D 71683

September 1982
65p.

up-to-date electronics tor lab and leisure

timekeeper

dark-room

computer

gas alarm

telephone

alektor September 1982 - 9-03

selektor

gas detector

Most people underestimate the damage that can be caused if a gas leak re-
mains undetected for a lengthy period of time. This gas sensor can raise the
alarm very quickly. It can also be used to trace a leak, should one occur.

rapid loading games

The major source of irritation to TV games owners is the time it takes to
locate a program on tape and load it. We found a solution to that problem:
using some further hardware, games can be stored in EPROM. A simple
program can transfer any desired game to the RAM within seconds!

9-20

the Elektor connection

F. Richter

As the old saying goes, 'it's simple, when you know how'. Most of the really
good ideas and inventions over the last century have been simple and so it is
with the solution to an old problem outlined in this article. A low cost
electronic connection between the main Junior Computer boards and the
interface!

inductive sensor

Anyone wishing to measure something will often require some kind of
converter. This article introduces a distance meter which uses the principle
of induction.

darkroom computer part 1

The darkroom computer described here is based on the 6502 and is capable
of dealing with virtually everything in the darkroom as far as measurement
and control is concerned. It is an exposure timer, a dual process timer, tem-
perature meter, photometer and contrast meter.

applicator

A full description with applications for the versatile ME 10.

home telephone system

Home telephone systems are fast growing in popularity with the availability
of telephone sets at a reasonable cost on the surplus market. The design
described here is a self-contained system and does not require a telephone
exchange.

synthesised sound animation

Sound animation or to put it in another way: the changing, delaying or
phase shifting of any periodic waveform, enhances any final result, some-
times quite dramatically. This article introduces an effective solution
which, although being relatively inexpensive, produces a rich ensemble
type sound.

missing link

9-28

9-30

9-42

9-46

9-52

As pP systems grow, they
tend to develop plug-in
extensions. The 'rapid loader
for TV games', portrayed on
this month's cover, consists
of three plug-in boards on a
basic board that plugs in to
an extension board of the
original computer!

time receiver for the Rugby MSF 9-54

This circuit was designed as an addition to the '6502 housekeeper' pub-
lished in the May 1982 edition. The two together provide an extremely
accurate time clock controlled by (60 kHz) transmissions from the Rugby
MSF transmitter that provide good reception throughout the U.K.

three phase tester 9-60

When connecting three phase motors to the power source, confusion can
arise if the cable markings are incorrect, illegible or non-existent. How do
you deal with this problem? The simple answer lies in the circuit described

here.

market 9-63

advertisers' index 9-74

Technomatic Ltd.

N London : 15. BURNLEY ROAD NW 10

(2 mins, from Dollis Hill Tube Station)
Ample parking space

YOUR ONE STOP SHOP FOR PERSONAL COMPUTERS. ACCESSORIES. SOFTWARE & COMPONENT REQUIREMENTS

Model B £399

inc. VAT
carr./unit £6.00

Upgrade kit
Model A-^B
£60

16K memory
upgrade £21.60

ZH8I ACCESSORIES

ONE PIECE 47 KEY KEYBOARD -FULLY ASSEMBLED AND TESTED. NO ADDITIONAL SOLDERING REQUIRED £33 + £1 PSP
ANODIZED METAL CASE TO HOUSE OUR KEYBOARD * ZX81 PCB £13 £ + £1 PSP
OS HIGH RESOLUTION GRAPHICS BOARD

6K RAM on board. Resident Software provides fast high res facilities which include mixed text and graphics. £69.50 * £1 PSP
QSSOUND BOARD CHARACTER GENERATOR BOARD USER PORT

£21.50 with Demo Cassette 8 CH INPUT and 8 CH OUTPUT PORTS

"1.50 * £1 PSP Kit £11.50 Built £14.95

SEE OUR INSIDE FRONT COVER PAGE ADVERTISEMENT FOR COMPONENT PRICES

Please add 40p p&p to all orders except where it is specified. Add 15% VAT to total order value.

For Export p&p will be charged at cost. Telephone orders (min. £5) can be placed using Access or Barclaycard
ORDERS FROM GOVERNMENT & EDUCATIONAL ESTABLISHMENT WELCOME

Resistors, Capacitors, Semi-Conductors et

TK

ELECTRONICS

11 Boston Road
London W7 3SJ

We have the SOFTY 2
EPROM programmer
stock at £169

9-12 - elektor September 1982

a new resolution for X-rays

In recent years the use of microfocal
X-ray equipment has become estab-
lished in the aerospace, steel, nuclear
and electrical industries for checking the
integrity of components and revealing
minute, incipient flaws. Now. real-time
imaging on television screens, using
microfocal sources, is emerging as a
high-resolution technique and its value
has already been demonstrated in
exposing small cracks and microporosity
in castings and welds.

Since the early days of X-ray radio-
graphy, workers have strived to improve
the resolution of the radiographic
system. The first recorded application
of microradiography in materials studies
was in the late 1890s when Heycock
and Neville produced images that
showed the fine structural detail of
alloys composed of two metals. Prior to
this, shortly after his discovery of
X-rays, Rontgen had himself produced
magnified radiographs of biological
specimens.

In medical radiography, the quest for
higher resolution has been, and still is,
hampered by the biological effect of
X-rays, which means that the X-radi-
ation to the patient has to be kept to
as small a dose as practicable. Involun-
tary movements of the patient during
exposure also introduces unacceptable
blur with all but the shortest of ex-
posures. To work within these con-
straints, conventional medical radio-
graphy usually uses extremely fast
film-screen combinations; but these
are themselves grainy and therefore in-
herently lacking in sharpness. Neverthe-
less, workers such as Buck I and-W right
of Guy's Hospital Medical School

in London are using direct, high-resol-
ution X-ray techniques with microfocus
X-ray equipment to examine the hands
and feet of patients and to investigate
pathological in-vivo specimens; they
report that they have observed radio-
logical information that was hitherto
unobtainable.

In industrial radiography, the con-
straints imposed by the health and
by the movement of the patient do
not apply. Moreover, in general, indus-
trial radiographs are produced on fine-
grained film with longer exposure times,
so they are usually able to resolve finer
detail.

Exposure and Projection
In the technique of microradiography,
a thin specimen is placed in contact
with an ultra-fine-grained film or photo-
graphic plate and exposed in a film
cassette using a normal X-ray tube.
The image is enlarged by optical tech-

Resolution is limited by the graininess
of the film, background fogging caused
by photoelectrons and scattered X-rays
from the irradiated specimen, and any
aberrations introduced by the optical
system. All these can largely be over-
come by using an extremely small X-ray
source, of the order of a few tens of
micrometres in size, and projecting the
image, thereby producing a primary
X-ray enlargement by beam divergence
which is virtually free of what is called
geometric 'unsharpness'. This reduces
any degradation of the image through
optical enlargements of the film grain
and completely obviates imperfections
caused by particles of dust, which often
mar radiographs that have been op-
tically enlarged.

The great improvement in resolution
through enlargement is not the only
important advantage of using a micro-
focal source and projection; perhaps
equally significant is the way it im-
proves the contrast of the radiographic
image. This comes about because
making the separation bigger between
specimen and film (or detector) reduces

the relative intensity of unwanted, non-
imageforming radiation that reaches the
film.

Radiographic contrast C can be written

0-43 (pi - Mi) G.lp
C 's+Iq

Where u , and Hi ate the linear attenu-
ation coefficients of the matrix and
embedded 'features', G is the gradient
of film or detector (density change for
a given step in exposure); l s is the
intensity of scattered or non-image-
forming radiation (noise) reaching the
detector, and Iq is the intensity of
attenuated radiation emanating directly
from the focal spot (image-forming
signal).

So, any reduction in l $ improves the
contrast, and the mere act of making
the space between the,specimen and the
film bigger ( up to between one and two
metres) significantly reduces the noise
level l s and thereby noticeably im-
proves the contrast in the radiographic
image. One disadvantage when using
the projection technique is that the
field size, that is, the area of the speci-
men in the effective beam, is smaller,
which restricts the amount of specimen
coverage on each film. However, the
contrast is further improved by this
reduction; because the intensity of
scattered radiation l s , is proportional
to the volume of irradiated sample, the
scattered radiation l s is proportional
to the volume of irradiated sample, the
giving an image which is cleaner and
therefore has better contrast.

Focal Spots

X-rays are generated by accelerating
electrons from a heated filament to hit a
tungsten anode (target). When the elec-
trons are decelerated by the target
atoms they give off energy in the form
of a continuous spectrum of X-rays; the
minimum wavelength of the radiation
depends on the maximum accelerating
voltage.

Some focusing of the electrons normally
occurs in the conventional X-ray tube.

X-ray equipment. A thin specimen is placed in contact with an ultra-fine-grained film or

isette using a normal X-ray tube. The image is then enlarged lb) by optical techniques.

the design of which is a compromise
between the area of the 'focal spot' at
the target and the flow of electrons
striking it. For radiology, a range of
focal spots from 0.5 mm to 4 mm
diameter is generally used both in
medicine and industry. The two-way
dependency between the area of the
focal spot and the electron-current
density is constrained by the melting
temperature of tungsten; nevertheless,
as the focal spot is made smaller, a
greater power loading into it, expressed
in watts/mm 2 , becomes possible and the
source is said to be more 'brilliant.

To achieve high resolution and enlarged
radiological images it is necessary for
the focal spot to be of the order of a
few tens of micrometres in diameter and
it is desirable that it should dissipate as
much power as possible, to allow short
exposures for film techniques and ample
real-time image birghtness for direct-
viewing systems.

During the past 30 years a range of
microfocal X-ray equipment has emer-
ged which is suitable for medical,
biological and many engineering appli-
cations. One early practical design
was developed and demonstrated by
Cosslett and Nixon of the Cavendish
Laboratories at Cambridge in the early
1950s, and later manufactured. The
Harwell El 2 X-ray unit is derived from
a series of X-ray units with small foci
based on designs by Ely (now Honorary
Fellow at Reading University) who has
contributed greatly to the development
of such specialized equipment and to
the increase in the application of
microfocus X-ray technology gener-
ally.

The Harwell E12 can operate at up to
100 kV and has a focal spot of roughly
15 /zm diameter. When used as a radio-
graphic source it enables an extremely
finely detailed array of X-ray infor-
mation to be recorded on film or other
devices. The X-ray unit comprises a
continuously-pumped Vacuum chamber,
with a demountable electron gun and
target assembly. Filament changes and
other maintenance can be carried out
without difficulty. The electron beam is
obtained from a heated tungsten fila-
ment which is maintained at —45 kV
negative potential; the beam is focused
electrostatically onto the cylindrical
tungsten target (set at +45 kV).

The filament itself is made from 0.1 mm
diameter wire bent into a sharp hairpin

Film or detector -J

When te projection distance (t) is long, it reduces the amount of scattered radiation that reaches
the film, thereby improving the radiographic contrast.

9-14 — elektor saptember 1982

(a)

—

Excessive and (b)

overlapping blur

7 A

Blur of phosphor
is small compared
with image detail

—

■— *

Typical direct-viewing X-ray system incorporating a particulate phosphor, which converts X-ray photons to electrons of similar spatial
distribution. A high voltage accelerates the electrons to a smaller, output phosphor, where the image formed can be viewed directly
using an optical device or a television camera.

shape. Operational life of the filament
is about 60 hours and it can be replaced
in a few minutes.

