro-
duces executable files in bytecode independently
of the processor used. The VM, specific to each

38 September 2013 www.elektor-magazine.com

Cardiyscope

device, then executes the application bytecode
which will behave in the same way regardless of
the target computer.

Under Android too, the bytecode produced by
the compiler can be executed on any terminals,
whatever processor they use.

Each Android application runs in its own process,
with its own instance of the virtual machine. Dal-
vik has been written in such a way that a device
can run several VMs efficiently.

Composing screens

Composing application screens with the help of
SDK Android before you have even written the
least line of code - what a gratifying step! Pro-
grammers have access to a library of graphic
components that they just have to arrange on the
screen as they choose (Figure 12). The arrows
represent some examples of drag-and-drop
between the component palette and the screen.
The screen already has its final appearance, but
the activity doesn't exist, as at this stage not a
single line of code has yet been written!

Events

An application in C always includes a main() func-
tion followed by an endless loop that calls in
turn the main functions to be processed, while
the architecture of an Android application is
event-based. In Java under Android, functions
are always executed following an event (touch
on the screen, reception of a text message, etc.)
and never include endless loops. Even the ini-
tialization function, when the application is run,
ends at the end of its processing and hands over
to Android. The execution engine is then able
to take care of the other applications running.
Events are managed by the operating system and
are easy to use in a development environment.

Activities

An Android application includes as many activities
as there are different screens displayed when it
is run. Each of these screen is formed from but-
tons, texts, graphs, the associated processing
for which is part of the activity. Since Android is
multitasking, an activity can have several states:

• active: currently running

• suspended: paused following a higher-prio-
rity event (e.g. displaying a text message)

• stopped: focus shifts to another activity.

The activity is finishing or
being destroyed by the system

i

onDestroy()

Activity
shut down

Figure 13.

Life-cycle of an activity.

The system remembers its state so it can go
back to it, but it can happen that it ends the
application to free up system memory.

Figure 13 illustrates the life cycle of an activity,
typical in a multitasking system.

Our ANDROECG application comprises three
activities:

• MainActivity start the application running.

It displays the main screen and the control
buttons (see screenshots) and sets up the
services the application requires.

• BtListActivity runs on demand to displays
the list of paired BT peripherals and select
the one for our interface.

• FileListActivity is run on a request to save
or read ECG data. It displays the list of exis-
ting files, along with an edit window for crea-
ting a file.

Services

These are background tasks that do not require
either a screen or any action from the user. Ser-
vices can communicate with activities via Intents.
In the ANDROECG application, for example, the
BluetoothService service looks after manag-
ing the BT module: establishing the connection,

www.elektor-magazine.com September 2013 39

•Projects

Figure 14.

Organization of the
activities, services, and
threads in our application

Screen touched

±

Menu

Screen touched

Screen touched

sending and receiving data, and shutting down
the link. The TimerlService service is a peri-
odic task responsible for displaying the battery
voltage every second.

Under Parameters on your Android phone , the
Applications menu gives a list of the services
running at any time.

Threads

The thread, or task, is the basis of concurrent
programming, which consists in developing an
application in which the tasks, from the user's
point of view, are running simultaneously. Each
task reacts to events (screen click, reception of
a BT message, etc.) independently of the others
and carries out the associated operations.

Each thread includes a run() method (function)
which acts rather like the main() function in C,
except in concurrent programming there are as
many run()'s as there are threads A service, for
example, runs in a thread. When an application
is run, Android creates the UI (User interface)
thread tasked with detecting all the events used
by the activity (e.g. button pushes) and acts
accordingly.

Each activity or service can create new threads
in order to carry out specific processing in them.
Our ANDROECG application includes the following
additional threads:

• ThreadGrapheYT taking care of the scrol-
ling ECG display. For smooth display move-
ment, this is given a high priority.

• ConnectThread establishes the connection
with the remote BT module.

• ConnectedThread manages the current BT
link, in particular the reception and trans-
mission of the data.

ANDROIDECG application organization

The organization of our application's activi-
ties, services, and threads, along with the links
between them (Intents) is less complicated than
one might fear at first sight (Figure 14). Refer
also to the screenshots (Figure 15).
MainActivity: Android creates this activity when
the application is runs and executes the onCre-
ate() method (Figure 13). This performs all the
necessary initializations and creates, among oth-
ers, the BluetoothService and TimerlService ser-
vices. The other methods (or functions) of the
activity take care of the actions on the touch
buttons and the menu functions. The last two
methods take care of the messages sent when
the other activities close and by the services in
order to react accordingly and/or inform the user
(e.g. loss of BT connection).

BtListActivity: This activity is created when you
press the "Paired BT Devices" menu button (Fig-

40 September 2013 www.elektor-magazine.com

Cardiyscope

ure 15). It opens a new window, queries the ter-
minal's BT adaptor, and displays a list of paired
peripherals (Figure 16). There's a button to let
you start a new search. The window and the
activity close when the peripheral is selected,
and a message including its identifier is sent to
the main activity. The main activity then starts
BluetoothService in order to establish the link
with our ECG interface.

BluetoothService: This service is created by
the main activity as soon as the BT adaptor is
activated. It is responsible for establishing the
link and then managing it. To do this, it creates
threads:

• ConnectThread is started as soon as the
peripheral is selected. This thread requests
the BT adaptor to establish a connection
using the SPP profile. When this is done (this
can take several seconds), this thread is
deleted before starting the next one.

• ConnectedThread remains active the whole
time the connection with the ECG inter-
face is open. It comprises in particular the
write and run methods respectively handling
transmission and reception of the data being
exchanged via BT. The run method detects
each sequence of samples in the received
stream, transmitted by the interface at a
rate of 250 Hz (Figure 6) in order to assign
in real time each of the 6 sample tables. The
size of these tables allow 10 min of cardiac
activity to be recorded. The thread is deleted
if the link is lost or when the application is
closed.

GrapheYT: instanced (that's Java jargon...) in
the main activity, this class comprises the vari-
ables declarations and methods needed to plot
ECGs. We can mention in particular:

• The 6 tables used to store the 10 min of ECG
traces

• The onDraw() method called periodically by
the ThreadGrapheYT thread, responsible for
plotting the selected ECGs, along with the
axes (Figure 17 in the box).

ThreadGrapheYT starts when the GrapheYT
class is created, so when the application is
launched. Within its run method, it includes the
call to the onDraw method mentioned above. It

Figure 15.

The menu functions:

BT connection, quit, saver
and load ECGs, erase ECG
memory.

Figure 16.

The BtListActivity displays
a list of already-paired
peripherals, and if so
demanded finds new
peripherals within range.

www.elektor-magazine.com September 2013 41

•Projects

ECG graph scrolling refresh algorithm

onDraw

Examine window size

In order to understand the algorithm properly, you need to have firmly in your mind the way the tables storing the last

10 minutes of cardiac activity are used:

• each of the DI, DII, Dill, aVR, aVL and aVF derivations includes its own table with 10 min of samples;

• each of these is assigned by a new ECG sample to each data sequence received by the BT module, i.e. 250 times a
second;

• in normal use (Mem cursor on right), the last sample acquired must always be displayed at the extreme right of the
screen;

• in order to obtain
a dynamic scrolling
graph, the display
function ( onDraw )
represents the last
samples memorized
in the tables, starting
with the most recent.

In a way, it's like going
back in time.

Hence the scroll speed

is 250 pixels per second

(Zoom xl).

“indexSample” attribution: index in ECG values table at r/h screen edge

Clear entire window

Display derivation names at r/h screen edge

Coordinates calculation for initial points of ECG at extreme r/h side of window

For all screen pixels, from right to left

indexSample = indexSample - zoom: index in ECG tables of next sample

Coordinates calculation for corresponding pixels on screen

Draw axes: solid lines every second, and dotted every 200

Draw connecting lines between two samples of each ECG

What work are the
Android terminal's
processors asked to do to
display a scrolling ECG?

In this example, the
size of the ECG graph is
722 x 403 pixels. Under
these conditions, each
time the onDraw method
is called, we need to:
erase the whole of the
screen, i.e. the 722 x
403 = 290,966 pixels,
plot the derivation
names;

• plot the axes that
move with the curves;

• plot up to three ECGs,
i.e. for each of the 722
right-hand segments;

• calculate the cardiac
rhythm, and display it!

ECG samples table

Most recent ECG sample index

Figure 17. The graph refresh algorithm.

That's all... The number of instructions executed by the processor, helped in some cases by its graphics co-processor, is
gigantic. What's more, to obtain smooth scrolling, the frequency at which the onDraw method is called must be significantly
higher than 10 Hz! Just a few years ago, a large office PC would not have been capable of sustaining that sort of working
speed. Today, one of these little wonders that fit in your pocket manages it easily, and also looks after the other active
applications...

42 September 2013 www.elektor-magazine.com

Cardiyscope

is given a high priority to ensure smooth move-
ment of the traces. However, the execution fre-
quency of its run method is determined by the
Android system. If other threads are making
heavy demands on the terminal's CPU, the traces
may move jerkily.

TimerlService: This class creates a service that
performs a relatively simple task every second:
displaying the interface battery voltage in a dig-
ital and graphical form (displayed at the top of
the screen).

A project to your
heart's content

FileListActivity : This activity is created when
the user chooses to save or load ECG reports
from the menu. It displays the list of existing
files, along with an edit window for editing the
name of a new file (Figure 18). The window and
the activity are closed when the file is selected,
after a sending a message including its name and
the nature of the operation ( save or load) to the
main activity. The main activity then performs
the operation requested.

My heart is going boom-boom

Without by any means exhausting the subject,
we've come to the end of our description of the
Elektorcardioscope. Next month we'll finally move
onto the practical side with construction, adjust-
ments, and some instructions. For the interface,
this will be quite quick, as the finalized circuit
board is now available from our Elektor PCB-
service in the form of an assembled, ready-to-
use module [7]. The adjustments won't require
any special skills either, but we shall be taking a
close look at the electrodes. For the intention of
this sophisticated device is indeed to bring ECG
within everyone's reach.

( 130227 )

I ? *

• • i ■ i

z

e

r

t

y

U

■

i

0

s

d

f

9

h

■

j

k

1

O-wxcvbn ’ «

Figure 18.

Selecting the file for saving
or loading ECG reports.

Internet Links

[3] www.elektor.com/120107 and www. elektor.
com/130227

[4] Android Apps | programming step by step, by
Stephan Schwark
www.elektor.com/android

[5] Le Site du Zero

http://www.siteduzero.com/informatique/tu-
toriels/apprenez-a-programmer-en-java
or http://goo.gl/OVZQY (in French)

[6] Author's website: http://electronique. marcel.
free.fr/

[7] www.elektorpcbservice.com/

www.elektor-magazine.com September 2013 43

•Projects

From BASIC to Python (3)

Communicating with the ElektorBus

The combination of
electronics, Python
and a PC is ideal for
extracting and visu-
alizing data from
homebrew or com-
mercial external hardware. And,
with the data flowing in the opposite direction,
we have the perfect way to control external hard-
ware. The communication itself can be imple-
mented using a common-or-garden serial inter-
face or, if something a little more sophisticated is
desired, over a bus. The ElektorBus makes for a
good demonstration of how such as system can
be put together.

In the first two parts of this series we
looked at the differences between BASIC
and Python in terms of mathematical
amusements such as Fourier synthesis,
plotting graphs, and graphical user in-
terfaces. In this third part we will describe
interfacing to the ElektorBus. We will steer clear
of dull theory and concentrate on colorful practical
applications.