To focus, the tip of the filament can be
positioned with precision within the
biasing cup under the control of a
stepping motor. Bias voltage, filament
position and spacing between filament
and anode can all be set for best con-
ditions with the high voltage switched
on, so precise adjustments can be made
to achieve the best focus within the
operational range of 30 kV to 100 kV.
For setting up, a series of meshes serve
as a test specimen and their images,
detected by a phosphor, are viewed on
a television monitor. A modified tele-
vision camera, fibre-optically coupled to
the phosphor, enables the focusing
routine to be carried out quite quickly.

Radiographic Techniques

For most conventional industrial radio-
graphy, used for examining castings,
welds and so on, it is normal to arrange
the radiographic geometry to give an
unmagnified image on the film. Indeed,
the practice is to place the film as near
to the specimen as possible to obviate
any lack of image sharpness, which
occurs if there is any separation when
using the customary foci. With micro-
focal X-ray techniques, however, pro-
jection enlargements of X5 to XI 0 are
common and enlargements up to as
much as X25 are sometimes used.

To obtain a X25 enlargement the
distance from the source of radiation to

film may be as much as three metres.
Because the radiation intensity falls
off inversely with the square of the
distance, the flux of X-rays with the
larger projection distances is low, so
long exposures are required if fine-grain
film is used. Typically, exposures up to
40 minutes may be needed to ensure
good enough penetration and obtain
a radiograph of a steel specimen six
millimetres thick. Exposure times as
long as this are quite unacceptable
for most applications, but when the
fine-grain film is replaced with a medi-
cal-type film and a fluorescent screen is
used, the exposure time can be reduced
to a few minutes without significantly
losing image details. Rare-earth fluor-
escent scieens such as those coated with
gadolinium oxysulphide are now avail-
able and, when they are used with
'green-sensitive' films, it is practicable to
use exposures of only a few seconds.
Until quite recently, micor-focal-projec-
tion techniques have been confined to
applications using film, so the full
potential of their use in real-time or
dynamic imaging has yet to be fully
realized. It was, however, predicted
as early as 1938 that the use of an
extremely small source may produce
interesting improvements in fluoro-
scopic resolution. X-ray tubes with
foci of 0.3 mm diameter were produced
by Seifert at about that time and
they were used in conjunction with
binocular image intensifiers in the early
1950s for medical engineering appli-
cations. But the combination never

achieved the resolution to match the
fine-grained-film, contact X-radio-
graphic techniques that were, and still
are, practised in industry.

Lack of Resolution

There are many positive advantages for
striving towards direct imaging systems,
including lower costs through elim-
inating film, the ability dynamically
to view the specimen from varying
aspects, and data that are more readily
adaptable to automation and to com-
puterized systems. Using a microfocus
X-ray unit, a great improvement in
image resolution can be made by
projecting the radiographic image
virtually devoid of geometric blurring.
Analogous to the use of fast film,
fluorescent screens combined with the
projection techniques allow fast image-
intensifier systems to be used. The
improvement in contrast found with
film techniques also holds good when
a real-time detector system is used.

A typical direct-viewing X-ray system
usually incorporates a particulate phos-
phor which converts X-ray photons to
electrons. With a spatial distribution
similar to the intensity distribution in
the X-ray image, the electrons are
accelerated by high voltage to a smaller,
output phosphor. The image on the
output phosphor can be viewed directly
with an optical device or with a tele-
vision camera and displayed on a
monitor.

It is convenient to express the resol-

I

-9-15

ution of a television image-intensifier
system in line-pairs/cm. The resolution
of a typical, modern image intensifier
with a fast caesium iodide phosphor
is about 25 line-pairs/cm. Radiographic
detail, such as a crack in the specimen
must then be greater than 0.4 mm
wide on the input screen before it can
be resolved. So, if cracks 0.1 mm wide
are to be resolved, a projected enlarge-
ment of four times is required.

Over the past few years, since suitable
microfocal X-ray equipment has been
available, projection radiography has
established itself as a valuable technique
in the UK in the aerospace, steel,
nuclear and electrical instrumentation
and generating industries. The technique
is now being applied routinely so small
weldments, aero-engine investments
castings including turbine blades, small
intricate assemblies, micro-electrical
components, plastic high-tension electri-
cal terminations, composite materials
and ceramics.

Application of real-time imaging with
microfocal X-ray sources is only just
emerging as a technique but already its
value has been demonstrated in the
examination of aluminium castings and
welds for microporosity, the study of
crack growth in composite materials and
the dynamic study of diffusion pro-
cesses in certain metals.

No doubt there is a need to produce
high-resolution radiological images that
can examine larger thicknesses of
material than can be accommodated
with the present X-ray units, even
though radiographs of up to 25 mm
of steel, with X8 projected enlarge-
ments, have been obtained using 'cold-
film' techniques. For real-time imaging
with an image intensifier, about 6 mm
of steel is the thickest that can be dealt
with. Good results have been obtained
in real time on aluminium alloy samples
up to 20 mm thick, with a better
sensitivity than is usual with many fine-
grained film techniques.

A marriage between microfocal X-ray
equipment and computerized image
processing techniques will probably
extend the thickness range and may
improve the image resolution.

With such developments, direct imaging
using microfocal X-ray equipment is
likely to become more widely applied,
both as a research tool and as a means
of checking the integrity of mass-
produced components.0733/1

BBC moves towards better TV
quality pictures

The quality of television pictures is
limited by the transmission bandwidth
and by the capabilities of display tubes.
There is no immediate sign of any large,
bright, higher definition display device
to take over from the shadow mask
cathode ray tube, but many workers are
in the field and some development is
expected during the next few years.
Assuming that a better display becomes
available there are possibilities for
matching wider bandwidth trans-
missions. Both satellite broadcasting and
optical fibre cable distribution offer
wider bandwidth and the BBC has been
considering how these could best be
exploited.

A key factor in any new transmission
system must be compatibility, whereby
existing receivers could continue to
work with new-standard signals,
although new receivers would be necess-
ary to derive full benefit. For at least
the early years of satellite or optical
fibre cable services it would be required
that existing receivers continue to be
usable, with appropriate converters. The
introduction of any non-compatible
system could require many years for
international agreement and new re-
ceiver development and hence seriously
delay the establishment of satellite
broadcasting.

The 5.5 MHz video transmission band-
width is adequate for 625-line mono-
chrome pictures. The limitations
become apparent when the colour
signals must be squeezed in with the
monochrome. Ingenious though the
PAL coding system may be, it is im-
possible to avoid some mutual inter-
ference between monochrome (lumi-
nance) and colour components. These
interferences show themselves as lumi-
nance appearing in chrominance chan-
nels (cross colour) giving rise to flashes
of false colour on striped suits for
example; and chrominance signals ap-
pearing in the luminance channel give
spurious dot patterns.

To reduce these effects to acceptable
levels, signals in the region of the colour
sub-carrier (4.43 MHz) are attenuated,
usually resulting in loss of all signal
frequencies from about 4 MHz up to the

5.5 MHz band limit. So the majority of
colour receivers roll off about 4 MHz
and show little fine detail whilst still
suffering from some degree of cross
colour aberrations.

Removing interference

A new proposal involves filtering
off high frequency components above

3.5 MHz. This gives a very slight re-
duction in picture definition, scarcely
noticeable on present-day display-tubes,
but virtually removes all possibility of
interference between luminance and
chrominance components so that cross
colour effects disappear.

In a wider-bandwidth satellite or optical
fibre channel there is room to transmit

the filtered off high frequency lumi-
nance components separately. The high
frequencies (3.5 MHz upwards) are
shifted in frequency to a higher band
(98 MHz upwards) and transmitted
together with the original low fre-
quencies and chrominance signals. The
upper limit of the separated high
frequencies could extend above the

5.5 MHz equivalent bandwidth of the
present transmission channel.

A new receiver, specially designed for
this wide bandwidth transmission
system, would shift the transmitted high
frequencies back to their original values
(3.5 MHz upwards) and hence display a
much-enhanced degree of fine picture
detail. The new receiver would also be
free from cross colour effects, since the
high frequencies would be re-inserted
after colour decoding had taken place.
The BBC has demonstrated experimen-
tal coders and decoders working on this
principle and has passed extended
bandwidth signals, with associated
digital sound channels, through an RF
link simulating a satellite channel.
Results were very satisfactory and
showed also that the proposed system
is entirely compatible with continuing
use of present-day receivers.

(786 S)

British Aerospace interested in
DISCO

British Aerospace Dynamics Group
Space and Communications Division has
recently received a £133,000 contract
from the European Space Agency to
conduct a Feasibility Study of DISCO,
one of five new space science projects
being studied before selection as
Europe's next major Scientific Satellite.
The work will take seven months.

DISCO (Dual-Spectral Irradiance and
Solar Constant Orbiter) is proposed as
a long-term solar observatory to monitor
variations of the sun's surface, the
heliosphere and the "solar wind" — the
particles that stream out through the
solar system. This will give scientists a
better understanding of the internal
workings of the sun, and of its effect
on earth's climate. Some of DISCO's
instruments are also intended to support
the observations of the International
Solar Polar Mission (ISPM).

ISPM will be launched towards Jupiter
in 1986 where the immensely strong
gravitational pull of Jupiter will be used
to accelerate the satellite out of the
ecliptic plane in order to observe the
sun's polar regions.

An interesting feature of the DISCO
project is that, to provide long-term
uninterrupted viewing of the sun, the
satellite would be placed in an orbit,
not around a physical body, but around
the Li Lagrange point, an empty point
in space about 114 million kilometers
from earth towards the sun, where the
gravitational fields of the earth and sun
balance.

(807 S)

iber 1982

Pure air is a rare commodity these days,
especially in urban areas. It is, therefore
hardly surprising that the average city
nose has become insensitive to pol-
lution. After all, generations have in-
hailed a mixture composed of exhaust
fumes, sewage smells, and industrial
chimney smoke. The result of all this is
that a change in the course of evolution
has taken place, practically destroying
(in a lot of us), our ability to sniff out
gas leaks, and household fires. It is
important to detect these possible
catastrophes in their early stage when
they can be effectively and economi-
cally dealt with. And just think about
the large amounts of odourising agent

24 hour protection
against gas leaks

detector

Gas is a very widely used
commodity in todays energy.

Most people underestimate the
danger of a gas leak and the
damage that can be caused if it
remains undetected for a lengthly
period of time. This gas sensor can
raise the alarm very quickly. It
can also be used to trace a leak
should one occur.

I that the gas board mixes with natural
| gas (without smell) every day at
great expense to usl! Waste of time
perhaps?

Efficient and sophisticated detection
equipment although being easily avail-
able is rather expensive. So, instead of
paying a high price for equipment
which makes your nose completely
redundant, it would be nice to have a
useful, cheap, but not so accurate device
to augment the human one. Let's face it
two noses are bound to be better than
one, and although some busybodies try
hard they still find it impossible to poke
their nose in two places at once.

The Figaro Engineering Company (no
not from Seville) from Japan have
come up with a low priced gas-sensor

which without too much difficulty
can be incorporated into home built
detection circuits.