By

Jean-Claude Feltes

(Luxembourg)

The ElektorBus was described in an eleven-part
series of articles [1] in Elektor magazine between
January 2011 and January 2012. The hardware
we will be using here is the experimental node, a
board carrying a microcontroller, LEDs and but-
tons described in part 6 of the series [2]. The
board can be connected to a PC using the USB-
to-RS-485 converter that was also described at
the time (Figure 1).

Hexadecimal help

First a little warm-up exercise: a script that pro-
vides an auxiliary function to display values in
hexadecimal and that shows how reusable code
can be encapsulated in a module. Data on the
ElektorBus are conveyed in binary, and so some of
the bytes in messages correspond to non-printing
ASCII characters: this is not ideal for display in a
terminal window. Instead, it is preferable to dis-
play received data bytes in hexadecimal format.
The code in Listing 1 from the file 'Hexfunctions.
py' is what we need. The function implemented
here can also be used by other programs, and
it can of course be extended if desired. In tradi-
tional Python style the module includes its own
test function. If it is run directly the variable

' name ' will have the value ' main ' and

the second part of the code will be executed. The
result is that an example string will be converted
into hexadecimal and displayed:

HELLO

48 45 4C 4C 4F 0A

The example 'Test_hexfunctions.py' shows how

44 September 2013 www.elektor-magazine.com

Basic to Python

the module can be used in another program:

from hexfunctions import *
s=”HELLO\n”

print s, translate2hex (s)

The imported module must be in the same direc-
tory as the main program or be found some-
where on Python's search path. Putting functions
in modules like this makes code easier to under-
stand at a glance and easy to reuse.

A GUI with wxPython

Many roads lead to an attractive graphical user
interface. However, Python does not really
have anything as easy to use as Visual Basic's
'Forms Designer', which gives full control over
the appearance of the result and which works
well when incorporated into the final program. I
first tried to use 'Boa Constructor', but debug-
ging seemed very tedious without a detailed
background understanding. 'Python Card' is
rather simpler, and very practical for straight-
forward applications. Unfortunately, however,
it requires Python Card to be installed on each
target computer.

'Tkinter' is the GUI library installed with Python.
It is very simple, and includes fewer objects than
wxPython. Since I needed to use the clipboard in
my application, I settled on the more complete
and powerful wxPython.

The code in Listing 2 gives the basic frame-
work for the graphical interface, which can be
extended step-by-step into a complete applica-
tion. Of course, you must install wxPython to
run it.

The main window is defined, in object-oriented
fashion, as a class 'MyFrame', which inherits all
the properties from the class 'wxFrame', includ-

Figure 1.

The PC receives data over
the (RS-485) ElektorBus
from a small microcontroller
board to which a light
sensor is connected.

Listing 1: Hexfunctions.py

def translate2hex (c) :

translate character string c to hex representation string
e.g ABC -> 41 42 43

for ch in c:

b=hex (ord (ch) )
b=b . replace (“0x” , ””)
b=b . upper ( )
if len(b)<=l:

b=”0”+b
h=h+b+” “

# iterate over all characters

# get hex value

# take away leading “0x for better overview”

# all in upper characters

# e.g. make “0A” out of “A”

# separate bytes by space

return h

# test:

if name == “ main

s=”HELLO\n”

print s, translate2hex (s)

www.elektor-magazine.com | September 2013 | 45

•Projects

Listing 2: G U It e m p I a t e . py

import wx

# GUI

class MyFrame (wx . Frame) :

def init (self, **kwargs) :

# create frame

wx. Frame. init (self, None, **kwargs)

# text box with fixed width font for nice data representati on
self . textbox=wx . TextCtrl (self , style = wx . TE_MULTILINE ,

pos = (5 , 5) , size= (300 , 200))

myfont = wx.Font(12, wx. MODERN, wx. NORMAL, wx.BOLD, False, u’Courier’)
self . textbox . Set Font (myfont)

self . button=wx . Button (self , -1, “TEST”, pos= (100 , 230) )

# Bindings

self . Bi nd (wx . EVT_IDLE , self . On Idle)

self . Bi nd (wx . EVT_WINDOW_DESTROY , self . OnDestroy)

Listing 3: Serialthread class

class Seri althread (seri al . Seri al) :

def init (self, port, baud, **kwargs) :

# Initialization of port + baudrate

seri al . Seri al . i ni t (self)

self.sCOM =seri al . Seri al (port)
self . sCOM . set Baud rate (baud)

# open port if not already open
if self . sCOM . i sOpen () “False :

self .sCOM.openQ
if self .sCOM. isOpen ()“True:

print „connected to“, self . sCOM . port
else :

print „Error opening port“

# Counter for received data blocks
self. ctr=0

# Create stop event (to terminate endless receiving loop)

# and message queue for thread (to transmit received text to TextCtrl)
self . stopevent=th read i ng. Event ()

self . msgQueue=Queue . Queue ( )

def di sconnect (self ) :

# set stop event so endless receiving loop can be interrupted
self . stopevent . set ( )

def connect (self ) :

# create a new thread object that runs serial thread

46 September 2013 www.elektor-magazine.com

Basic to Python

self . Bi nd (wx . EVT_BUTTON , self . On Button)
def OnIdle( self, event):

# if nothing else to do, update text from message queue
pass

def OnDestroy (self , event):
print “Exit”

def OnButton (self , event):

self .textbox. AppendText (“Button pressed\n”)

#

# Main program

if name == “ main ”:

app = wx . App ( redi rect = False)

frame = MyFrame (ti tle=”GUI” , size = (320,270))

frame . Show(T rue)

frame . Centre ( )

app . Mai nLoop ( )

# to read serial characters

self . seri althread = threading. Thread (target=self . readSerial)

# clear stopevent and Connect thread
self . stopevent . clear ( )

self . seri althread . start ()

def readSeri al (self ) :

# endless receiving loop

while not self . stopevent . i sSet () :
data=““

# read from port

c = self . sCOM . read (1)

# synchronize

if ord(c) == OxAA:
self.ctr += 1
rest = self . sCOM . read (15)
data=c+rest

# format c to 16 bytes output

datastri ng=str (self .ctr) + „\t“ + translate2hex (data) + „\n“

# update message queue

self . msgQueue . put (datastri ng)

wx . WakeUpIdle ( ) # wake up to update text

# end serial thread
print „di sconnected“
self . sCOM . close ()

www.elektor-magazine.com | September 2013 47

•Projects

Figure 2.

The window created by the
code in Listing 2.

eter or light-dependent resistor can be connected
to the ADCO pin on the expansion connector, as
described in [2a] and [2b].

Before proceeding it is necessary to identify the
correct serial port on the PC: in the Windows
device manager look to see which new COM port
appears when the converter is plugged in. Linux
users can use the following command

Is /dev/tt*U*

which will give a result similar to the following:

ing functions to increase and reduce the size of
the window, to move the window, and so on.
The properties of the window are specified in the

function ' init ()'. This is where the graphical

elements are defined: in this case a text box and
a button. In the interests of simplicity the sizes
and positions of these elements are fixed. With
the help of so-called 'sizers' it is possible to cre-
ate a dynamic layout.

The next step is to create three event handlers,
although two of these will not be not needed
until later. Traditionally the name of an event
handler starts with 'On'. The 'Bind()' function
attaches these functions to specified events: for
example, 'OnButtonO' is called in response to a
button press.

With the class 'MyFrame' defined it can be used
in the main program. This first creates a 'wx.
App' object, which is mostly concerned with the
processing of events. We then create an instance
of our window, centered and made visible on the
screen. Events are then handled in the infinite
loop y app.MainLoop()'.

When the program is run the window appears as
shown in Figure 2. It already responds to the
button being pressed and even supports exten-
sive editing functions: pressing the right mouse
button brings up a menu for selecting, copying,
deleting and inserting text.

The program framework can easily be extended,
for example to include a function for storing text
in a file.

ElektorBus: read operations

The experimental node is connected to the PC
via the USB-to-RS-485 converter. We fit an
ATmega328 microcontroller to the small printed
circuit board and program its flash memory with
the hex file downloaded from [4]. A potentiom-

/dev/ttyUSBO

The Python script for scanning the serial ports
from part 1 of this series [3] can also be used.
If any of the scripts given below fails to work,
the first thing to check is that the serial port
is configured correctly. While experimenting it
is possible under some circumstances for the
operating system to surreptitiously change the
port number. For example, this can happen if a
script is using /dev/ttyUSBO and the converter is
unplugged and then plugged in again: the con-
verter will then be assigned to /dev/ttyUSBl,
and the script will not be able to receive data. In
normal use this very rarely happens.

We can now start to extend the GUI frame-
work in Listing 2. First we have to load all the
modules required and set the various interface
parameters:

COMport = “/dev/ttyUSBO” # adjust to suit
Baud = 9600

import threading, Queue
import serial
import time

The module 'threading' is needed because serial
data reception is carried out in a separate thread
from the main code. This in turn is because the
receiver needs to run in an infinite loop and this
would create a conflict with the main loop in 'wx'.
Indeed, this is the most complicated aspect of
the program.

We create a special class called 'Serialthread'
to access the serial interface (Listing 3). An
object in this class opens the interface, reads the
incoming data bytes, formats them and sends the
result via a message queue to the other parts of
the program. This runs in an independent infinite

48 September 2013 www.elektor-magazine.com

Basic to Python

loop until the thread is stopped.

A 'Serialthread' object inherits all the proper-
ties and methods of the base class 'Serial': the
port name, baud rate, functions for reading and

writing, and so on. The procedure ' init ()'

creates a Serial object that implements all the
port operations. The port is opened if it is not
already open, and the baud rate is configured.
The counter 'self.ctr' is not important for now,
but later will be used to number the data blocks.
There are two further objects of interest used
by the serial thread: a stop event to cause the
thread to terminate and a message queue to
send data to the GUI.

The external interface for starting and stopping
the thread is via the methods 'connectQ' and 'dis-
connect()'. The 'disconnectO' method simply sets
the stop event. The 'connectO' method launches
a new thread in which the function VeadSerial()'
is executed. This function in turn continuously
reads bytes from the interface in an infinite loop
until the stop event is received. The thread then
terminates and the port is closed.

Within YeadSerial()' there is a mechanism for
synchronizing to the incoming data. In the Ele-
ktorBus protocol each data packet starts with
an OxAA byte. When this value is seen, the data
packet counter is incremented and a further fif-
teen bytes are read in. These are then formatted
into a string and placed on the message queue.
The function 'wx.WakeUpIdleO' is then called to
signal to the GUI part of the program that there
is an entry in the message queue that can be
read when there is no other work to be done.
The 'Serialthread' class thus allows data to be
read continuously without affecting the execution
of other code. In effect it runs in parallel with the
other parts of the program. The class must be
instantiated and started from the main program,
which means that we have to extend the code in
'MyFrame', which corresponds to the main win-
dow, created in the GUI framework script, by
adding the following code (after the bindings)
to the procedure ' init ()':

class MyFrame (wx . Frame) :

def init (self, **kwargs) :

1 # T □ GUI

29

AA

00

00

OA

00

05

40

47

40

00

00

00

00

00

00

00

30

AA

00

00

OA

00

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00

J 31

AA

00

00

OA

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AA

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AA

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AA

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?• 35

AA

00

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AA

00

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OA

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3 7

AA

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OA

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05

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00

00

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00

00

00

00 1

1 TEST

Figure 3.