As opposed to using radioactive el-
ements this sensor is in the form of an
NPC resistor. In other words, a resistor
which has a 'negative pollution coef-
ficient'. This is a highly technical term
used to hide a very simple principle. The
higher the concentration of undesired
gas or compound in the air, the lower
the resistance of the sensor. The ad-
dition of a comparator and display
circuit creates a reasonably sensitive
electronic nose.

There are two main types of NPC sen-
sors which are suitable for the circuit
described, the 812 and 813. The only
difference between them is that the 812
is highly sensitive to carbon monoxide,
ammonia, alcohol and benzine, making
it ideal for smoke/fire detection and
breathalisers, whereas the 813 is better
at recognising propane, butane and
methane, which is just right for finding
gas leaks. The choice is left to the
constructor.

The circuit

Readers will be pleasantly surprised by
the electronics in figure 1, or rather by
the lack of them. As can be seen very
few components are required.

To the left of the circuit diagram, a
standard 5 V stabilised power supply is
shown. As the gas detector consumes
rather a lot of current, simply con-
necting an ordinary dry battery won't
do. Further on in this article the use of
lead/acid batteries is described and
discussed, but, to be quite honest, the
circuit should be mains powered.

Resistor R1, potentiometer PI are
connected in series with the gas sensor
GS1 . This network constitutes a voltage
divider, the operation of which is
related to the amount of pollution in
the air. To put it in more down to earth
terminology: the stronger the pong,
the higher the voltage applied to the
positive input of the comparator IC2.
The reference voltage is applied to the
negative input of IC2 and is determined
by P2.

Assuming that the circuit is adjusted in
a relatively clean environment to start
with, the voltage at the negative input
of IC2 (reference) will be initially higher
than at the positive input. When the
sensor detects something out of the
ordinary this situation should reverse
itself. The voltage applied to the posi-
tive input will now be higher. As a result
transistor T1 will be turned on (con-
duct) causing the LED D7 to light and
the relay Rel to be switched on. Ob-
viously the relay can be used to activate
any kind of alarm system chosen, such
as tweeters, bells or of course a venti-
lator. Looking at the circuit it may
seem at first sight, that one of the two
potentiometers is superfluous. This is
not really the case, because it is a good
idea to use PI to determine the oper-
ating range of the sensor, with the

9-18 - elektor September 1982

3

Figure 3. The circuit diagram of the mains power supply and trickle charger for use with sealed lead acid accumulators. This circuit is not
mounted on the printed circuit board.

reference voltage and therefore the
sensitivity being set by P2. IC2's oper-
ation is basically independent of the
'absolute' levels at either of its
inputs because it has a common mode
range.

Components D5 and S2 are marked
with an asterisk for reasons that deserve
an explanation. Any alarm system using
bells and LEDs is all very well, but
they are only effective as long as there is
someone to hear or see them. That is
certainly not the case when you are out
for the day or on holiday. With most gas
leaks after a short interval of time the
actual pressure under which the gas
escapes, automatically drops. This can
cause the sensor to stop detecting the
presence of it. When you return home
after a day out, it is no good finding out
that a gas leak exists the hard way,
because that will be the last time you
will read our magazinel A memory
function has therefore been incorpor-
ated into the circuit. This is where diode
D5 comes into the picture. As soon as
the circuit 'smells' something, the com-
parator flips over and D5 conducts,
causing regenerative feedback around
IC2. The comparator will remain in this
condition (staying high), irrespective of
any eventual change to GS1. This latch
function is maintained until the reset
switch S2 is depressed.

Putting the electronic nose
together

Figure 4 shows the printed circuit board
of the circuit. No provision for mount-
ing the relay onto it was made, because
we know from experience that most of
you have your own ideas as to the type
and size to use. Keep in mind that the
current level required by the relay
should not be more than transistor T1
can handle. It should be below 100 mA,
This will also keep the total consump-

tion of the circuit down to under
200 mA.

Although the emitter voltage of T1 will
either be 0 V or around 4 V, it still
cannot be connected directly to a TTL.
For if the output of the TTL goes low,
current would also enter this output
causing an excessive voltage drop across
the relay. In other words the relay
would either never switch on or if
already on it would certainly drop

There are only two possible ways of
inserting the gas sensor GS1 into its
corresponding socket, and for once both
are correct. It really does not matter
which way it's inserted as the pin
assignment is symmetrical and non
polarised.

Either the 812 or 813 is suitable as the
only real difference between them is
their overall operational range which
incidentally is set by adjusting the two
potentiometers.

Before finally calibrating the circuit it is
best to operate the gas sensor for a
certain amount of time. This may sound
ludicrous (can you do one without the
other?), but the point is, that GS1 needs
a warming-up period before it will
behave correctly. We suggest leaving the
circuit switched on, permanently, for a
period of two to three days. This time
frame depends on the accuracy required
by the user. In practice, for most
domestic purposes twelve hours is suf-
ficient.

Check whether GS1 is drawing current
for the filament by touching IC1 and
GS1 . If everything is correct both these
should feel warm. Initially set P2 so that
a voltage within the 1 to 3 V range is
measured at the junction of PI and R2.
After the warming-up period the circuit
can be calibrated precisely, obviously in
a clean environment if at all possible.
Anyone with a private jet should fly the
circuit to Lapland, where the air is still

relatively uncontaminated. Those of us
who are prepared to settle for less will
have to make do with the fresh air of
the workshop.

In order for the calibration procedure to
be successful a voltmeter is needed, so
as to measure the voltage levels at the
junction of PI and R2 and at the junc-
tion of P2 and pin 2 of IC2. The absol-
ute values at these points are not
important as the object of the procedure
is to ensure that the level at P2/pin 2 is
higher than at P1/R2. The point is by
how much? The smaller the difference
the more sensitive the circuit and as
Murphy's Law states; the more sensitive
something is the more likelyhood of a
false alarm! After all you don't want the
alarm to sound every time someone
lights a cigarette or when the gas central
heating switches on?

In theory the only accurate way to
calibrate the circuit is to use a fully
equipped laboratory. Unfortunately not
all of us have one so we cannot take
everything into account like ambient
temperature and humidity and so on.
By a process of elimination we found
the prototype worked well when PI
was set to give a reading of between
1 ... 3 V, with P2 set to approximately
0.5 V higher. Anyone wishing to base
their rule of thumb on a difference of
only 50 millivolts be warned!

The circuit operates along digital lines.
In other words, it detects something or
it does not. By way of an experiment,
this can be translated into an analogue
indication by connecting a multimeter
or separate voltmeter across the net-
work PI and R1. The voltage level will
be seen to rise in the presence of a
harmful concentration of gas or com-

And the best of luck with the circuit,
keeping in mind that it is not going to
tell you when your son has been
smoking in the outside loo again.

Figure 4. The printed circuit board for the
gas detector circuit.

Parts list

R1,R3,R4 = 1 k
R2 = 4k7
R5 = 390 n
PI = 22 k (25 k) preset
_P2 = 4k7 (5 kl preset
Capacitors:

Cl = 1,000 p/10 V
C2 - 330 n
Semiconductors:

D1.D2.D3.D4 = 1N4001

D5 (see text),D6 * 1N4148

D7 = LED

T1 = BC547B

IC1 - 7805

IC2 = 3130

GS1 = Figaro gas sensor,
type 812 (orange) or 813 (black)

(see text), with socket (Watford
Electronics)

Trl = 6 V/300 mA transformer
FI = 315 mA fuse

51 = dp mains switch

52 = push-button make contact (see text)
Rel = 5 V/300. . . 650 £2;

relay; make contact (not on pcb)

General points of interest and
information

Methods which detect gases by elec-
tronic means range from gas chromato-
graphy to complex circuits using radio-
active elements. The Japanese company
Figaro set out to prove that good results
could also be obtained using semi-
conductor material, in this case N doped
tin oxide (Sn O:)-

The basic principle used, is that the elec-
trical conductance of semiconductor
material is reduced when it absorbs
oxygen. The quantity of oxygen ab-
sorbed and the absorption rate relates
directly to the temperature of the
sensor. Therefore by maintaining the
sensor at a fixed temperature in normal
air, its resistance should remain con-
stant. This gives the reference value Rs.
When the sensor comes into contact
with gases such as carbon monoxide,
hydrocarbons and so on, the molecules
of these gases are also absorbed. This
reverses the oxygen reaction, increasing
the conductance of the sensor's ma-
terial, thereby decreasing its resistance.
In order to achieve a reasonably fast
response the whole process is speeded
up by heating the sensor's surface by
several hundred degrees celcius.

Figures b and c show the decrease in
resistance of the 812 and 813 corre-
sponding to their exposure to certain
gases. The reference value Rs has an
equivalent which can be determined by
using the formula shown in figure 2a. In
this case U is the constant supply volt-
age, U r , being the voltage measured at
the junction of the sensor and resistor R.
The sensitivity and performance of the
sensors is greatly affected by the voltage
drop across the filament (figure d). the
supply voltage (figure e), the tempera-
ture and relative humidity of the
environment, (figure f). The graphs
shown in figures d to f are only meant
for information and general interest.
This is because very few home construc-
tors are going to have a fully equipped
laboratory at their disposal, which is
needed in order to calibrate the sensor
accurately.

The operation of the sensor is based on
lengthy absorption processes. Conse-
quently, it takes quite a time to react to
changes in environmental conditions
and to different gas concentrations.

If the sensor has not been in use for
a while (however short), the whole
warming-up and calibration procedure
must be repeated.

A few useful specifications

RS (1000 ppm)

filament dissipation

An alternative supply in the event
of mains failure

The electronic nose consumes a con-
siderable amount of current, so there is
no point in just connecting any old

'Gas-tight' lead accumulators (and we
do not mean the type normally found
under the bonnet of a car), are ideal. An
average type rated at 6 V/4 Ah measures
approximately 66 x 33 x 1 27 mm and
weighs 700 gramms, which to put it
mildly is not very large. 12V/4.5Ah
types are also available and although
being larger and heavier, they are still
suitable.

This kind of battery requires a con-
tinuous trickle charge. 6.9 V for the 6 V
type and 13.8V for the 12 V one. The
circuit shown in figure 3, uses a 723
voltage regulator and can be used for
independently charging the battery, or
as a trickle charger/stabiliser for an
alarm system switching on the battery
supply in the event of a mains failure.
The 723 is also useful as a stabiliser
should you wish to power the gas detec-
tor by battery only. In this case every
component to the left of IC1 shown in
figure 1, should be omitted.

The choice of whether a 6 V or 1 2 V
battery is used is left to the construc-
tor, but we suggest that a 6 V one is
better, but then I Cl has to be substi-
tuted for an LM2930 from National.
This National 1C has the advantage that
the difference between input and out-
put voltage need only be 0.6 V making
it very efficient (low dissipation).
Constructors should refer to the Car
Stabiliser article which appeared in
Elektor's summer circuits issue in 1980,
where they will find a detailed descrip-
tion of the LM 2930. The pin assign-
ment is identical to the 7805.

When using a 1 2 V battery the 7805 can
be retained, with the proviso of
adequate cooling.