# serial thread Received data displayed in

self . seri alrecei ve = hexadecimal.

Seri althread (COMport , Baud)

self . seri alrecei ve . connect ( )

We now have an object called 'serialreceive' which
is connected to the port specified by the variable
'COMport' and which writes all incoming data to
the message queue. To see the results we have
to extract the text from the message queue and
write it to the text box. It is at this point that
we make use of the function 'OnldleO' that we
prepared earlier:

class MyFrame (wx . Frame) :

def init (self, **kwargs) :

def OnIdle( self, event):

# if nothing else to do, update
text from message queue

while not self . seri alrecei ve .
msgQueue . empty ( ) :

msg=self . seri alrecei ve .
msgQueue . get ( )

self . textbox . AppendText (msg)

The 'pass' command was simply a placeholder
and can now be deleted. With these changes the
incoming data should now be displayed in the
text box with incrementing packet numbers as
shown in Figure 3.

To avoid the appearance of unsightly error mes-
sages when closing the window, an extra touch
is to add an 'OnDestroyQ' procedure:

# Bindings

def OnDestroy (self , event):

self . seri alrecei ve . di sconnect ( )
ti me . sleep (1)

www.elektor-magazine.com | September 2013 | 49

•Projects

This will ensure that before the window closes the
serial thread is forced to finish. This can take a
moment, which explains the need for the 'time.
sleep(l)' command. The end result of all the mod-
ifications we have described is the new program
'Serialreceivel.py', which, as usual, is available
for free download from the Elektor web pages
accompanying this article [4].

In the interests of modularity and code clarity
the definition of the class 'Serialthread' can be
stored separately from the module 'Serialthread.
py', as soon as no further changes are planned.
It can then be imported into the main program.

This requires a further small modification, as the
modules required in turn by 'Serialthread' have to
be imported from within that module rather than
in the main program. The file 'Serialthread.py'
therefore also needs to include the following code:

import threading, Queue
import serial
import time

from hexfunctions import *
import wx

class Seri althread (seri al . Seri al) :

def init (self, port, baud,

**kwargs) :

# Initialization of port +

baudrate

seri al . Seri al . i ni t (self)

• • • •

# end serial thread
print “disconnected”
self . sCOM . close ()

The resulting main program 'Serialreceive2.py'
now dispenses with the entire definition block
for the class 'Serialthread'. The 'import' lines
are also no longer required, as they now appear
in 'Serialthread.py'.

It must be admitted that this division of the code
is not ideal, as the module 'Serialthread.py' is
written in a way rather specific to the project
rather than being aimed at more general use.

ElektorBus: write operations

The experimental node includes a red LED. We
would like to be able to turn this LED on and off
using two buttons on the PC. The byte sequences
required on the ElektorBus are as follows:

• Turn on: AA 00 00 05 00 0A 00 00 00 00

60 01 00 00 00 00

• Turn off: AA 00 00 05 00 0A 00 00 00 00

60 OO 00 00 00 00

We will need to add a second button and a sec-
ond event handler to the GUI program, and give
the buttons appropriate labels. To do this we edit
' init ()' in the 'MyFrame' class as follows:

def init (self, **kwargs) :

# create frame

• • • •

buttonOn=wx . Button (self , -1, “LED
ON”, pos= ( 100 , 230) )

buttonOff=wx . Button (self , -1,

“LED OFF”, pos= (200 , 230) )

# Bindings

• • • •

buttonOn . Bi nd (wx . EVT_BUTTON ,
self . OnButtonOn)

buttonOf f . Bi nd (wx . EVT_BUTTON ,
self . OnButtonOf f )

The event handler calls the functions 'self.OnBut-
tonOn()' and y self.OnButtonOff()'. These have to
be expanded:

def OnButtonOn (self , event):

self . textbox .AppendText (“LED

ON\n”)

data=b”\xAA\x0O\x0O\x05\x0O\x0A\
X00\X00\X00\X00\X60\X01\X00\X00\X00\X00”
self. seri al_thread . sCOM .
wri te (data)

def OnButtonOf f (self , event) :

self . textbox .AppendText (“LED

OFF\n”)

data=b”\xAA\x00\x00\x05\x00\x0A\
xOO\xOO\x0O\xOO\x6O\xOO\xOO\xOO\xOO\xOO”
self . seri al_thread . sCOM .
wri te (data)

If you look at the source code for the 'Serialthread'
class, you might wonder where the function
'write()' is defined. What is happening is that the
base class 'serial. Serial' defines this function which
the derived class inherits: it therefore does not
need to be defined explicitly in the derived class.
The result of these changes is the program
'Receive_send.py', which allows the red LED on
the experimental node to be turned on and off
from the PC. The software is not yet perfect: in

50 September 2013 www.elektor-magazine.com

Basic to Python

particular it does not implement synchronization
with the node. If the node and the PC happen to
transmit at the same time, a collision occurs on
the bus and data will be lost. This can be avoided
by having the PC only transmit a message on the
bus immediately after a packet has been received
from the node (the ElektorBus 'direct mode': see
the ElektorBus specification at [1]). For example,
a flag could be set to indicate that a message is
to be sent but the actual sending of the appro-
priate byte sequence would be delayed until the
next data packet is received.

Graphs

Next we would like to convert the incoming data
from textual to graphical form, so that we can
for example visualize the output of the A/D con-
verter on the node.

The standard library for creating graphical output
is called 'Matplotlib', and you will need to install
this library before proceeding. In the first part of
this series we used the simple 'pyplot' interface
which provides a quick and easy way to generate
graphs. Things get more complicated if we want
to embed a graph in a GUI as we have to use the
object-oriented interface, which has rather more
options. Indeed, the range of options offered can
be overwhelming at first. The documentation for
Matplotlib can be found at the project homep-
age [5], and an example of embedding it in a
'wx' interface can be found at [6].

Before embarking on changes to the 'Serialthread'
module make a copy of the code under a different
name, for example 'Serialthread_diagram.py'.
Now, the values from the A/D converter that we
need are in bytes 5 and 6 of the received data
packet. The values have to be extracted and then
passed to the main program. There are several
different ways this might be done: one approach
is to add arrays 'x' and 'y' to the 'Serialthread'
object as attributes to carry the values. The extra
code in ' init ()' is then as follows:

class Seri althread (seri al . Seri al) :

def init (self, port, baud,

**kwargs) :

This sets up two empty arrays for the x and y
values. The additional attribute 'starttime' allows
the total run time so far to be calculated, so that
the x-axis of the graph can be labeled in seconds
rather than by packet counts.

The receive function is modified to extract the A/D
converter data, calculate the value and the time-
stamp, and place these in the 'x' and 'y' arrays:

def readSeri al (self ) :

# infinite receiving loop
while not self . stopevent . i sSet ( ) :

# synchronize
if ord(c) == OxAA:
self.ctr += 1
rest = self . sCOM . read (15)
data=c+rest

*256

startti me

output

## update x,y
Ibyte = ord(rest[6])
hbyte = ord(rest[5]) & 7
adc = Ibyte + hbyte

t=ti me . ti me ( ) -self .

self . x . append (t)
self . y . append ( adc)
print t,adc

# format c to 16 bytes

wx . Wakellpldle ( ) #

wake up to update text

With these modifications the module provides
the values for graphing in addition to the hexa-
decimal output.

Now let us turn to the changes in the main pro-
gram. The name of the 'Serialthread' module
has changed:

from seri althread_di agram import *

To draw the graph we have to import the neces
sary libraries:

## init arrays and timer for data
self . x= []
self . y= []

self. starttime=time . time()

from matplotli b . backends . backend_wxagg
import FigureCanvasWxAgg as FCanvas
from matplotli b . figure import Figure

www.elektor-magazine.com | September 2013 51

•Projects

Take care with the indentation when copying from the listings:

incorrect indentation will result in errors
or strange program behavior.

Matplotlib works with a number of 'back ends'
which actually carry out the drawing commands.
Different back ends plot graphs on the screen,
for example with wx, or output to printers, or
to files. The first line above creates an object
'FCanvas' for wx on which Matplotlib can draw.
'FCanvas' inherits methods and properties such
as size and position from 'wxPanel'.

The object 'Figure' is a container within which
drawing occurs. However, 'Figure' is not the graph
itself: instead, the graph is an 'Axes' object, and
a 'Figure' object can contain one or more 'Axes'
objects. This might seem bewildering, but it is
all part of the power of Matplotlib.

Some changes are also needed in the layout and
in text sizes to obtain a satisfactory appearance:
details can be found in the Listing.

The graph window (see Figure 4) is constructed
in the ' init ()' function of the 'MyFrame' class:

class MyFrame (wx . Frame) :

add_subplot (111)

self. canvas = FCanvas (self , -1,
self. figure)

self .canvas. Set Posit ion ( (450,5) )
self .canvas. SetSize( (300,250) )

• • • •

First we create a subplot within the 'Figure'
instance. One 'Figure' can contain several sub-
plots, for example if there are several time series
to display. The syntax is as follows:

figure. add_subplot (numrows , numcols,
fi gnumber)

In our case there is only one graph (fignum-
ber = 1) and hence only one row (numrows = 1)
and one column (numcols = 1). The last three
lines above the drawing surface for the wx back
end is created using the 'FCanvas' object. It is
then positioned and its size set. We are now in
a position to draw a graph.

def init (self, **kwargs) :

# create frame

wx. Frame. init (self, None,

**kwargs)

The second change affects the function 'OnldleQ'.
This is called whenever the program has nothing
else to do, in particular when new data become
available.

# text box with fixed width font
for nice data representati on

self . textbox=wx .TextCtrl (self,

style

200 ))

wx . TE_MULTILINE ,

pos = (5 , 5) , si ze= (420 ,

myfont = wx.Font(10, wx. MODERN,
wx. NORMAL, wx.BOLD, False, u’Courier’)
self . textbox . Set Font (myfont)

def OnIdle( self, event):

# if nothing else to do, update
text from message queue

while not self . seri al_thread .
msgQueue . empty ( ) :

msg=self . seri al_thread .
msgQueue . get ( )

self . textbox . AppendText (msg)

buttonOn=wx . Button (self , -1, “LED
ON”, pos= ( 100 , 230) )

buttonOff=wx . Button (self , -1,

“LED OFF”, pos= (200 , 230) )

## diagram

self. figure = FigureQ
self. axes = self. figure.

## display values in diagram
self. axes . plot (self . seri al_
thread. x, self . seri al_thread . y)

self .canvas. draw ()

This code plots the most recently received x and y
values: recall that the 'x' array holds timestamps
in seconds. The graph is finally displayed using
the command 'canvas. drawQ'.

52 September 2013 www.elektor-magazine.com

Basic to Python

The changes described above lead to the pro-
gram 'Diagram. py'. In this simple example we
have not included code to fix the scaling of the
graph: instead the scaling continuously changes
to match the data displayed. Another side-effect
of the simple implementation is the continually-
changing color of the curve each time a new graph
is plotted. If you wish to modify the presenta-
tion to suit your particular requirements, you
will need to immerse yourself in the Matplotlib
documentation.

Conclusion

You should now have an initial impression of the
basic principles involved in creating a GUI. The
software we have described is far from perfect:
error handling is rudimentary and if anything
does go wrong the interpreter will simply report
the error immediately.