Before any battery is connected to the
circuit the output voltage of the 723 1C
should be adjusted to 6.9 V or 13.8 V.
The battery should have an average life
span of 20 hours (without recharging). K

812 813

38 t 3 Jl 30 i 3 n

1 ... 10k 5. ..15k

(isobutane) (methane)

1 5 mW max. 15mWmax.

24 V max. 24 V max.

650 mW 830 mW

5V i 0.2 V 5 V ± 0.2 V

9-20 - ele

iber 1982

rapid loading gar

Nowadays, people tend to hate anything that seems to be a waste of
time. To TV games computer owners, a major source of irritation is the
time it takes to locate a program on tape and load it. The whole
procedure can take as long as two or three minutes! Terrible. Two or
three seconds would be infinitely better.

Faced with this problem ourselves (yes, and irritated!) we started
looking for a solution — and found one. Using some further hardware,
games can be stored in EPROM; a simple program can transfer any
desired game to the RAM area within seconds. This is a major
improvement, as you can imagine!

for the TV games
computer

To be quite honest, this circuit was
originally designed for purely selfish
reasons: we wanted it at home, and
thought it would be quite useful at
electronic exhibitions. However, TV
games owners who saw it in operation
were so enthusiastic that we decided it
would be unfair not to publish it!

So what is this circuit supposed to do?
Before we get to that, let's see what the
existing situation is. Programs for the
TV games computer are stored on tape.
As required, they can be read into the
RAM area after which the game can be
started. This works nicely, and tape is a
relatively inexpensive kind of 'memory'.
However, the whole procedure takes
time: you've first got to locate the tape,
and the position of the desired file on
that tape. Then, finally, the program
can be transferred from tape to com-
puter. Another time-consuming process!
Furthermore, it is not as reliable as one
might wish. Interference pulses, tape
drop-outs and other 'nasties' can cause
the computer to reject the incoming
data. A second or third try may be
required before the program will load
properly. Fortunately, this does not
happen often — but even so, when it
does happen it is infuriating!

For 'popular' games or programs - the
ones that are used most! — it would be
nice to have a reliable rapid-loading
facility. Something that is as quick as
plugging cartridges into a 'commercial'
machine (but, preferably, not quite so
expensive . . . ) An obvious solution is to
use EPROMs. However, there is one
problem: many programs for the TV
games computer will only run if they
are stored in RAM. So what do you do?
Store them in EPROM, and transfer
them to RAM when you want to run
them!

The basic idea

The TV games computer (even the
extended version), uses only a small
section of the available address space.
The 2650 can handle memory from
0000 to 7FFF, but we are only using
the addresses to 1FFF. All higher
memory area can be used for storing
programs in EPROM.

To make this system work, we need
three things: some hardware for address
decoding, a set of EPROMs to cover
the higher address range, and a little
program to transfer the desired data
from EPROM to RAM area.
Basically , the upper address
range (24K) is sufficient
for five or ten programs.
This may well be more
than enough, but we pre-
ferred to make double sure.

The basic address-decoding hard-
ware is mounted on one p.c. board,
andthe EPROMs are mounted (in groups
of four) on plug-in extension boards.
This does tend to get complicated, since
the basic board is designed for plugging
into the extension board for the TV
games computer ... In other words: the

loading

games

EPROM boards are plugged into a board
that is plugged into the extension board
that is connected to the basic TV games
computer. Confusing? Yes, for now.
Easy to do? Yes, definitely!

Before going into this further, we must
apologise: we are going to add to the
confusion! The three plug-in extension
boards each contain four EPROMs,
making a total of twelve. Using 'normal'
2716s, this would seem to fill the avail-
able 24K address space. However, one
EPROM must contain the program-
transfer routine. Therefore, we are one
EPROM short, and the last 2K ist left
unused. If this is deemed unacceptable,
we have a solution to offer: the exten-
sion boards will also accept 2732s. Six
of these will cover the total area! Plus
one for the transfer routine, of course.
However, the boards will only cater for
one type (as determined by one wire
link on each plug-in board and four
links on the basic board) and 2716s
cannot be mixed with 2732s.

The details

As mentioned earlier, programs for the
TV games computer will only run
correctly if they are loaded into RAM,

in the 'normal' address area. This means
that they can be stored in EPROM, at
higher addresses, but not run 'up there'.
Before running a program, it must be
copied into the RAM area: furthermore,
it is useful if the program counter (PC)
is set correctly at the same time. This
transfer is accomplished by means
of a short auxiliary routine; to avoid
problems, this routine is stored from
1C00 to 1C7F and from 1E00 to 1E7F
- two unused areas, until now. This
program is also stored in EPROM, on
one of the plug-in boards. Note that
this must be the first board (connector
X) and the first position! The 'hex
dump' is given in table 1.

The procedure for loading a new game
is as follows: Enter 'PC = 1C00'. When
the '+' key is operated, the computer
will request a 'file number'; as soon as
this is entered (again followed by a
'+') the desired file will be located and
transferred to RAM (provided the
corresponding EPROMs are plugged in,
of course!). Then the program counter
will be set to the correct start address;
hitting the '+' key will start the game.
And that's all: rapid (a few seconds) and

A few more points require explanation,
unfortunately. Lets start with an easy

The hardware

This part is easy for two reaons: a it

9-22 - i

1C00 76 60 75
1C10 95 3F 02
1C20 04 20 CC
1C30 86 02 0E
1C40 00 3B 04
1C50 44 F8 81
1C60 IE 04 19
1C70 20 84 01

08 06 24 3F 06
OE 3B F4 9A 7C
08 00 OE E8 00
E8 00 Cl OE A8
18 1C IB 5B C2
17 CC 08 05 3B
CC 08 96 04 11
51 9A 7B CC 08

02 3F IE 00
06 08 20 CE
9A 05 05 FF
00 EB D4 18
45 07 85 00
6F CC 08 04
OD 08 03 18
97 3F 02 OE

1 A 7B CC 08
48 00 5A 7B
20 F8 OE C3
17 77 09 84
75 08 08 D3
A6 02 IF IE
09 2D 08 02
IF 1C 00 FF

1E00 20 CC 08
1 E 1 0 40 18 73
1E20 00 9A 06
1E30 98 02 74
1E40 67 A6 D9
1E50 E8 00 CC
1E60 98 65 09
1E70 08 A2 OD

9F 77 02 12 9A
01 1 A 02 05 OF
44 7F CC 08 03
40 05 FA OE A8
72 OF 08 00 86
88 A 4 3F 03 9F
El 02 B4 40 1C
08 A3 3F 05 29

7D 3F 00 55
45 IF OD 61
20 68 81 CC

00 CO CO CO

01 98 04 87
EE 08 05 98
1C 3D 06 04
04 OD CC 08

3F 01 81 F5
22 17 OE E8
08 02 E8 F6
CO CO CO CD
01 CB F6 OE
6A EF 08 04
3F 02 E3 OE
9A IF 00 38

0800. -1: basic EPROM address (indexed R2 in program)

0804] 5: last EPROM address of current section

0895 : file number

08A4.-5: current RAM address

Required data format in EPROM:

.001 : section indicator (note 1)

. 002. -3: last EPROM address of this section
. 004. —5: PC start address of program
. 006, —7: first RAM address for this section
. 008 : program data

Note 1 : for one EPROM section, the section indicator is 80;

for two EPROM sections, the section indicators are 01 — 81 ;
for three EPROM sections, the indicators are 01 - 02 - 83;
for four sections, the indicators are 01 - 02 - 04 - 87.

Note 2: Programs must be densely packed in EPROM. In other

words, there should be no unused bytes between the 'last
EPROM address of this section' and the file number for the
following program.

Table 1 . This routine transfers programs from EPROM back to RAM. as required. When using
2732s, it is advisable to store 450F at address 1C48. and 44F0 at 1C50.

is relatively unimportant, and b it is

The circuit for the basic extension
board is given in figure 1. In essence, it
consists of two address decoders with
two further (combined) outputs. The
main chip (IC1) provides 'chip enable'
signals for the EPROMs, according to
the address that is output by the 2650;
the second address decoder (IC2) selects
the two address ranges (1C00 . . . 1C7F
and 1E00...1E7F) for the EPROM
that contains the program transfer
routine. Two further multiple-input
gates combine the various outputs,
and provide the necessary feedback
to the extension board, as shown in
figure 3.

The plug-in boards with the EPROMs
are even simpler: witness figure 2.
Address, data and chip-enable inputs are
passed to the EPROMs. A single wire
link determines whether 271 6s or 2732s
are used. And that's all!

Figure 3, as mentioned above, shows
where the DBE1 and DBE2 connections
from the basic extension board should
be connected. This entails a bit of
scratching on the p.c. board, to break
the connections to the corresponding
pins of this 1C.

On the same 'extended TV games'
board, four additional connections must
be made to the first connector position;
these are shown in figure 4. While we're
at it, figure 5 shows a copper track that
must be broken on the same board,
adjacent to this connector. And the
OPREQ connection from the main
board is shown in figure 6. The new
boards are shown in figures 7 (basic
board) and 8 (EPROM plug-in board).
What does all this accomplish? In a
nutshell:

• the basic extension board (that is to
carry the EPROM plug-in boards) is

connected to the 'Interton' position on
the "extended TV games' board. This
provides the bulk of the necessary
signals.

• the OPREQ signal from the 2650 is
connected to the new hardware.

• pins 1 1 and 1 2 of N23 on the 'TV
games extension board' are dis-
connected from positive supply, and
connected to the DBE1 and DBE2
signals. This ensures that the Data-Bus
buffer is Enabled when transferring data
from the new EPROMs.

• finally, the 'missing' address lines
(A14, A13 and A12) are also applied

to the 'Interton' connector — at pins
26. 27 and 28. Address line All is also
required, and this entails disconnecting
it from supply common.

The software

There are three points that can be
considered, when discussing software:
'How to use it' (very important!), 'What
it does' (interesting) and 'How it does it'
(of limited interest, for enthusiasts
only). In this article, we only intend to
deal with the first two of these points.

alektor September 1982 — 9-23

There are actually three programs in-
volved (tables 1 . ..3). Taking it from
the top: table 1 gives the 'hex dump' of
the routine that transfers programs from
EPROM into the RAM area, when you
want to run a stored program. The
'instructions for use' are simple: start
the program at PC= 1C00. Then enter
the file number of the desired game;
after a few seconds, 'PC = . . . ' will
appear. Operating the '+' key will start
the program. If a program was stored in
more than one EPROM (more on this
later), an error indication will be given
if one EPROM was not plugged in:
'F I L = X-N', where X is the file number,
and N is the number of the missing
section. An 'L' in this position indicates
that the last section was omitted, or
that the file number was not used at
all.

In essence, this routine is little more
than a 'block transfer' with a few refine-
ments. It scans the memory area, from
2000 on, looking for the requested file
number. If it hits 'FF' (or any negative
number, for that matter) in a file
number position, it assumes that the
remainder of that EPROM is unused and
jumps to the next one. This does mean
that EPROMs must be densely packed:
a new program must follow its prede-
cessor without leaving any unused gaps!
The position of the EPROM(s) in the
upper address range is unimportant:
they can be plugged in anywhere. The
'RAM scratch' assignment and 'data
format in EPROM' are summarised
below table 1 .

One important point should be noted.
When using 2732's for program storage,
it is advisable to store 450F at address
1C48 and 44F0 at 1C50.