A further disadvantage is that the program can
only run for a certain length of time. The prob-
lem is that the experimental node will continue
to deliver data twice a second and eventually the
'x' and V arrays will become full. Drawing the
graph takes longer as more data points accu-
mulate, and if new data points arrive while the
graph is being drawn then Matplotlib will no lon-
ger be able to keep up. The window will go gray,
indicating that the GUI thread is overloaded. It
is interesting to note that the terminal window
still displays the activity in the receiver thread,
since this continues to run independent of the
GUI thread.

It should be possible to work around this problem
by modifying the program so that the graph is
only redrawn every few seconds. Why not give
it a try?

( 120744 )

The Author

Jean-Claude Feltes is an electronics
engineer teaching at the Lycee Technique
des Arts et Metiers in Luxembourg, a
specialist technology and arts school
that offers courses leading a range of
vocational qualifications. In his spare time
he keeps himself busy with electronics and
programming projects (see [7]).

Internet Links and References

[1] ElektorBus web site: http://www.elektor.
com/elektorbus

[2a] Here comes the bus! (6): http://www.
elektor-magazine.com/1 10258

[2b] Here comes the bus! (8): http://www.
elektor-magazine.com/1 10428

[3] From BASIC to Python (1): http://www.
elektor-magazine.com/1 10483

[4] From BASIC to Python (3): http://www.
elektor-magazine.com/1 10483

[5] Matplotlib documentation: http://matplot-
lib.org/contents.html

[6] Sandro Tosi: Matplotlib for Python
Developers

[7] Author's home page: http://staff.ltam.lu/
feljc/home.html

[8] 'Python Programming and GUIs for Elec-
tronic Engineers' by Andrew Pratt: http://
www.elektor.com/products/books/pro-
gramming/python-programming-and-guis-
for-electronic. 1320886. lynkx

Figure 4.

Terminal window with
packet counts and a
graphical display.

www.elektor-magazine.com | September 2013 53

Using Libraries

By Neil Gruending So far the previous DesignSpark Tips and Tricks

(Canada) articles have discussed how to setup and con-

figure DesignSpark from a new installation per-
spective. In this installment we will look at how
to use libraries to create a schematic and PCB
design in DesignSpark.

Component

Figure 1 . What are libraries?

Properties of a component When we created schematic title blocks we started

the design software needs. by creating a schematic symbol in the schematic

symbol library and then created a component in
the component library that referenced that sche-
matic symbol. That was an example of a sche-
matic documentation component that didn't need
any PCB design information associated with it.
But normally you would probably want to create
components that would incorporate the informa-
tion as shown in Figure 1.

design information in multiple components. For
example, you could make multiple resistor com-
ponents by creating one schematic symbol and
then reusing it in the other components. Because
all the components refer to the same symbol,
any changes to it automatically propagate to the
components that use it. The same is also true
for PCB footprints and the 3D CAD models. The
DesignSpark website has a good tutorial about
libraries and how they're used, see [1].

Organizing them

The libraries that come with DesignSpark are usu-
ally installed into 'C:\Users\Public\Documents\
DesignSpark PCB 5.0\Library' and are a good
example of how to organize a large component
library organized by manufacturer. I prefer to
organize my libraries by component type because
I also use the libraries as a component part num-
ber database. For example I have a 2N3904 in
my transistor library, but I have multiple man-
ufacturer part numbers associated with it so I
don't have to remember which transistor man-
ufacturers I've used previously. I also try and
reuse schematic symbols and PCB footprints so
those go into generic libraries which make my
library structure like this:

• Grouped component libraries (transistors,
resistors, capacitors, etc)

• A generic schematic symbol library (resistor
symbol, capacitor symbol, etc)

• A generic surface mount PCB footprint
library (0603 footprint, LQFP footprints, etc)

• A generic through hole PCB footprint library
(DIP footprint, 1/4W resistor footprint, etc)

The schematic symbol is stored in a schematic
symbol library file (*.ssl), the PCB footprint is
stored in a PCB symbol library and the 3D CAD
model is stored in a 3D view library file (*.pkg).
The top level component is stored in a component
library (*.cml) along with the part number and all
the technical information. The component library
also stores the references to the other libraries
needed to complete the component.
DesignSpark uses different files for the different
types of library data so that it's easy to reuse the

So now that we've talked about libraries, let's
learn how to use them starting with Modelsource.

ModelSource

If you haven't heard, Modelsource is an online
database of components that's available to use
in many different PCB software packages includ-
ing DesignSpark. I like that DesignSpark directly
connects to Modelsource so you can find compo-
nents without leaving the application (a tutorial is
available at [2]). It's also a great resource to find

54 | September 2013 | www.elektor-magazine.com

Tips & Tricks

IPC compliant PCB footprints which meet stan-
dard manufacturing guidelines. To open Model-
Source in DesignSpark, click on the ModelSource
button or go into the 'View->ModelSource Bar'
and you will see the ModelSource screen shown
in Figure 2.

Let's search for a surface mount MMBT3904 NPN
transistor using the parametric search engine.
Click on 'CLICK TO CHOOSE' and login if neces-
sary. To find a list of the available bipolar tran-
sistors by choosing 'Semiconductors->Discrete
Semiconductors^ Bipolar Transistors', which lists
740 different transistors like in Figure 3.

Now let's narrow down the search results by
adding some filters to the columns of data. For
'Transistor Type' select 'NPN', 'Mounting Type' to
'Surface Mount', 'Package Type' to 'SOT-23' and
'Maximum Collector Emitter Voltage' to '40V'. The
second transistor listed is an MMBT3904 which is
exactly what we were looking for. After pressing
the 'Load Preview' button you will get the follow-
ing screen where ModelSource will show you the
schematic symbol, PCB footprint and some key
component design parameters (see Figure 4).
You can also find components using the 'Part
Number Quick Search' field if you already know a
portion of the part number. Now that we've found
our transistor click on 'Use Component' to use
the component in your design and DesignSpark
will download the component to a library in the
downloaded libraries directory (you can find the
full path in the Folders tab in the Library Man-
ager). DesignSpark will tell you the name of the
library after the component is downloaded. You
can now add the transistor to your design by
dragging it from the ModelSource window into
your design or you can use the usual 'Add Com-
ponent' toolbar button.

But what do you do when ModelSource doesn't
have the part you want or you want to change
something about the component? For example,
I would change the MMBT3904 component we
found to have a more conventional schematic
symbol that shows the emitter. That's when it's
time to use your own custom libraries.

Custom libraries

I always like to create my own set of libraries but
that can be a lot of work. So I like to copy com-
ponents from other sources when possible and

DesignSpark PCB 5.0 wired by RS - [Schematic Design: temp *]

a] File Edit View Add Settings Qutput look y^rdow Help BOM Quote PC8 Quote

D0HO iiUfffliSS* <9

- & x

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15

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View Datasheet! Use Component

m

[<]

[>][

ModelSource

Abs

33030.88

34115.24 thou

then modify them. For our MMBT3904 example
that would mean copying the downloaded com-
ponent information into our own libraries using
the Library Manager and then editing the com-
ponent as required. This is also a good time to
double check everything in case there's an error.
The most important part of setting up your own
libraries is to use common attributes for every
component, so that it's possible to generate

Figure 2.

The ModelSource screen just
after opening it.

Figure 3.

The ModelSource screen
displaying search results.

www.elektor-magazine.com | September 2013 | 55

DESIGNSPARK PCB

Figure 4.

Detailed information of a
transistor in ModelSource.

jlj DesignSpark PCB 5.0 wired by R$ - [Schematic Design: temp *]

^ File Edit View Add Settings Output Tools Window Help BOM Quote PCB Quote

m

■U! OESICNSPARK PCB

L

- 6> X

io *i •

^ ModelSource (Logged in as neilg)

A o

■0 x

Semiconductors

L D iscr ete S emiconductor s

A

i3

n

ED

Bipolar Transistors

Logout

Clear

Help

Part Number Quick Search:

Go

MPN=MM8T3904LT1G

RS Number=5450343

Ptice=0.099

Qly«50

RoHS-Y

Manufacturer=ON Semiconductor
Package Type=S0T -23
Mounting Type=Surface Mount
Pin Count=3
Transistor Type=NPN
Maximum Collector Emitter Voltage=40 V
Number of Elements per Chip=1
Conf iguration-S ingle

Load Preview

View Datasheet

Use Component

MPN

RS Number

Price

Qty

RoHS

Manufacturer

Package Type

E

Mounting Type
Surface M... ’ r

Pin Count

SELECT

SOT-23

SELECT

MMBT2222AL... 5449400 0.080 1

MMBT3904LT... 5450343 0.099 50

ON Semicond... SOT-23

BJy

OT-23

Surface Mount 3

nr

ki [«i u\ [>1 ® r>n

Products per page:

30 -

Page 1 of 1

Viewing 1 • 6 of 6 products

’‘fc ModelSource (Logged in as neilg)

AbS 18815.14 23286.58 thou
—

reports like bill of material (BOM) reports. For
example, I prefer to store all of a component's
manufacturing information in the component
library. This means that I usually have multiple
manufacturer part numbers, so I use the attri-
butes as shown in Figure 5 where I've added
three additional manufacturer part numbers. Note
that the other attributes are required if you want
to use the DesignSpark BOM quoting function.

Conclusion

ModelSource and DesignSpark's libraries are
a great resource when creating our own set of
libraries and they can save a significant amount
of time. At this point we can create a schematic
and next time I will talk about some tricks when
editing a schematic and how to generate a bill
of materials.

( 130207 )

Component

Package

Gate

Values

}'

Vendor«ON Semiconductor
FI Manuf act urer_Part _Number ■ M M B T 3904 LT 1 C
Q Manufacturer_Name=ON Semiconductor
QRS Part Number* 5450343
AJIied_Number=

Manufacturer! =On Semiconductor
Manuf actunerl _Part = M M BT3904LT 1 G
Manuf actuner2 = Fairchild
[H Manuf acturer2_Part = M M BT3904
PI Manufacturer3=NXP

Manufacturer3_Part =MMBT3904.21 5

Delete

Edit...

Up

Down

All Packages

Internet References

[1] www.designspark.com/tutorial/
components-library-structure-library-manager

[2] www.designspark.com/eng/tutorial/compo-
nents-downloading-from-modelsource-build-
ing-up-libraries

Figure 5.

Useful attributes of a
component.

\ 7 \ New Values are added to all other Packages

OK

Cancel

Apply

Help

56 | September 2013 | www.elektor-magazine.com

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•Labs

.Labs Tips & Tricks

By Clemens Valens

(Elektor.Labs)

Summer is over, projects have been wrapped up, now is the time to document it
all on Elektor.Labs. Here are some tips and tricks to get more out of your online
publications.

Proposals

Finished

In Progress

Active Popular

Mains Gate:
Programmable Relay &
Enerqy Monito...

4,540 views

Active Popular

Clektor.POST No. 12
(Low-cost Allergy
Fiqhter)

1,154 views ★ ★★★★

Intelligent Cuclight
System for Theatres and
Event...

758 views

EDITOR'S

CHOICE

efficient solenoid valve
[13U258-1]

810 views

Active Popular

Wireless battery charger
(Receiver POQlRX)
[ 130168 ...