Given a program that will transfer data
from EPROM to RAM, the next ques-
tion is: how did the data get into
EPROM in the first place? Easy! It was
transferred from RAM to EPROM, by
means of the program given in table 2!
This routine is a variation on the one
given in chapter 21 of the TV games
computer book. It uses the plug-in
EPROM programmer, as described in
the book - in fact, programming can be
simplified considerably by using several
programmers in consecutive EPROM
sockets.

As before, 'instructions for use' come
first. The EPROM with this program
is inserted in the same position as
the previous one, since it uses the
same address range (1C00...1C7F
and 1E00 . . . 1E7F). This is the only
unused address range on the same 'page'
as a RAM area, so there is no other
solution! The program is started, as
before, at 1C00. Based on the data
that is located from 1BD0 on (details
are included below table 2), it will first
display the next EPROM range that
it intends to program. If this meets the
programmers approval, he can operate
the WCAS key. Normally speaking, the
EPROM will than be programmed.
Possible errors are indicated: if the

9-26 - elektor September 1982

rapid loading games

Table 3 2000 IB C
2010 AC C
2020 21 ;
2030 94 $
2040 FD '
2050 04 f
2060 40 C
2070 C6 C
2080 BF 2
2090 OB E
20A0 CF E
20B0 CF E
20C0 IB E
20D0 2A C
20E0 51 (
20F0 03 C
2100 FB C
2110 Cl 1
2120 15 C
2130 BC :
2140 CA £
2150 FB C
2160 EC '
2170 70 1
2180 07 C
2190 IB *
21 AO FA C
21 BO 9C l
21C0 16 C
21D0 FO '
21E0 D1 C
21F0 3F C

Table 3. This calculati'

I FF 76
I C8 AA
■ 1C 3B
. B4 40
! IB FF
! F3 07
I 77 09
) C4 59
) 07 OB
r El 49

> 82 CF
I 01 CF
! FF IB
l 04 CO
! C8 E5
I 01 CE

> 05 04
I 78 3F
I E6 AA
! ID OA
l 00 17
: 3F 06
I 7D 3F
f OD El

I 3F 05
I 22 08

> DA 00
) C8 9A
) A4 04
! IB F2
) C6 F9
) 3F 21

60 75 18 07 04

04 04 C8 A8 04
FA 3F 21 D5 C9
98 12 74 40 IB
IB D4 08 84 07
OB CB EB IB 09

05 02 20 8D CO
71 08 C7 DO C8
C2 09 B8 60 1 A
CF EO 45 81 CF
EO C8 77 01 A5
A1 49 87 04 CB
EE IB EC IB D2
C8 A8 3F 21 56
08 D7 50 50 C3
AO CO 1C 1C 00
77 10 3F 02 CF
02 OE 17 08 90
00 10 00 8A 17
8F 05 FE OD El
IB FF 07 A4 IB
02 77 02 20 C8
00 55 3F 01 81
92 9A 05 E4 EO
2F IB 53 D1 1 A
A 1 08 95 06 18
07 06 3B 2E 77
75 08 06 24 07
C8 82 IB 4F IB
OB F4 05 02 77
75 CB E3 75 09
OC 07 17 05 08

eatly simplifies the job of lo

3F 21
FF C8
90 06
62 08
OA 05
B4 40
C4 CD
C4 E4

03 61
EO C8
01 A4
87 IF
IB FC

04 80

05 80
IB 71
75 10
15 OA
8A 15
47 CE
DO 08
FO 08
F5 40
14 IB
50 C8
3F 21
09 09
08 60
FF IB
09 OE
17 00
3F 21

03 04 AC C8
A6 06 18 3F
20 3B FI 08
A2 08 7E IB
06 3F 21 05
1C 20 CC 76
El D3 OD EO
20 1C 21 EC
98 68 77 09
Cl OF Cl 49

00 75 08 C2
20 5A IB FD
3F 05 29 04

06 24 3F 21

01 63 E3 18
CO IF C6 IB
OF 41 1 A CD
AA E6 OE 14
OE 14 OE OA
A 1 49 D9 78
A7 08 9F IB
FO 44 7F C8
18 73 01 9A
5E 09 DO 1 A
89 3F 02 OE
31 06 1C 3B
AO A5 07 C9
1 A 03 61 98
FE IB FI IB
Cl 49 AE El

07 OF 05 05
05 IF 20 00

EPROM section is not 'empty', the first
programmed (EPROM) address, with its
data, are displayed on a red screen; if
programming fails at some address, this
is displayed on a purple screen. If all
goes well, however, this (section of the)
program will be stored correctly: file
number, section indicator, last EPROM
address for this section, PC start address
for the program, first RAM address for
this section and program data — all as
shown below table 1 . If further program
sections are required (as described
later), the next section to be stored
will now appear on the display. If
the next EPROM is inserted, on a
programmer board, WCAS can be oper-
ated again. Alternatively, the current
data from 1BD0 . . . 1BFF can be stored
(operate Reset to return to monitor!);
switch off, not forgetting the 25 V
supply (!), and plug in the correct units;
switch on and reload the program and
the data from 1 BD0 on; restart at 1 C00.
When the last section of program is
stored, the file number will be displayed
on a green screen.

We now come to the burning question:
where does the data from 1BD0 on
materialise? Admittedly, you could
work it out yourself. But it's much
easier to let the computer do all the
hard work! It can hardly miss, when
you use the program given in table 3.
Operation is simple. First, enter the
RAM area for the program. Then the
start address - unless two RAM areas
are required, in which case '+' steps
through to a second RAM area entry.
Then enter the next vacant EPROM
area (Beg=, End=). If this is sufficient,
well and good; if not, the program will
request a further section. Up to four
sections of EPROM can be used (or
three, if two RAM areas must be
loaded). As soon as the EPROM area is
sufficient, the End address will be
modified to the actual End address that
is required (on a green screen). After
making a note of this (I) and operating
the '+' key, the file number can be
entered (00 . . . 0F). The program then
branches to 1C00, which is just right if
the EPROM with program 2 is installed!

In conclusion

This useful extension for the TV games
computer consists of quite a few bits
and pieces of hardware and software. A
brief summary may prove helpful:

• The basic extension board (figures 1
and 7) is plugged into the Interton

position on the TV games extension
board. Two further connections are
made to the extension board (DBE1
and DBE2, see figure 3), four connec-
tions are made on that board (figure 4)
and one copper track is broken (figure
5).

• The OPREQ connection is taken
from the main TV games computer
board, as shown in figure 6.

• Up to three EPROM plug-in boards
(figures 2 and 8) can be inserted in
the basic extension board.

• One or more (up to four) plug-in
EPROM programmers are built, as

described in chapter 21 of the TV games
computer book. Connections to the
main board of the TV games computer
are also described there; it should be
noted, however, that a multiple-input
OR gate must b e used to combine the
various OPACK signals, if more than
one EPROM programmer is connected.
A suitable circuit is shown in figure 9.

• The calculation routine (table 3) is
stored from address 2000 . . . 21 FF.

In other words: the second EPROM
position on the first plug-in board. The
EPROM programming routine (table 2)
is located in the first EPROM on this
board: addresses 1C00...1C7F and
1E00...1E7F. (Note that these
EPROMs can be programmed by means
of the plug-in programmer! A suitable
routine is given in the book, as table 48;
however, the instruction at 191C must
be deleted - C0, C0 - to program the
EPROM from 2000 on.)

• Programs can now be stored in
EPROM. First enter the RAM and

EPROM addresses, using the calculation
routine as described earlier; then initiate
the actual programming by means of the
WCAS key.

• When programming is completed,
replace the first EPROM on the first

plug-in board: this position should now
contain the transfer routine given in
table 1. It will transfer a program from
EPROM to RAM, as soon as the file
number is entered. Note that the pro-
grammed EPROMs can be mounted in
any position on any of the plug-in
boards: the transfer routine will locate
all relevant sections without difficulty.
One final note, for those who hate
'wasting' EPROM area. The routines
given in tables 1 and 2 must be located
in those address ranges, as mentioned
earlier, and the remaining space in these
two EPROMs must unfortunately be
left unused. However, the program
given in table 3 is initiated in such a
way that the area from 2200 on can be
used for storing games programs! That
EPROM, therefore, can be used to the
full. M

die EMtor
connection

a link between the Junior and interface board

As the old saying goes, 'it's simple, when you know how', most of the
really good ideas and inventions over the last century have been simple
and so it is with the solution to an old problem outlined in this article.
The idea is so straightforward that all our design staff were astonished!
A low cost electronic connection between the main J.C. boards and
the interface!

Once more an active and resourceful reader has come up with a
relatively brilliant idea!

F. Richter

Despite all the solutions presented by
our design staff in the May 1981 issue,
and in the Junior Book 3, problems still

The main disadvantages of the original
system were:

• Too costly.

• Relocation of the interface board
presented problems.

• The bus board for the interface was a
relatively long distance away from

the main construction.

• The way the bus board was designed
could easily lead to short circuits.

Without doubt the solution presented
by our reader gets over most if not all
these problems.

Figure 1 clearly illustrates how it is
done. Female connectors with wrap-
around pins are used. One multiway
connector also serves as the physical
foundations of the sandwich type con-
struction.

First of all a connector is mounted onto
the interface board and obviously
soldered into place. The wrap-around
pins protruding from the copper side are
then plugged into a second multiway
connector, which in turn has been
mounted onto the main base board.
Could it be simpler?

The distance between the boards should
not be less than 1.5 cm otherwise some
of the components will snag the con-
struction. The tallest components to
watch out for are switches SI and S2.
And please remember that the quarz
crystal must be remounted at 90 degrees
to the vertical (flat), over IC6.

When screwing the 'sandwich' together
some of the plastic surrounds of the
connectors (at each end), may get in the
way of the spacers. This problem can be
solved very easily by sawing off a small
piece of the connector (see figure 2).

By the way, we certainly don't have any
objection against receiving more of
these ingenious ideas from our readers.
Get your thinking caps on! K

9-28 - elektor September 1982

Most readers will remember the old
principle of induction, forced upon us
by enthusiastic teachers in our school
days. No? Never mind we had to look it
up as well!

The universally accepted principle is
that when a current flows through a
conductor, a magnetic field is created
around it. Winding the wire (conductor)
into a coil will mean that the magnetic
field of each individual winding is added
to the relatively homogeneous field at
the core of the completed coil. The
result is that a type of electro-magnet is
created which has a negative and a posi-
tive end or pole, very similar to a perma-
nent magnet. On we go with the physics
lecture!

The coil's inductivity can be calculated
by using the formula:

L = Po-p r .N J • D/I.

So, besides the number of windings N

inductive

sensor

and the geometric dimensions (D/I),
the inductivity (L) is also dependent
on the relative permeability (p r ). What |
we mean by this is best explained by the |
fact that an iron rod which fills the
inner space of the coil completely (no
air gap), gives the maximum possible
induction. When compared with a coil
without a rod the induction is approxi-
mately 6000 times greater! This maxi-
mum value will never be realised with
this circuit simply because we do not j
push the rod all the way. We will |
explain this later on in the article.

As most of you have already guessed,
the principle of the inductive sensor is
based upon the fact that the inductivity
of the coil will vary according to the
space taken up by the rod.