165 views ★ ★★★★

Icons

We keep enhancing the Elektor.Labs website by adding
useful features. It is an evolutional process, and features
are added as soon as we discover that we need them.
One of the latest additions is in the form of icons that you
probably have noticed on the homepage. Currently there
are four icons available for drawing attention to a project.
Two of these icons, the blue Post icon and the yellow
Editor's Choice icon are controlled by Elektor editors. The
Dead End and SOS icons are available to all posters. They
can be activated to let other users know that you need
help or that you are stuck.

Please note that when you activate the Dead End icon,
your project may be moved to the Finished column, so
use it with care.

The Elektor.Labs homepage now boasts icons.

View

Edit

Speed Projecting Display for Car

Save

Help wanted:

(ft) N/A

O Help warded
O Deadend

Main project picture:

Remove

Title:

Soeed Proiectina DisDlav for Car

Here is where you activate
an icon for your project.

58 September 2013 www.elektor-magazine.com

elektor labs

Project visibility

Even though project header illustrations are optional, I
highly recommend that you upload a photo or drawing
anyway for the simple reason that nicely illustrated
projects have a higher priority and end up higher in the
lists. Every time you update your project— when you click
the Save button— it is moved to the top of the list if it has
a project header photo. The website provides a nice default
picture of a dirty scribbled-on coaster (or "beer mat" if
you prefer), but it is ignored by the rating mechanism.

Make sure to replace the default project header
photo by a nice photo of your own project. It will
improve the visibility of your project.

www.elektor-labs.com

Active users

Some people are really prolific on Elektor.Labs and we like that a lot. Because these users are important to us, we have devised
a scoring mechanism that allows us to identify active posters in an objective manner. Every once in a while Elektor staff have
some hardware, books or other goodies to give away and the active users will be first in line to receive these freebies. The
scoring system is simple: posting a project is worth four points, a contribution is two points and every comment earns you one
point. The next step is of course to make the scores visible on the website— we are working on it.

P.S. Note that scoring is not completely automated, real people are involved, so posting rubbish projects, contributions or
comments - spam in short - will not get you anywhere. We may even decide to block your access.

Passwords and email addresses

This is an awkward subject and, understandably, we keep receiving questions about it. The main thing
for you to know as an Elektor Member is that, for historical reasons, we currently have two independent
systems with two different login domains. We are working on unifying it all, but, unfortunately, that takes
time. Your Elektor.com, Elektor.Post and Elektor.Store account is not the same as your Elektor.Labs and

Elektor.Magazine account. You can make things simple for yourself
by using the same email address and password for both accounts,
but that's not mandatory.

If you want to change your email address just contact us by sending
your old and new email addresses to service@elektor.com or labs@
elektor.com and we will do it for you.

Two accounts require two sets of login credentials.

elektor labs

I ogin

Email:

Password:

| Login *

Fornnt password?

“Participating on Clektor LADS is reserved to Clektor members, an Clektor account
Is required to sign on.

Elektor member;.: please use your account details to enter Uie Elektor LABS
website.

I wont to become an Elektor member

* PAcewrvrri fnrgnfTpn?
r Register

Shoppinq cart Vr 1

blektor Credits : U

www.elektor-magazine.com September 2013 59

•Labs

90 Degrees and Rising

By Thijs Beckers

(Deputy Editor)

It wasn't just the temperature outside that rose
significantly this summer @ Elektor House. While
testing his prototype of the soon to be published
battery testing circuit, lab worker Tim Uiterwijk
was surprised to measure the temperature of a
7-watt series resistor (the big white one in the
picture with the thermometer sensor held on it)
at well over 90 °C!

This wasn't expected, as the calculated dissi-
pated energy (P djss ) was well below the 7 watts
the power wire wound resistor is allowed to dis-

sipate according to its specifications. The dissi-
pated power could be calculated using the well-
known formula:

^cliss X R.

While the upper limit of the current through this
resistor was calculated to be 8 A, during a test it
had been limited to 4.5 A. So in this case (with I
= 4.5 A and R = 0.1 ft), with just (4.5 2 x 0.1 x)
2 watts this relatively big ceramics clad resistor
got very hot fast. Too hot, actually. In general,
any component exceeding a body temperature
of 80°C is regarded bad practice in our labs, so
a solution had to be found.

As can be seen in the pictures, the circuit is
mounted on a heatsink. This heatsink is a 'stan-
dard' CPU cooler (with fan) intended for an Intel
P4 processor. Mostly due to their active air flow,
this type of heatsink boasts a very low ther-
mal resistance to air; of the order of 0.40 K/W.
So they're excellent for dissipating lots of heat
— depending on the model a PC CPU can easily
generate 125 W or so — so the heatsink obvi-
ously has to be able to "digest" that. Another
shunt resistor has already been mounted on the
heatsink (on top in the top photo), but there's
enough room for a second one.

Tim swapped the ceramics covered resistor with
an aluminum housed wirewound power resis-
tor rated at 50 W, and he mounted both power
resistors on the sides of the heat sink where
the airflow was highest, see the bottom photo.
This solution proved adequate, with the previ-
ously hot headed power resistor now reaching
only 33°C under identical circumstances. With a
maximum (software limited) current of 8 A, the
resistor now heats up to about 50 degrees C,
satisfying the Nothing-Hotter-Than-80 rule and
even providing a little headroom. Theoretically,
the 50-W resistor should be able to withstand
currents of up to 22 A and temperatures of up
to 250 °C(!), but those extremes will never be
met in our application.

Problem solved. Now keep an eye on our upcom-
ing editions— an article with the full schematic and
description of the circuit will be published soon.

( 130055 )

60 September 2013 www.elektor-magazine.com

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MIFARE is the most widely used RFID technology, and this book
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standards, RFID antenna design, security considerations and crypto-
graphy. The complete design of a reader’s hardware and software is
described in detail. The reader’s firmware and the associated PC
software support programming using any .NET language. The
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•Industry

The new
868 MHz NT Series RF
transceiver module from Linx makes it
simple to send and receive digital data in the 863
to 870 MHz band. The module offers the option of
UART or True Transparency™ interfaces which lets
the user create a wireless wire for use with nonstan-
dard data rates, custom protocols, or encodings such
as PWM and Manchester. The module features best-
in-class receive sensitivity (up to -113 dBm) and low
power consumption (only 19.2 mA in receive mode
and 16 mA in transmit mode at 0 dBm).

Unlike so-called transparent modules that require a
UART interface and buffering scheme, the True Trans-
parency™ feature of the NT Series reduces latency by
transmitting raw data. All the user has to do is con-
nect circuitry to the input line of the sending trans-
ceiver and the output line of the receiving trans-
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urable up to 8 channels. Although programming is

NT Series Transceiver
now Available in 868MHz for
European Market

not required, the NT Series has a UART interface that
enables more advanced configuration with a micro-
controller, RS-232 interface or USB interface. The
868 MHz version offers 68 channels, which lets
the user implement a frequency agility scheme
such as frequency hopping or listen before talk.
The module has a maximum output power of
+12.5 dBm and a receiver sensitivity of -113 dBm.
This gives the module a typical line of sight range
of up to 2.5 miles (4 kms) at the maximum output
power with typical monopole whip antennas. Regu-
lations in the country of operation dictate the maxi-
mum legal output power, so the final system range
may be less depending on the country of operation.
Typical range at 0 dBm is up to 3,000 feet.

To aid rapid development, the NT Series transceiver is
available as part of a master development system. The
system comes with two development boards for bench-
marking and prototyping, each of which is populated
with a transceiver. The boards include lights that show
you which areas are active as well as other enhance-
ments that simplify the development experience.

To help you get started, the system also includes
antennas, a daughter board with a USB inter-
face, demonstration software, extra modules and
connectors.

www.linxtechnologies.com (130285-11)

Mini-ITX-board for Intel Atom

The HB131 is a small form factor mini-ITX board based on low power Intel Atom
Cedar Trail platform. The dual-core Atom D2550 processor, which is offered with
Intel's NM10 chipset, is primarily known for its lower power consumption &
graphics enhancements compared to earlier Atom processors. The Mini-ITX board is
equipped with dual Gigabit LAN ports and rich I/O. This environmentally responsible
embedded mini-ITX board exceeds its predecessors in both performance speed
and graphics capability all to offer greater scalability in a smaller footprint fanless
PC solutions. The mini-ITX board features rich I/O interfaces: 8 USB 2.0, 6 COM
ports, 2 LAN, 1 VGA up to 1920x1200, 1 LVDS up to 1440x900 (24bit). On board
are 1 PCI and 1 mini-PCIe slots for richer connectivity, lx SO-DIMM supports DDR3
800/1066MHz RAM up to 4GB, 2 SATA II. It is designed to be a high performance,
reliable, secure and easy to manage board. Making it an ideal platform for point of sale, self-service terminals,
queue machines and digital signage. Utilizing the small footprint of 170mmxl70mm, the HB131 offers greater
connectivity with dual Gigabit Ethernet with options of expansion via one Mini PCIe slot.

www.habeyusa.com (130285-IV)

62 | September 2013 | www.elektor-magazine.com

0-120ml/min
Liquid Flow Sensor

The newest liquid flow sensor from Swiss
sensor manufacturer Sensirion excels thanks
to its ultra-pure materials and outstanding
precision. The small SLQ-QT500 sensor is
designed for the needs of the semiconductor
industry specifically.

The SLQ-QT500 covers flow rates from 0 -
120 ml/min. As with all Sensirion liquid flow
sensors, its flow channel is absolutely straight
and has no moving parts. The sensor is based
on the patented CMOSens® Technology. The
microthermal flow measurement is performed
through the flow channel wall, which separates
the chip from the measured liquids. Therefore,
only the PFA tubing and the quartz flow
channel are in direct contact with the liquid.
This guarantees that the sensor has a superb
chemical resistance. Thanks to these features
as well as the RS485 digital interface, the
sensor is able to achieve an exceptionally
reliable measurement with a sample rate of up
to 1 ms.

With this unique technology even liquids with
a very high viscosity (100,000 cP and more)
are not a problem. Andres Laib, Director of
Sales Liquid Flow Products says: "The sensor
is suitable for measuring hydrocarbon-based
solvents such as photoresists, as well as water-
based liquids such as TARC and FI202. With the
SLQ-QT500, liquids with virtually any viscosity
as well as liquids which contain particles can be
measured. This makes the sensor unique in the
liquid flow sensor industry."

www.sensirion.com/slq-qt500 (130285-III)

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www.elektor-magazine.com | September 2013 | 63

•Industry

DesignSpark PCB Version 5.0 is Here

RS Components (RS), the trading brand of Electro-
components pic, has unveiled the latest release of
DesignSpark PCB, the company's award-winning profes-
sional software for schematic capture and PCB layout.
DesignSpark PCB Version 5.0 integrates two additional
features within the free design tool — online Design
Rule Checking and buses — which have been introduced
to further reduce design times for engineers and to
minimize errors during the design process.

This new release builds upon the previous version of

DesignSpark PCB announced in October 2012, which
provided access to the industry-leading ModelSource
component library, PCB quote service and BOM quote
functionality.

Design Rule Checking determines whether the physi-
cal layout of an integrated circuit satisfies a series of
recommended parameters called design rules. DRC is
an important step during the physical verification of a
design, which is normally carried out in the post-design
phase. Online DRC is an advancement of this process
that enables the engineer to detect errors in real-time
during the design stage, highlighting any issues before
the design is finalized and layout completed.

The inclusion of online DRC within DesignSpark PCB Ver-
sion 5.0 is a significant addition to the software's func-
tionality, introducing considerable time saving benefits
to the user as any errors can be rectified immediately
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ensuring maximum yield and reliability.

The increasing adoption of digital design has resulted in
the use of on-chip buses, or bundles of related wires, to
transfer data, enabling the engineer to combine multiple
data signals into a bus in multiples of 8 (e.g. 8, 16, 32).
Instead of drawing and/or labeling the individual wires in
a schematic, a single 'bus wire' can be used to represent
related wires, thus simplifying the final schematic. This
also enables simulation and debugging to be carried out
with minimum errors. Buses have been added directly
into DesignSpark PCB Version 5.0, allowing the engineer
to refine the schematic during the design phase.

www.designspark.com (130167-III)

Ultra Stable Surface Mount TCXO

Bliley Technologies' has released industry standard 5 mm x 7 mm
TCXO in an SMD hermetically sealed package. This series of TCXOs
threatens OCXO Frequency vs. Temperature stability without the power
consumption or cost. The frequency vs. temperature stability starts as
low as ±50 ppb.

Options available for customer selection include supply voltage;

+3.3 VDC or +5 VDC, HCMOS, LVCOS, or clipped sine wave output, and
operating temperatures as wide as -40 °C to +85 °C. This series can
be offered as a TCXO or can be configured with electronic frequency control range of ±5 ppm.

In addition to outstanding frequency vs. temperature stability this series of TCXO offers excellent phase noise at
10 MHz of -125 dBc at 100 MHz offset and a floor of -150 dBc. The T85H Series products are well suited for all network
timing and general precision applications where optimum reliability and performance meet excellent price targets.

www.bliley.com (130167-VI)

64 | September 2013 | www.elektor-magazine.com

CD

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Miniature Switch
Saves Valuable
PCB Space

C&K Components has developed the
new PTS 530 Series ultra low-profile
top actuated SMT switch that is ideal
for consumer electronic applications.
Utilizing C&K's unique symbol
line identification system, design
engineers can quickly and easily
identify actuation force ratings,
particularly valuable for designs that incorporate multiple switches. The marking system
defines the model number and actuation force on the switch itself, which eliminates the
need for designers to rely on confusing schematics.

The PTS 530 Series SMT switch is only 4.5 mm x 4.5 mm with a 0.55 mm thickness
(0.65 mm for high force versions), saving valuable PCB space. The small size combined
with its extensive lifespan of up to 1,000,000 operations makes the PTS 530 Series
satisfies multiple needs that other miniature switches cannot.

The momentary action, top actuated SMT switch features a gullwing or J-type terminal and
can be soldered via infrared reflow in accordance with the IEC61760-1 standard. The PTS
530 Series switches are available in six different varieties, dependent on actuation force.
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www.elektor-magazine.com | September 2013 | 65

Tech The Future

Internet

the Physical Layer

By _ The Internet consists of some 40,000 administratively separate networks linked

Tessel Renzenbrink

(Elektor ttf Editor) together. How does this system-of-systems operate at the physical layer? Is it

flakey and unreliable as some corporations hanging on to their private connections
seem to think? Can it handle the persistent growth of data volumes? Is it expand-
ing to reach the billions of poorly connected people in developing countries? Let's
ask the specialists.

I discussed these questions with Henk Steen-
man, CTO of the Amsterdam Internet Exchange
(AMS-IX) and James Cowie, co-founder and CTO
of Renesys, an Internet measurement and ana-
lytics company, in an interview.

Internet exchange

In the early 1990s much of the local European
Internet traffic was routed over submarine cables
across the Atlantic to Virginia, USA. There MAE-
East, one of the world's first Internet Exchanges
(IXs), hosted physical connections to route traf-
fic from one network to another. For many small
European Internet Service Providers (ISPs) it was
the only exchange point available.

In 1997 twenty competing ISPs and carriers
established AMS-IX to interconnect their networks
locally [1]. AMS-IX brought down the cost of data
exchange, reduced latency and eased traffic con-

gestion on the heavily overloaded American hub.
Henk Steenman has been part of the Dutch non-
profit organization from the beginning. With his
help AMS-IX has grown out to be one of the larg-
est Internet exchange points in the world. Con-
stantly in a close race for first place with DE-CIX
in Frankfurt, AMS-IX currently comes second with
595 participating networks, and traffic peaking
at 2.3 Tb per second.

Internet intelligence

Renesys is an American company that collects
and analyzes data about both the logical and
the physical structure of the Internet [2]. "The
logical map tells you how the Internet believes
it should be routing traffic", says James Cowie.
"It basically says if you needed to reach this per-
son and you were someplace else, what chain
of organizations would help you carry it there.

66 September 2013 www.elektor-magazine.com

Internet @ the Physical Layer

The physical map is more detailed and involves
figuring out which IP-addresses, which routers,
are connected to each other and which ones of
those are actually the most useful in moving
traffic closer to its destination. We take active
measurements of millions points of hundreds of
places worldwide to make an accurate map of
what the Internet is doing.

"We use that information for customers who need
to figure out how to use the Internet effectively as
a tool of business. People tend to study their own
part of the Internet very carefully. But nobody
worries about what is over the horizon. One of
the things we provide is that big picture because
more and more companies have a global inter-
est. The Internet is not a managed system so
we provide some of that missing transparency."

Physical layer

"It's interesting what we learn about the physical
layer from the logical grid and these performance
readings on the sensors", says Cowie. "There
is a good example of a case where we saw a
number of networks in Iran and Iraq disappear
simultaneously. We thought this was strange so
the next day we looked at the media to see what
this might have been. It turned out there is a gas
pipeline that goes from Iran across the frontier
into Turkey and then on to European markets
for the energy. Pipelines take a lot of effort to
negotiate because you have to get the rights of
way, secure it and bury the pipeline. So when
people do that they typically also put some fiber
optics next to the pipeline because it's basically
zero marginal cost. That must have been the
case here because that day there was a roadside
bomb that had broken the pipeline."

"The heartening thing is that the Internet did
not permanently get impaired by that because
the Internet works around things like that all the
time. There was probably another fiber route that
could be used and was failed over to. So in our
data you see a problem and then a recovery. The
Internet is much more resilient to damage even
than it's fabled."

Government regulation

Governments the world over increasingly want
to regulate the Internet at the end user level. I
asked the two specialists if they see the same
trend at the infrastructural level.

Cowie: "The ITU, the UN agency responsible
for global telecommunications regulations, took

their eye off the ball during
a critical window of inflation
where the Internet stepped
beyond something that
could be lightly regulated.
Which was— in my opinion—
an excellent stroke of luck
because it now becomes
much more difficult to ret-
roactively put things back
in the bottle.

It is always possible govern-
ment intervention will cause
a mounting regulatory bur-
den, you're always operat-
ing in some jurisdiction. But
I think that governments
realize that the Internet's
fluidity makes it possible for
IT services to go anywhere.
The people are going to be
reluctant to do things that
will cause their local market
to look less favorable from
an investment standpoint.

I often am asked by peo-
ple if their part of the Inter-
net can be taken offline as
happened during the black-
outs in Egypt and Syria. I
think that in Western Europe
and the United States there
aren't really many threats
left to the Internet. The
Internet has grown so won-
derfully diverse in these
places that in terms of being
attacked or people taking
the Internet offline it really
can't happen anymore. It's
beyond that stage."

The AMS-IX CTO isn't keen
on increasing regulations
either: "Currently the Dutch
regulators are keeping
their distance with respect
to AMS-IX, but that could
change. If regulations and
bureaucracy were imposed
on us, it would be at the cost
of the flexibility and simplic-
ity with which we run our
operation. One of our most

Henk Steenman, CTO of AMS-IX

James Cowie, CTO of Renesys.

www.elektor-magazine.com September 2013 67

•Tech The Future

important qualities is that we're a carrier-neu-
tral IX which means that any ISP can connect
to exchange traffic. We'd like to propagate our
neutrality as much as possible, and I am afraid
that if the Government steps in we will lose some
of that."

Data flood

AMS-IX deals with a doubling of traffic volume
roughly every two years. The challenge for Henk
Steenman and his colleagues is to find technical
solutions to deal with that growth. "We're now
implementing 100 Gb/s Ethernet equipment which
became available last year", says Steenman. "Up
until then we used the 10 GbE standard so we're
increasing the throughput rate of our network
by a factor 10. As one of the largest exchanges
we're always running up against the limit of what
is technically possible. We're participating in the
IEEE standard body where the next standard is
being developed which is going to be 400 GbE.
The data rate of each new Ethernet standard has
always been increased by a factor 10 but the
technology simply isn't ready to make the jump
to 1 Tb. Although in terms of growth we could
really use that. On the other hand, growth is a
two-way street, traffic can't grow faster than the
available infrastructure allows, so I don't foresee
any serious shortage."

James Cowie isn't worried about capacity either.
"If you look at the total amount of subsea fiber
that interconnects the various continents, a very
small portion of that is actually lit and available
for use. There is enormous amounts of reserved
bandwidth. And inside continents, especially in
Europe, there is just amazing amounts of avail-
able bandwidth that could be lit if the traffic
grows. I don't think that is ever going to be a
problem."

Digital divide

In most developed countries a well-established
Internet infrastructure provides fast and cheap
connections. In developing countries, however,
infrastructure has lagged behind, causing a dig-
ital divide. Is the gap closing?

James Cowie: "The trend is that the countries
that had the least Internet do the best as soon
as the Internet does arrive. East Africa is a per-
fect example. The only Internet that had been
available was very limited, much of it was over
satellite connections which are very slow and
super expensive. Until the submarine cables came

on shore. Within three months we could see the
whole market turning upside down. People were
canceling their satellite contracts and they were
moving all of their things onto this cable, and
the data rates went from tens of kilobits per
second to a gigabit network. Exactly like that in
the space of weeks and months.

What happened there ultimately was that peo-
ple were leapfrogging generations of technology.
Maybe they never get a desktop computer— they
just move right ahead an get a smartphone. Not
having had the continuous evolution through all
the stages that Western Europe went through,
they get to cherry pick the very best technology
at the very lowest prices. So it is actually very
positive. The digital divide is still quite deep but
the Internet is a great leveler."

Henk Steenman: "Now that the networks are
rapidly growing in East Africa the next thing
needed in terms of infrastructure are regional
IXs. In Kenya, for instance, much of the local
traffic destined for neighboring countries is routed
via Europe for lack of a well-established regional
exchange point. They are facing the same prob-
lem that motivated us to establish AMS-IX in the
1990s. So we thought: "we've done this before,
why not do it again?" We're now developing an
IX in Mombasa in collaboration with the Kenyan
Telecommunications Service Providers Associ-
ation of Kenya (TESPOK) to improve regional
connectivity."

When I asked James Cowie where he thought the
Internet would go from here he answered: "Yeah
that's the part where I stop making predictions.
The only thing we can be sure of is that it will
be unexpected. It will be something completely
different. We're always wrong. I am guessing the
innovation is going to come from all these people
in East Africa who are on the Internet and have
real need. And it will be something that would
have never occurred to us because we don't need
things, really. We have most of our needs met.
So these folks will figure this out."