By using this principle we can now
make physical distance proportional to
an electrical signal. Couldn't be simpler, I
well, maybe! Some of you may be sur-
prised to learn that the prototype was
accurate to ± 0.01 mm over a distance
of a few centimeters, even though we
found that the accuracy did depend on
the form of the coil.

How to combine the sensor to the cir-
cuit as shown in figure 7 will be
explained later on in the article. We
thought that a close look at the circuit
would be better at this stage.

have coil will travel

Anyone wishing to measure something by electronics means will
often require some kind of converter.

This article introduces a distance meter which uses the principle of
induction. The result is an easy to build, relatively simple to calibrate,
measuring circuit, with a wide range of applications.

The circuit

A Wien bridge oscillator together with
an amplitude stabiliser (A1) produces
a sinewave signal with a frequency of
approximately 13 kHz. This signal is in
turn fed to a Wheatstone bridge via a
power stage mainly consisting of T1 and
T2. The Wheatstone bridge is made up
by the two partial resistors of PI and
two identical coils, one of them being
the coil of the sensor.

The formula to use, in order to achieve
a balanced bridge, is

Figure 1. The circuit diagram of the sensor, showing the points at which the voltage are to be measured for correct calibration. A DVM or

analogue voltmeter is all that is required to complete the instrument.

Xl./Xl, = Rpa/Rpb-
In the circuit, the A.C. voltage levels of
the differential amplifier inputs are
identical. Consequently, the A.C. volt-
age level at its output will be approxi-
mately 0 V.

As already explained the inductivity
of the coil changes with the position of
the iron or ferrite rod. By moving the
rod, the bridge will become unbalanced
and the input voltages of the differential
amplifier will no longer be identical.
The result of all this is that pushing the
rod further into the coil will result in a
higher A.C. voltage level at the output
of A2. The output of the differential
amplifier is then rectified ready to be
applied to a DVM or analogue volt-
meter.

The coil

The circuit just described is suitable for
any type of coil. However, tests with
the prototype have shown that a coil
identical to the one shown in figure 2,
produced the best results. Therefore, for
simplicity's sake we have based all our
calculations and calibration procedures
on this type.

The coil consists of a plastic housing of
approximately 8 mm. We actually used
the case of an ordinary ballpoint pen.
Three hundred turns of 0.2 ... 0.3 mm
enamelled copper wire are wound on to
the housing over a length of 6 cm. Keep
in mind that fewer windings would
increase the load on the power stage of
the circuit (T1/T2). Two identical coils
have to be made (LI and L2). As long as
the coils are made along the lines speci-
fied you will find that the induction of
each coil will have a value of 95 pH.
One of these coils can serve as the dis-

tance meter. An iron or ferrite rod,
which must be longer than the coil
housing has to be inserted into it. The
prototype used is an iron rod 13 cm
long and 4 mm in diameter.

Obviously different coils can be con-
structed for other purposes, but, as the
applications are many, we have left that
to the reader's discretion. An example
of an electronic scale or, we should say
a pair of scales is shown in figure 3.

Calibration

In order to calibrate the circuit cor-
rectly an induction curve has to be
drawn as shown in figure 4.

First of all the circuit has to be cali-
brated in a quiescent state. In other
words with the rod fully extracted.
Check that the oscillator is operating
correctly (giving a 13 kHz sinewave),
by measuring the A.C. voltage at point
Ml as shown in the circuit. If everything
is correct then the voltmeter should
read approximately 1 V rms . The next
step is to connect the voltmeter to point
M2 on the circuit. PI should be adjusted
until there is a minimum output from
the differential amplifier A2. The proto-
type gave a reading of approximately
0.074 V rms . A DVM or analogue volt-
meter set to the D.C. range is connected
to the output of the circuit (M3). t
Before going further, just like the
famous professor, who had a biological
problem to solve. We need pencil and
paper to work it all out, plus a ruler.
Push the rod in one mm at a time taking
a note of the voltage reading at the out-
put. From the results a graph should be
drawn (see figure 4). You will notice
from the graph that the relationship
between the movement of the rod and

the voltage level at the output is only
linear over a specific range. Armed with
all these data the sensor can now be
calibrated precisely.

Push the rod into the coil until the start
of the linear range and take a note of
the voltage reading at the output. Now
push the rod a further 1 cm exactly, and
adjust P2 to give a reading precisely 1 V
higher. Finally return the rod once again
to the start of the linear range and set
P3 to give a reading of 0 V. This com-
pletes the calibration procedures as the
aim is to achieve a linear relationship of
1 V per cm.

Applications

The circuit described is meant as a
starting point for different designs. It
serves as the basic ground work needed
before the constructor can go on to
design sophisticated measuring instru-
ments. In other words we found it to be
a good teaching aid. Obviously some
applications come immediately to mind.
These are, the scales as shown in fig-
ure 3, for the measurements of thick-
nesses, level indicators and even in the
study of earthquakes. A more down to
earth application is for measuring the
depth of tread on a tyre, and so on and
so forth.

When designing a particular measuring
instrument using the principles outlined
in this article, there is one fact that
must be taken into account at all times.
You must ensure that the magnetic field
of the coil does not saturate the iron
rod! Apart from that, obviously any
change in the specification of the coil
will require appropriate changes in the
component values, but, the basic prin-
ciples will always remain the same. H

9-30 - elektor September 1982

darkroom computer part 1

darkroom
computer

The darkroom computer described
here is capable of dealing with
virtually everything in the
darkroom as far as
measurement and
control is con-
cerned. It

is an ex- ^

posure ^

several sections, each
mounted on a separate
printed circuit board and
* there are a tota| ° f seven
' | boards in all:

• The processor board. A
small 6502 system that
forms the heart of the
circuit.

• The display board. Obviously
LED-displays are necessary for the

Hr

• The keyboard. A capacitive keyboard
especially designed for this appli-

/ cation. It can be lit from behind and the

top is covered with a protecting layer.

• The keyboard interface board con-
tains the necessary electronic com-

_ • ponents for the capacitive keyboard.

a miCrOprOCcSSOr • The process timer board. The 25
LEDs are used as the timing indi-

timer, a dual

process timer, temperature
meter, photometer and contrast
meter.

The darkroom computer is based
on the well-known 6502 micro-
processor and a capacitive key-
board designed specifically for
this application. Construction is
relatively easy while overall cost
is far less than the equivalent
commercial systems.

processor

• The photometer board, with which
the light and contrast can be meas-

• The temperature meter board. For
accurate temperature measurements

of the several baths.

The project is fairly complex and for
this reason it was decided to divide
it into two separate articles. In this
issue we will give a description of the
computer itself together with its display
and special keyboard. Instructions for
use are also included. A closer look at
the accesories; the process timer, the
light meter and temperature meter will
follow in the next article.

PART 1

Figure 1 . The circuit diagram o* the microprocessor section. The system mainly consists of a 6502 microprocessor, a 2716 EPROM and a 6532
RIOT (which contains a RAM. I/O lines and a timer).

followed by the 2716 EPROM IC3, decoding consists only of two inverters. All the columns are pulled low in turn
which contains the necessary software. N5 and N6, with which the complete by IC2. The capacitance of the key pad
The third major 'box' in the circuit, memory range is divided into three will transfer the pulse to the 4 mono-
IC2, consists of a 6532. This 1C is the blocks ( IC2, IC3 and IC4 ). Finally, the stable multivibrators ( MMV's I consis-
interface between the computer and the supply voltage for the complete dark- ting of gates N1 . . . N8. If no key is
outside world. It contains 16 l/Q lines room computer is produced by the two touched, a logic 1 will appear on each of
and takes care of the keyboard, display, voltage stabilisers IC8 and IC9. the PA4 . . PA7 lines, via the transistor

process timer and the light and tempera- The readout consists of the four dis- stages T2 . . . T5. However, when a key

ture meters. The 6532 also contains a plays in figure 2, which are multiplied pad is touched, the scan pulse will be
timer which is used for the two process by PA0 . . PA3 via the BCD-to-decimal diverted to earth. The MMV associated
timers and the enlarger. The 128 byte decoder IC2. The displays are multi- with the row will not receive a scan
RAM in the 6532 is used to store the plexed and the data inputs to them are pulse and the microprocessor then

temporary data and the process and TO . . . T3. The hexadecimal code on knows which key has been touched. A

alarm times given by the keyboard. these lines is converted into the seven complete keyboard scan takes about

The 1MHz clock signal required by the segment code by IC1. Each display is 10 ms.

processor is supplied by a 4MHz crystal activated for about 25 ms. The footswitch SI is shown in figure 2.

oscillator and a divider-by-four, con- The capacitive keyboard is in the form This switch is connected in parallel to
sisting of FF1 and FF2. The circuit of a printed circuit board. It consists the START/ST. key and allows the

around T1, T2 and N3 and N4 provides of 20 keys, arranged in 5 rows of 4 keys timer to control exposure time while

a reset signal when the power supply is and each key pad only needs to be leaving the hands free,

initially switched on. The address touched with a finger tip to operate. It may be useful to have the safe light

9-32 - .

amber 1982

darkroom computer part 1

operate in conjunction with the enlarger
lamp. This facility is provided by the
relay RE1 shown in figure 2. Transistor
T1 will switch the relay on when a
logic 0 appears on the PB5 line. When
this line goes to logic 1, the relay will
switch the safe light off and the enlarger
on. The enlarger can also be controlled
manually by means of switch S2 in
order to refocus or change the enlarge-

This is as far as we go with the descrip-
tion this month, more on the accesories
will follow in the next article.

Construction

The basic darkroom computer consists
of four printed circuit boards:

• The microprocessor board.

• The display board.

• The keyboard interface.

• The capacitive keyboard.

It is strongly recommended that the
printed circuit boards are used in order
to greatly simplify construction. How-
ever, it is possible to use an ordinary
keyboard if preferred. In this case the
following modifications must be made:
The A/B wire link on the display board
must be linked in the B position.
Resistors R9 . . . R13 must be replaced
by wire links. The normal keyboard
(using 'make' contacts) is then moun-
ted between the junctions of lines
CO LI . . . COL5 and PA4 . . . PA7. Ex-
cept for the four resistors R31 . . . R34,
all the components situated between
. PA4 . . . PA7 and the keyboard may
now be omitted. Obviously the printed
circuit for the keyboard is not required.
A heatsink with a thermal resistance of
7°C/W must be used for the regulator
IC8. In practice it may be possible to
mount the regulator onto the inside of
a metal case (if used). The pins of the
regulator must be soldered directly onto
the board. This would be ideal, provided
that a mica washer and heat conductive
paste are used. It is even possible to cut
off the power supply section of the
printed circuit board and mount it
elsewhere, if that happens to be more
convenient. In any event, ensure that
the case is well ventilated or IC8 will get
hot under the collar. There is a minor
modification to the board with respect
to IC9 (the second regulator) .The track
between the common terminals (centre
lead) of this regulator and the earth
plane at the edge of the board (the wide
track) must be cut. The section of track
left, connecting the common terminal
of IC9 and the negative end of Cl 2,
must now be linked with a short length
of wire to the +5 V output of IC8. This
modification must be made because the
board was disigned for the 6502 'house-
keeper' which needed two 5 V rails.
Here we need +5 V and +10 V. If a
7810 can be found it can be soldered
directly into the board in the position
for IC9 without the need for any
modifications, but they are very thin on
the ground. No heatsink is required for
IC9. Do not forget to check that the

9-36

elektor septer

1982

power supply functions correctly before
inserting any expensive ICs into their
sockets. It is also a good idea at this
stage to check for any short circuits on
the printed circuit board.