( 130130 )

Internet References

[1] www.ams-ix.net

[2] www.renesys.com

68 September 2013 www.elektor-magazine.com

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•Magazine

Radiometer
PHM22 /
PHA928a
Blood pH

/ o 2 / co 2

Analyzer

System

Please wait for
the results of your
blood test

By Seppo Lindeman (Finland)

Is it a bright green instrument? No, more like
greenish grey. Must be made by Radiometer,
Copenhagen then, as that was their favorite
(and only?) color for over 50 years of producing
high quality electronic test gear back in the 20 th
century. The first time I came across Radiometer
equipment was in 1961, at my first job at the
HelvarTV and radio works in Helsinki. There were

many green Radiometer vacuum tube voltmeters
and signal generators around at the place— easily
noticed of course as they have a tendency to flock
in small groups near AC outlets. It wasn't until
later that I noticed that all Radiometer gear was
"that" green, be it medical-analytical or electronic
test instruments.

It was at a time when:

70 | September 2013 | www.elektor-magazine.com

^efagmcA

• transistors entered competition commercially
with vacuum tubes, and nobody had ever
heard of ESD (electrostatic discharge);

• Germanium transistors often broke down
"mysteriously";

• vacuum tubes lovers kept joking that the
single advantage of transistors over tubes
might be formulated as: "taking less space
in the bin when defective".

That time, roughly.

Muppet Beaker's delight

The hematic pH/0 2 /C0 2 analyzer discussed here
is not a single instrument— it consists of many
individual assemblies. The first models appear
to have been made in the 1950s. The equipment
discussed here however is mainly 1960s. These
instruments appear in various guises, like bench
models or 'mobile' for moving around on carts
(trolleys). The instrument constellation shown
in Figure 1 consist of a blood pH meter type
PHM22t with glass and calomel electrodes, a Micro
Electrode Unit, a type PHA928a Oxygen Monitor
with a p0 2 electrode and a D616 thermostat
controlled cell, a tonometer and two humidifiers
made from glass, a type VTS13 thermostat, a
meter scale expander and two gas bottles.

pH Meter 22 (v. 1966)

The heart of the analyzer system is the PHM22t
(model year 1966) pictured separately in
Figure 2. There are five vacuum tubes in the
meter (Figure 3). The circuit diagram pictured
in part in Figure 4 allows seven sections to be
identified roughly: input amplifier, chopper, AC
amplifier, demodulator, meter, power supply and
compensation voltage divider. Three .jpg files
constituting the complete schematic can be
downloaded from [2].

The output voltage rate of the glass electrode is
61.54 mV pH-i at 37 °C (approx. 100 °F). The
meter has a resolution of .001 pH, equating to
61 microvolts. A pure DC amplifier built from
vacuum tubes is not easy to keep stable at such
low voltages. Also, the glass electrode output
current is so low as to call for an amplifier input
impedance in the range from 50 to 500 megohms.
If you look at Figure 5, there's the principle
of operation of an older Radiometer pH meter
type PHM12. It incorporates vacuum tubes
configured as a high gain amplifier. There is also a
'Weston Pair' type reference element ('Normal').
Before starting any measurement, users should

press the Test button and register the analog
measurement needle with a mark on the scale.
This is to ascertain that any drift observed is not
due to the amplifier.

Inside PHM22t

The input amplifier of the pH meter comprises a
chopper circuit. The chopper converts the input
DC signal into an AC signal, which can be amplified
more easily and drift-free. The chopper is similar
to the Brown Converter pictured in Retronics , April
2013 [1]. The original chopper of this meter was
mechanical, but later it got changed to a photo
chopper (VR7). Sadly, humming away at 50 Hz
a mechanical chopper can be relied on to fail
after a few years due to contact
problems.

The AC amplifier is a 3-stage
vacuum tube amplifier with
feedback from the cathode of the
output stage to the cathode of
the input stage. The output of
the AC amplifier is transformer-
coupled to the demodulator
circuit. The demodulator
converts the AC signal into a DC
signal virtually proportional to
the direct voltage at the input of
the pH meter (Figure 6).

One interesting thing to note
concerns VI, the first tube
in the PHM22t (not shown in
Figure 4). Its filament voltage
is lower than 6.3 V due to a 3-Q.
series resistor, R48— and VI is
a rectifier. The intention is to
raise the tube's input impedance
by keeping the cathode of the
tube 'colder' than normal. On
the down side, reducing cathode
emission is likely to shorten the
tube's lifetime owing to cathode
poisoning. When I opened the
rear cover of the meter for the
first time, I noticed that one tube
glowed dimmer than the others,
and mistakenly thought it was
defective.

The DC supply voltage to the
amplifier comes from a full-
wave vacuum tube rectifier. The
input stage operates off an 85-V
voltage stabilizer tube.

www.elektor-magazine.com | September 2013 | 71

•Magazine

PHA928a
oxygen monitor

The oxygen monitor is
an all-passive instrument
containing only feedback
circuits that get connected
into the amplifier and
meter circuitry of the
PHM22 pH meter and the
external meter.

The p0 2 electrode is
operated at a polarization
voltage of 630 mV
generated off a 1.35-V
mercury battery. The
circuitry around it is
all low impedance, low
current. Figure 7 shows
the simplified diagram of
the p0 2 channel. Figures
6 and 7 show how a
single pH meter may be
used for two different
measurements.

Carbon dioxide
levels (pC0 2 )

The instrument
constellation does not
feature a pC0 2 electrode.
The C0 2 levels in blood are
measured indirectly using
three blood samples.
The first measurement
is direct pH. The other
samples are placed in
separate chambers of
the tonometer, which also
accept a feed of 4% C0 2
gas and 8% C0 2 gas (Figure 8). After about
4 minutes of equilibrating, the pH of both samples
can be measured. The pC0 2 value can be obtained
from Astrup's (eta/.) method using Siggaard-

Anderson's nomogram
correlating arterial pH
and pC0 2 [3]. There's a
lot more to glean from the
same nomogram in terms
of human physiology,
but we are not medical
students here and the
matter is well beyond the
scope of this article.

IESP 20041

Retronics is a monthly section covering
vintage electronics including legendary
Elektor designs. Contributions, suggestions
and requests are welcome; please telegraph
editor@elektor.com

Micro Electrode Unit

The main cause of inaccurate measurement
results is drift. The electrodes are sensitive to
temperature and static buildup due to the very
high impedance. The blood samples leave stains
on the glass and also degrade (to a degree) the
polypropylene membranes of the electrodes. The
electronic parts also cause drift. All electrodes
are protected with glass jackets and surrounded
by thermostat-controlled water. The water is
lightly saline, creating a liquid shield around the
electrode— like a metal cover protects a sensitive
amplifier (Figure 9).

Because the electrode sensitivity is subject to
change, calibration is necessary using reference
solutions for pH, and zero and saturated liquids
for p0 2 .

Fifty years on

Today, hematology lab workers have an array
of control ampoules at their disposal, and have
the comfort of massive quality check systems.
Nothing of the sort was available in the 1950s
and 60s when the Radiometer PHM22/PHA928a
kit was used in hospitals. One hospital in Helsinki
reportedly ran their very own quality check and
calibration "Retronics Style": if there was so much
of a hunch of incorrect measuring results from the
PHM22/PHA928a equipment, laboratory staff were
quick to turn their brand new Rolodex, telephone
a certain lady on the hospital cleaning staff, and
wait for her shift to start. She was strong, stout
and permanently healthy— in a word: Known
Good. A nurse would simply take samples of the
good lady's blood and have them analyzed. If the
lab staff thought the pH, p0 2 and pC0 2 results
made sense then the entirety of Radiometer blood
analyzer gear was declared beyond suspicion and
good to go for most patients— except possibly for
one Jackson, Michael Joseph (b. 1958).

( 130132 )

Internet References

1. The Curse of the Collector, Elektor April
2013. www.elektor.com/120753

2. www.elektor.com/130132

3. www.anaesthesia.med.usyd.edu.au/resourc-
es/lectures/acidbase_mjb/description.html

72 | September 2013 | www.elektor-magazine.com

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•Magazine

Hexadoku Puzzle with an electronic touch

It's irrelevant if it takes you 20 minutes or 3 weeks to solve our popular Hexadoku puzzle— what matters is the
achievement of actually solving this brain teaser. If you believe you are successful at finding the solution in the gray

boxes, submit it to us online, and you automatically enter the prize draw for one of four vouchers.

The Hexadoku puzzle employs numbers in the hexadecimal
range 0 through F. In the diagram composed of 16 x 16 boxes,
enter numbers such that all hexadecimal numbers 0 through F
(that's 0-9 and A-F) occur once only in each row, once in each
column and in each of the 4x4 boxes (marked by the thicker

black lines). A number of clues are given in the puzzle and
these determine the start situation.

Correct entries received enter a prize draw. All you need to do
is send us the numbers in the gray boxes.

Solve Hexadoku and win!

Correct solutions received from the entire Elektor readership
automatically enter a prize draw for one Eurocircuits PCB voucher
worth $140.00 (£80.00) and three Elektor book vouchers worth
$60.00 (£40.00) each, which should encourage all Elektor readers to
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1

F

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B

6

0

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D

3

5

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2

2

4

7

0

9

A

B

C

E

B

8

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F

6

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5

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A

2

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The competition is not open to employees of Elektor International Media, its business partners and/or associated publishing houses.

74 | September 2013 | www.elektor-magazine.com

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Gerard's Columns

ConFused

By Gerard Fonte (USA)

The lowly fuse (or circuit breaker) is an important part in
the safety of any electrical device. Primary fuses (for the AC
mains) protect against fire, shock and destroyed equipment.

Secondary fuses (in the DC power supply) are generally used
to safeguard the device. Unfortunately, there seems to be
considerable confusion about these basic parts.

Ratings

There are two numbers associated with a fuse: a current
and a voltage. Nearly everyone knows the current controls
the fuse. More current than specified and the internal link
melts and the fuse blows. The fuse is a current device, so it
doesn't matter what the voltage is. It could be one volt or
100 volts (as long as it is below the rated voltage).

However, it can take a couple of hours for slow-blow fuse
types to open at 135% of rated capacity. At 200%, it's typi-
cally a couple of minutes. The fast-acting fuses may still take
up to an hour at 135% of rated current but only a few seconds
at 200%. Most people are quite surprised at this much delay.

At ten times the current, the blow times are about 0.1 seconds
(slow) and 0.01 seconds (fast). For modern electronics, this
is still plenty of time to do significant internal damage. (The
speed specification is provided in the data sheet as Ft. The
lower the value, the faster the fuse.)

The voltage specification is the confusing one. It means
that the blown fuse will present an open circuit at the rated
voltage. That is: the gap in the link will be physically large
enough so that there will be no arcing between the internal
remains. Obviously this is a very important consideration. If there is
arcing between the fuse elements, then the fuse is not really protecting
anything. Current is still flowing into the device.

from shock and vibration. Mounting your fuse on a thin
panel next to a big motor is just asking for trouble.

The fuse is also a thermal device. If you are using sur-
face-mount fuses, or soldering fuses directly into your
circuit (instead of using fuseholders) you must be very
careful with the application of heat. It is quite possible
to blow the fuse with improper assembly.

Sizing (mains power)

If you are wiring a house the fuses (actually circuit-break-
ers) are sized according to the appropriate building code.
This code is often based on the thickness of the down-
stream wire rather than the actual current the circuit is
expected to handle.