A 1 MHz symmetrical square wave must
appear at pin 8 of IC7. A multimeter
together with the test circuit of figure 7
can be used to measure this clock
frequency. The meter must indicate
0 V on both the Q and 0 outputs when
a square wave is present. Of course it
is also possible to use a frequency meter
if one is at hand, in which case the
frequency can be set accurately with
C2.

Check that the output of N3 (pins 9 and
10 of IC6) goes high after switching the
power on. The code AA (10101010)
must now be put on the data bus by
means of the small test circuit in figure
7b. The circled numbers refer to the
board connector pins (between IC1 and
IC3). Now IC1 can be fitted into its
socket (ensure that the power is off
when doing so). The connector must
now have symmetrical square wave
signals at the following points: Pin 29
A0 250 kHz, A1 1 25 kHz, A2 62.5 kHz
and so on down to a frequency of
7.6 Hz at A 15.

Pin 14 of the connector (R/W) must be
logic 1, If a fault exists it must be
verified at first that AA really is present
on the data bus (by means of a multi-
meter). The easiest method of checking
all the frequencies is with the aid of an
oscilloscope. However, the circuit in
figure 7c together with a multimeter can
also do the job. The circuit is connected
to a pair of adjacent address lines
(A15 and A14; A13 and A12;...A1
and A0). The meter will indicate either
0 V or 5 V if all is well. Any other
value will indicate either a short circuit
or an open circuit on one or other of
the two lines. If everything is in order,
AA can be removed from the data lines.
Remember to take IC1 out of its socket
before using the soldering iron on the
board.

The above tests should ensure that the
printed circuit board assembly has been
completed correctly. All the ICs may
now be fitted into their sockets.

Only one point needs particular atten-
tion on the display board. This is the
wire link A mentioned earlier on in this
text when an ordinary switch type
keyboard is to be used. As stated, the
link must be in position B in this case.
The connections between the keyboard
and the other boards must be kept as
short as possible whatever type of
keyboard is used. The capacitive key-
board itself needs some attention
before being wired in. It is manufac-
tured from printed circuit board
material with a red colour on the
underside. However, in contrast to a
normal printed circuit board the top is
covered in a thin layer of hard plastic
to prevent damage and oxidisation of
the key pads. On the underside the row
contacts are already interconnected

Figure 6. The keyboard. The front is covered by a protective layer,
can be illuminated from behind.

1982-9-37

during manufacture. The same is not
true of the column contacts. These
connections must be made carefully
using thin enamelled copper wire.
Bearing in mind that the keyboard is
capacitive and therefore good operation
is only guaranteed when the wire links
are as near identical as possible. The
photograph illustrates the completed
keyboard and can be used as a guide.
The footswitch is connected between
SI on the display board and SI on the
keyboard.

The finished printed circuit boards can
now be mounted in a case and wired
as shown in figure 8. The drawing also
illustrates how the display board and
the keyboard interface must be placed
in relation to the keyboard if optimum
results are to be achieved. It is im-
portant that the leads between these
three boards are kept very short. Allow
a space of at least 3cms behind the
keyboard for the illumination. More
of this in a minute! Normally the
keyboard will not require screening
but if the keyboard is not mounted
parallel to the front panel of the case
it may be necessary. In this case a sheet
of thin aluminium will have to be
placed behind the board and earthed.
It may be preferable to complete the
wiring and check the operation first
in order to see if screening is required.
All connections to the outside world
can be made via sockets on the rear
panel of the case. One 14 way connec-
tor will cater for all the external cir-
cuits , but it may be more convenient
to use separate sockets if the process
timer, the light meter and the tem-
perature meter are not all required. Two
sockets for the enlarger and the safe
lights will be necessary and these
should be positioned as far away from
the keyboard as possible. This also
applies to all 220 V wiring for obvious
reasons.

When using an enlarger with a halogen
lamp (together with a transformer) it
is recommended that a filter network,
consisting of 100J2 and 100n/400V
in series, is wired between the relay and
the enlarger. This will keep interference
to a minimum.

The keyboard illumination

It is obviously very necessary that the
keyboard is made visible for it to be
used with any great succes in the
darkroom, and we went to great pains
to make this possible.

Four or six 6 V/50 mA miniature bulbs
can be uniformly distributed behind
the keyboard. These can be mounted
in miniature sockets fitted into a sheet
of white (or red) plastic or perspex
placed underneath the keyboard. Sides
can then be glued to form a box to
prevent any stray lights from escaping.
While the box must remain 'light tight'
it must definitely not be air tight, since
these bulbs can generate a surprising
amount of heat.

The lamps can be fed with an unstabil-
ised d.c. voltage and their brightness
can be adjusted be means of series
resistors. These will need to be of a
fairly high voltage. The lighting system
can be made even more attractive if
the lamps together with the displays
could be dimmed to cope with the
changing conditions. The circuit in
figure 9 will provide this facility. It can
be constructed on a small piece of
Veroboard. The output must be connec-
ted to pin 1 of IC2 on the display
board. The value of R4 must be reduced
to 10^2 if 6 lamps are being used
instead of 4. The maximum brightness
can be set by P4. The supply voltage for

the lamps and the dimming circuit is
derived across C9. Before inserting the
bulbs into their sockets ensure that
their supply is set to 6 V by PI . This is
important since the voltage level across
C9 is about 18 V. Transistor T3 of the
dimmer circuit must be provided with
a heat sink.

Practical tests

When construction and wiring are
completed (for this section) it will be
possible to check that all the operations
are correct. Before going any further the
darkroom computer will only operate
correctly if the EPROM, IC3, contains
the correct program. The listing of this

circuits shown here together with a multimeter.

darkroom computer part 1

program is shown in the accompanying
table 1.

The links PA5, PA6 and PA7 between
the processor board and the keyboard
interface must be disconnected. Check
that presets P 1 . . . P4 are set to zero,
before the power supply is switched on.
The display should now read 000.0.
This brings us to the calibration pro-
cedure of the keyboard printed circuit
board. For this purpose potentiometer
P4 must be turned anti-clockwise very
slowly, while intermittently touching
the MEAS key. At a certain point the
sign 'd‘ will appear on the display.
When this occurs do not rotate P4 any
further. When touching the RETURN
key, 000.0 must appear again on the
display. The display must show 000.9
when touching the keys SET/CLR and
9 respectively. When touching the
START/ST. key the relay must be
activated and switched off again after
0.9 seconds. If one of the keys does not
operate correctly, P4 must be rotated
slightly further and the procedure
described above repeated until all keys
react equally well.

The darkroom computer must then be
switched off and PA5 reconnected.
Switch the computer on again and turn
P3 until key SET.PR.T. reacts correctly,
'd ' appears on the display. Repeat this
procedure with links PA6 and PA7 and
the potentiometers P2 and PI. Before
doing this it is best to take a look at the
instructions for use in order to become
familiar with the functions of the
various keys. This helps to avoid mis-
takes. For example, the START/ST.
key will not react at all if it has been
touched after the MULT. key. Only
keys 0 ... 9 and RETURN react after
MULT, has been touched.

Ready for use

Adjustment of the four potentiometers
completes the construction and cali-
bration procedure of the basic dark-
room computer. It can already be used
in this form in the darkroom. However,
not all functions will be operational of
course.

To reach this aim, the constructor must
add the three circuits being published
in the next article. Anyway, the dark-
room computer can already be used as
exposure timer, as memory to store the
various times and as a process timer
with ten different times at its disposal.
Last, but not least, it also works as
continuous brightness control for the
displays!

Instructions for use of the
darkroom computer

This chapter deals with the operation
of the darkroom computer. Since it
describes the function of each key it
may be an advantage to have the com-
puter at hand, so that the theory can be
'converted' into practice right away.
The instructions for use deal with the
complete computer system and include
the circuits that will be published later.

The keys that will function with the
basic set-up are marked with a * :
DIM* : The brightness of the seven
segment display is controlled by this
key. They will be at maximum bright-
ness when the computer is first switched
on. Touching and holding this key will
cause the displays to become dimmer
until they go off altogether. If the key
is still held they will gradually return to
maximum brightness. The light level
will remain constant at the level oc-
curing when the key is released.

STORE* : The time period shown on
the display can be stored in memory
with this key. There are ten different
time periods available (0...9). The
time is stored as follows; for example,
the time is to be stored in memory 4.
Simply touch STORE and then key 4.
A d ' will appear on the display when
the STORE key is used to advise that
the computer is 'waiting' for a number.
When a number is entered it will appear
for one second on the display. The
number is stored when the display
blanks. The ten memories available are

also used for the second process timer.
RECALL* : This key is used to recall
the data from the memories. The
memory address and then the memory
data will appear on the display when
the RECALL key is followed by a
number key.

SET/CLEAR* : The display will read
000.0 if this key is touched. A time
between 0 and 999.9 seconds can now
be selected by the number keys.
START/ST." : The enlarger can be
switched on and off by means of this
key. The lamp of the enlarger will be
switched on by the relay after a time is
fed in and the START/ST. key is
touched. As mentioned before, the safe
light is switched off when the enlarger
is switched on and vice versa. This
happens automatically. The time orig-
inally set will then appear on the dis-
play and can be used again by touching
the START/ST. key a second time. The
enlarger can be switched off at any time
by this key. The START/ST. key is also
used to start and stop the second
process timer (see SET.PR.T. 2).

September 1982 - 9-41

RETURN* : This key is used to return
from a certain function to the main
program, in order to select another
function. It can be used when a key has
been touched by accident, which applies
to the CLR.PR.T.; SET.PR.T.; MEAS;
STORE; RECALL and MULT. The
'old' data appears on the display again
after the RETURN key is used (except
for the RECALL key).

0 ... 9': These keys are used to read
in a certain time and to choose a par-
ticular function with keys that have
more than one function.

SET.PR.T. (SET PROCESS TIMER):
The three functions of this key set the
process and alarm times. A 'd ' will be
displayed (indicating that a decimal key
must be used now) after this key has
been touched. The following choices
are:

—0 : The time can now be entered. The
time shown on the display re-
mains there for 3 seconds after the last
key was touched and then disappears,
indicating that it has been stored.

— 1 : It can now be determined at
which LED of the timing indi-
cator the alarm must sound, as follows:
After the command SET.PR.T.— 1 has
been given, the code 02 will appear on
the display. The number on the display
is increased by 1 per second until
number 25 is reached. This will be
followed by a return to the 'old' data
(02) on the display. The number dis-
played indicates the number of a
certain LED. For example, it is required
to sound the alarm at the 6th LED. Any
key touched when the number 06 is
shown will add an 'A' to the display.
This indicates that the alarm will go
off at this LED number. The alarm can
be set 15 times in this way. After the
25 numbers have been scanned, the
timer returns to the main program.
Giving the command SET. PR. T— 1 again
causes all alarm numbers (with the 'A')
to appear on the display again. It is now
too late to make any changes. To be
able to do that alarm registers must be
cleared again.