Unfortunately, for electronic projects there does not appear to be
a real standard. My old ARRL Radio Amateur's Handbook says to
add 10% to 20% more than the expected maximum current draw
and then use the fuse with the next largest current rating. So, if
your project is expected to draw one amp (of AC mains current) you
would probably choose a 1.25 amp fuse. However, 200 pages later,
the book says to fuse at 150% to 200% of maximum.

It is my suspicion that no one wants to be too specific because of legal
concerns. If someone publishes a 'standard' (or even a suggestion) and
someone else gets hurt using that standard or suggestion— lawsuits
will flow. (I'm certainly not going to make any suggestion or define a
standard!) I also suspect that many products that include stand-alone
AC power supplies ('wall-wart') do so to eliminate AC safety testing and
certification. It may be a bit more expensive to do so but there is much
less legal exposure.

Of course, if you are making a project for your own use, you only have
to consider your own risk. You can do whatever you want. But as soon
as you start selling your device, you must consider the risk to the users.

Application

For AC mains, it is absolutely essential that the fuse be placed in the
hot lead rather than the neutral lead. This makes sense when you stop
and think (always a good idea). A blown fuse in the hot lead stops the
current before it gets any farther into the device. If the fuse is in the
neutral lead, there is still voltage present in the device, even if the fuse
is blown. So if a different ground path is available— like a test lead or
your finger— current will flow. What's more, if the fuse is in the neutral
lead and there is a short to ground (rather than neutral), the fuse won't
blow at all. In this case, the fuse is not in the current-carrying circuit
and is useless.

A fuse in the secondary of a transformer can be in either lead. If there are
multiple transformer windings, or if the transformer has a center-tapped
winding, several fuses may be required.

The standard fuse is a mechanical device and is susceptible to damage

PTC

PTC (Positive Temperature Coefficient) devices are very different from
fuses. They are also thermal devices that operate about the speed of
slow-blow fuses. However, one huge difference is that they do not actu-
ally interrupt the power. Rather, they change to a higher resistance and
limit the current. Thus, some current still flows. The second big differ-
ence is that they 'self-reset' when they cool down. Usually, this requires
the removal of AC mains power. However, if the short-circuit goes away
when the current is reduced (fairly common for some solid-state designs),
the power can come back on unexpectedly. This can be very shocking.
Choose wisely when you select your protection.

( 130231 )

76 | September 2013 | www.elektor-magazine.com

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CIRCUIT CELLAR

ADuC841 Microcontroller Design Manual:

From Microcontroller Theory to Design Projects

If you’ve ever wanted to design and program with the ADuC841
microcontroller, or other microcontrollers in the 8051 family, this is the book
for you. With introductory and advanced labs, you’ll soon master the

many ways to use a microcontroller. Perfect for academics! Q

Buy it today!

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•Store

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170/0 DISCOUNT + FREE SHIPPING

www.elektor.com/tubeamplifiers

Concept, implementation and assessment

E Designing Tube Amplifiers

This book looks at tube amplifiers from more than
just a theoretical perspective. It focuses primarily
on the design phase, where decisions must be taken
with regard to the purpose and requirements of the
amplifier, and it addresses the following questions:
How do these aspects relate to subjective and
objective criteria? Which circuits sound the best,
and why? If you want to develop and market an
amplifier, what problems should you expect? What
are the significance and meaning of measurements?
Are they still meaningful, or have they lost their
relevance? Thanks to the enormous processing power
of computers, we can now measure more details than
ever before. How can these new methods be applied
to tube amplifiers? Menno van der Veen will give you
all the answers!

188 pages • ISBN 978-1-907920-22-6
£29.50 • € 34.50 • US $47.60

Display, buttons, real time clock and more

[ Elektor Linux Board
Extension

This extension board was developed to further
propel our Embedded Linux series of articles and the
matching GNUblin board. It has a display, buttons,

a real time clock and 16 GPIOs. Linux devotees,
switch on your solder irons. The Linux extension
board includes everything needed to provide the user
interface for a wide variety of projects!

Module, SMD-populated and tested board, incl.
LCD1, XI, K1-K4, BZ1, BT1 for home assembly
Art.# 120596-91
£31.10 • € 34.95 • US $50.20

Learning to fly with Eagle

( EAGLE V6 Getting
Started Guide

This book is intended for anyone who wants an
introduction to the capabilities of the CadSoft's EAGLE
PCB design software package. This book will quickly
allow you to obtain an overview of the main modules
of EAGLE: the schematic editor; layout editor and
autorouter in one single interface. You will apply your
knowledge of EAGLE commands to a small project,
learn more about some of the advanced concepts of
EAGLE and its capabilities and understand how EAGLE
relates to the stages of PCB manufacture. After reading
this book while practicing some of the examples, and
completing the projects, you should feel confident about
taking on more challenging endeavors.

208 pages • ISBN 978-1-907920-20-2
£29.50 • € 34.50 • US $47.60

140 Minutes video presentation and more

DVD Feedback in
Audio Amplifiers

In this Masterclass we address several aspects
of feedback in audio amplifiers. The focus of this
Masterclass, although not entirely math-free, is on
providing insight and understanding of the issues
involved. Presenter Jan Didden provides a clear
overview of the benefits that can be obtained by
feedback and its sibling, error correction; but also of
its limitations and disadvantages. Recommended to
audio designers and serious audio hobbyists!

ISBN 978-907920-16-5
£24.90 • € 29.95 • US $40.20

Ultrasensitive wideband E-smog detector

E TAPIR Sniffs it Out!

Attention boy scouts, professionals and grandfathers!
This ultrasensitive wideband E-smog detector offers
you two extra senses to track down noise that's
normally inaudible. TAPIR — short for Totally Archaic
but Practical Interceptor of Radiation — also makes a
nice project to build: the kit comprises everything you
need. Even the enclosure, ingeniously consisting of the
PCB proper! Using the TAPIR is dead easy. Connect the
headphones and an antenna and switch it on. Move it
around any electrical device and you'll hear different

80 | September 2013 | www.elektor-magazine.com

Books, CD-ROMs, DVDs, Kits & Modules

Open Source Electronics

inirh

gfektor

Technology

Vjiiir nt, Hin-fJ "

j ^ektor LabWorX 1

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noises with each device, depending on the type and
frequency of the emitted field.

Kit of parts, incl. PCB

Art.# 120354-71

£13.30 • € 14.95 • US $21.50

OS Hard- and Software for Electronics
Applications

. Open Source
* Electronics on Linux

If you have ever wanted to take advantage of the
expanding field of open source software for electronics
and everyday applications, this book is for you. Using
the Linux OS, Warwick A. Smith guides you through the
world of open source hardware and software, teaching
readers to use EDA tools and software that is readily
available online, free to download. The hardware
projects inside can be built using easily obtainable
parts, in the comfort of your own home, on single sided
PCBs, or professionally manufactured with output files
generated by you. Open Source Electronics on Linux
is about changing today's electronics enthusiast into
empowered, savvy, discerning engineers capable of
building and modifying their creations, be it solely on
Linux or in tandem with your current operating system.
272 pages • ISBN 978-1-907920-19-6
£29.50 • € 34.50 • US $47.60

LabWorX 2

Mastering Surface
* Mount Technology

This book takes you on a crash course in techniques,
tips and know-how to successfully introduce surface
mount technology in your workflow. Even if you are
on a budget you too can jumpstart your designs
with advanced fine pitch parts. Besides explaining
methodology and equipment, attention is given to
SMT parts technologies and soldering methods. Many
practical tips and tricks are disclosed that bring
surface mount technology into everyone's reach
without breaking the bank. A comprehensive kit of
parts comprising all SMT components, circuit boards
and solder stencils is available for readers wishing to
replicate three projects described in this book.

282 pages • ISBN 978-1-907920-12-7
£29.50 • € 34.50 • US $47.60

UK /ROW

Elektor International Media
78 York Street

London - W1H 1DP United Kingdom
Phone: +44 20 7692 8344
E-mail: service@elektor.com

Ideal reading for students and engineers

, Practical

p

Digital Signal Processing
using Microcontrollers

This book on Digital Signal Processing (DSP) reflects the
growing importance of discrete time signals and their
use in everyday microcontroller based systems. The
author presents the basic theory of DSP with minimum
mathematical treatment and teaches the reader how
to design and implement DSP algorithms using popular
PIC microcontrollers. The author's approach is practical
and the book is backed with many worked examples
and tested and working microcontroller programs.
The book should be ideal reading for students at all
levels and for the practicing engineers who may want
to design and develop intelligent DSP based systems.
428 pages • ISBN 978-1-907920-21-9
£44.90 • € 49.90 • US $72.50

USA / CANADA

Elektor US

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East Hartford, CT 06108 USA
Phone: 860.289.0800
E-mail: service@elektor.com

Further Information and Ordering: WWW. elektor. COm/store

or contact customer service for your region

www.elektor-magazine.com | September 2013 | 81

•Magazine

NEXT MONTH IN ELEKTOR MAGAZINE

8x8 Two-color LED Matrix
with an ATmega328P

This project aims to explain programming for
Atmel microcontrollers in an easy way. The use
of an 8x8 array of 2-color LEDs is mainly for the
fun of it. Hopefully it also helps you understand
the way the "bitshift" operation works for the
purpose of LED driving. Plus we have a go at
game programming! Some elementary knowl-
edge of C/CT+ programming is helpful here.

Xmega Webserver

Due to lack of space in the current edition,
we've had to reschedule publication of this AVR
powerhouse to the October 2013 magazine. In
terms of I/O we have 4 LEDs, 4 pushbuttons
and a (separately installed) display. For inter-
facing, you can choose between RS485 and
various UART/TTL connectors, allowing our BOB
USB-TTL converter to be connected, for exam-
ple. The Embedded Extension Connector makes
the board pretty versatile. The board also has
a Micro SD connector, and there is room for a
TCP/IP module that allows web server and other
network applications to be realized.

Wind Speed

and Direction Meter

The most widespread way to measure wind
speed and direction is with a wind vane and
an anemometer. In this project we take a dif-
ferent approach— without moving parts and
using a circuit based on a thermal mass flow
meter. A heating element heats the air, which
depending on the wind direction and speed
gets directed across sensors fitted around the
element. Measured values are interpreted in
software, which is no easy task.

Article titles and magazine contents subject to change; please check the Magazine tab at www.elektor.com.
Elektor October 2013 edition is published to Members on September 10, 2013.

See what's brewing
@ Elektor Labs 24/7

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www.elektor-labs.com

and join, share, participate!

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82 | September 2013 | www.elektor-magazine.com

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ORDERING INFORMATION

To order, contact customer service for your region:

USA / CANADA

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PLEASE NOTE: While we strive to provide the best
possible information in this issue , pricing and availability
are subject to change without notice. To find out about
current pricing and stock , please call or email customer
service for your region.

COMPONENTS

Components for projects appearing in Elektor are usually
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that the source(s) given is (are) not exclusive.

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accepts no responsibility or liability for failing to identify
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Copyright

All drawings, photographs, articles, printed circuit boards,
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carriers published in our books and magazines (other
than in third-party advertisements) are copyrighted
and may not be reproduced (or stored in any sort of
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www.elektor-magazine.com | September 2013 | 83

International System-on-Chip Conference

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Dr. John Paul Shen,
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SPANSION

CRC Press

For More Information or Questions, Please Contact the SoC Conference Organizing Committee at:

SoC@SoCconference.com or (949) 851-1714
www.SavantCompany.com & www.SoCconference.com