—2* :This key initialises the program-
ming of the second process timer.
This timer can store a maximum of 10
different time periods ranging from 0.1
to 99.9 minutes. Three of the four dis-
plays show the first time period in
minutes. The fourth (left) display
becomes dim and flashes very quickly.
This indicates the memory location in
which the number shown on the other
displays is stored (0...9). The time
period will be stored in this memory ad-
dress when the STORE key is touched.
The number of the next memory
location will then be displayed and the
same procedure can be repeated. As
stated before, this can be done 10
times. The following must be carried
out if less than 10 process times are
used, when the last required time is
stored (for example, the third), the
command 00.0 must be entered. The
first time period will then re-appear on

the display when the STORE key is
used. The second process timer can now
be started by operating the START/
ST. key. The left display will behave
'normally' again. Now the countdown
for one process time begins. When
00.1 appears on the display, the buzzer
announces that the last 6 seconds (of
that particular time) have been reached.
At the end of the period the buzzer
produces one long tone and the count-
down for the next process time begins.
The first time period re-appears on the
display and the left display starts to
flash again, after the last process time
has passed. It is now possible to either
start again (START/ST.), change the
process time or return to the main
program (using the RETURN key). The
process timer can be stopped whenever
required. In this event the timer jumps
back to the first process time and
remains there until the START/ST. or
RETURN key is touched again.
CLR.PR.T. (CLEAR PROCESS
TIMER): This key also combines several
possibilities. Again 'd' is displayed
when this key is touched to indicate
that a number key must be used next.
Now, there are several possible options:
—0 : The LED that is furthest to the
right on the process timer is now
cleared.

-1 : If lit, the second LED goes out
when touching this number. If
only one LED is lit, nothing wil happen
at this command.

—2 : Both LEDs will go out. Further-
more the LED running period is
then wiped out.

—3 : All alarm points for the process
timer are cleared, in other words,
all alarms are silenced.

-4*: All ten process times for the
second process timer are cleared.
In all 4 cases, the number entered is
displayed for approximately 1 second,
after which the computer returns to
the main program.

MEAS. (MEASURE) : All measuring
functions are controlled by this key.
The three possibilities are:

—0 : Light measurement. The enlarger
is switched on as soon as the '0'
key has been touched. The '0' remains
visible on the display for a moment
before it disappears. The display is
blank for two seconds while the com-
puter measures the amount of light that
falls on the light sensor. This value is
converted into an exposure time and the
result appears on the display. The
enlarger then switches off. The calcu-
lation made by the computer is based
on the brightness of the enlarger lamp
(the more light falling on the sensor,
the shorter the exposure time) and the
multiplication factor that can be added
by means of the MULT. key. We will
come back to that later on. An incorrect
(light) value will be indicated on the
display by EEE.E.

—1 : Contrast measurement. The re-
lationship between the lightest
and darkest spot on a negative. First

place the light meter on the lightest,
part of the negative being projected.
Then touch the keys MEAS. and 1
respectively. The display will blank for
2 seconds after which 'd' will appear
(the enlarger remains on). The meter is
then placed on the darkest area of the
negative and key 1 is touched. After 2
seconds, the left display will indicate a
C and the others a number relating to
the contrast. The contrast ratio is
indicated in light values; This is the
logarithm to the base 2 of the ratio
between the lightest and darkest spot.

The value obtained in this way can help
in choosing the right kind of paper for
a certain negative (the bigger the con-
trast ratio, the softer the paper will need
to be).

The enlarger lens should not be fully
open, but be on, for example f 5.6,
when taking the measurements. Ensure
also that the scale of enlargement is
not too big, otherwise the measurement
of the dark areas would fall out of the
measuring range.

The minimum contrast ratio that can be
measured is 1.0, which relates to a
light/dark relationship of 2 : 1. C 00.0 '

will appear on the display with any
lower ratio. The maximum contrast
that can be measured is 12.0, a value
that is only very, very rarely reached!

-2 : Temperature measurement. About

1 second after the '2' key has 1
been touched, the temperature will be
indicated on the display. This value is |
accurate to within 0.1 C. The display
flashes very weakly showing that it is
temperature that is being displayed. j
Returning to the main program can
only be done by means of the RETURN

MULT. (MULTIPLIER) : This key en-
ters the multiplication factor. A three-
figure number (which is always 01.0
when the computer is initially switched
on) appears on the display after this key
has been used. A number entered will
now appear on the display.

The multiplication factor is used in the
light measurement (see MEAS. -0), the
exposure time internally measured by
the computer is multiplied by this I
factor and the final result is displayed.

The multiplication factor depends on
the type of paper being used and
sometimes on the scale of enlargement. i
More details will follow in the darkroom |
computer part 2.

Again the constructor can only return
to the main program by using the
RETURN key.

There are still two 'ordinary' switches
that need to be described.

START PR.T. (START PROCESS
TIMER) : This switch is situated on the
process timer containing the 25 LEDs.

The first LED starts to 'run' when this
switch is used. Operating the switch
once again causes the second LED to
run as well.

FOCUS*: The enlarger can be switched
on and off by means of this switch.

M

a full description with
applications for the
very versatile MF 10
by National

realised with only half an 1C. By connecting
both halves 4th order filters (24 dB/octave)
can be realised.

Obviously filters having an even higher steep-
ness can be constructed by cascading a num-
ber of MFIOs. Any of the classical filter
networks (such as Butterworth. Bessel. Cauer
and Chebyshev) can be formed.

Modes of operation

First of all we advise the reading ol

Features

Besides the worthwhile qualities already men-
tioned. the MF 1 0 is extraordinary in as much
as the clock to centre frequency ratio is accu-
rate to ; 0.6%. This kind of accuracy, which
incidentally remains the same irrespective of
the number of times you duplicate the circuit,
can never be realised by conventional filters
using OTAs. The accuracy of these filters
really is useful, especially when considering
them for use with microprocessors, for auto-
matic testing and measuring equipment, and

The stability of the filter cutoff frequency is
directly proportional to the stability of the
clock frequency. The highest input frequency
is 30 kHz with the highest clock value at
1 MHz with around 1 .5 MHz being typical.

So far so good! At first sight it may appear
that the high price of the MF10 is rather

little that you v

rry, by the time
s 'narrative' there w

forward versior

i of operation
most straight-

t least, figure 4, whicl

September 1982 - 9-45

•l-Is! -V®M

'.".(ife-VSlM

F5r

Definition of terms

fCLK : the switched capacitor filter external
clock frequency.

f Q : center of frequency of the second

order function complex pole pair.
f Q is measured at the bandpass out-
put of each %MF 10. and it is the
frequency of the bandpass peak
occurrence (figure A).

Q: quality factor of the 2nd order func-

tion complex pole pair. Q is also
measured at the bandpass output of
each % MF 10 and it is the ratio of f 0
over the —3 dB bandwidth of the 2nd
order bandpass filter, figure A. The
value of Q is not measured at the
lowpass or highpass outputs of the

ible amplitude peaking at the above
outputs.

HqbP : *8 gain in (V/V) of the bandpass
output at f = f 0 .

HOLP : ths gain in (V/VI of the lowpass out-
put of each ViMF 10 at f-*0Hz,
figure B.

HoHP- the gain in (V/VI of the highpass out-
put of each 54MF 10 as f “HcLK/ 2,
figure C.

Q z : the quality factor of the 2nd order

function complex zero pair, if any.
(Q z is a parameter used when an all-
pass output is sought and unlike Q it
cannot be directly measured).

f z : the center frequency of the 2nd order

function complex zero pair, if any. If
f z is different from f 0 , and if the Q z
is quite high it can be observed as a
notch frequency at the allpass out-

f notch 1 the notch frequency observed at the
notch output(s) of the MF 10.

H ONt : the notch output gain as f-»0 Hz.

HqN 2 : the notch output gain as f -tfCLK^ 2 -

Pin description

LP, BP, N/AP/HP

These are the lowpass, bandpass, notch or all-
pass or highpass outputs of each 2nd order
section. The LP and BP outputs can sink typi-
cally 1 mA and source 3 mA. The N/AP/HP
output can typically sink and source 1.5mA
and 3 mA, respectively.

This is the inverting input of the sumi
opamp of each filter. The pin has static
charge protection.

SI

SI is a signal input pin used in the all
filter configurations (see modes of opers
4 and 51. The pin should be driven wi
source impedance of less than 1 k.

Sa/B

inputs of the filter's 2nd summer eithe

analog ground (Sa/b low to Va ) or tc

lowpass output of the circuit (Sa/b hig
Va + ). This allows flexibility in the var
modes of operation of the 1C. Sa/b ■*
tected against static discharge.

V A + . V D +

Analog positive supply and digital pos
supply. These pins are internally conne

through the 1C substrate and therefore Va +
and Vq + should be derived from the same
power supply source. They have been brought
out separately so they can be bypassed by
separate capacitors, if desired. They can be
externally tied together and bypassed by a
single capacitor.

va\v 0 -

ively. The si
Vq + apply hs

LSh

Level shift pin; it accommodates various clock
levels with dual or single supply operation.
With dual ;5V supplies, the MF 10 can be
driven with CMOS clock levels (±5V) and
the L Sh pin should be tied either to the
system ground or to the negative supply pin.

If the same supplies as above are used but

T 2 L clock levels, derived from 0V to 5V

supply, are only available, the L Sh pin should

be tied to the system ground. For single
supply operation (0 V and 10 V) the Vq ,
Va pins should be connected to the system
ground, the AGNO pin should be biased at
5 V and the LSh pin should also be tied to

the system ground. This will accommodate

both CMOS and T 2 L clock levels.

CLK(AorB)

Clock inputs for each switched capacitor
filter building block. They should both be of
the same level (T 2 L or CMOS). The level shift
(LSh) pin description discusses how to
accommodate their levels. The duty cycle of
the clock should preferably be close to 50%
especially when clock frequencies above
200 kHz are used. This allows the maximum
time for the opamps to settle which yields
optimum filter operation.

50/100/CL

By tying the pin high a 50:1 clock to filter
center frequency operation is obtained. Tying
the pin at mid supplies (i.e., analog ground
with dual supplies) allows the filter to operate
at a 100:1 clock to center frequency ratio.
When the pin is tied low. a simple current
limiting circuitry is triggered to limit the over-
all supply current down to about 2.5 mA. The

AGND

Analog ground pin; it should be connected to
the system ground for dual supply operation
or biased at mid supply for single supply
operation. The positive inputs of the filter
opamps are connected to the AGND pin so

'clean' ground is mandatory. The AGND pin

is protected against static discharge.

a home

telephone*

system

up to nine extensions without an exchange

The telephone is a familiar and accepted
part of our everyday life. For the vast
majority of users it forms the basis of
their livelihood, while for many others
it is a necessary convenience. The
telephone is so much a fact of life that
not too many people, even many
electronic engineers, fully understand
how it works. It is generally assumed
that since it has been around for so
long, it can't really be that complicated.
Not true, the average telephone set is a
hybrid concoction of electro-mechanical
bits and pieces, all held together with
3 BA nuts and bolts. After all, it did
originate in the era of valves and intern-
ally does present a somewhat dated
appearance. The reasons for this are
many and none of them have anything
to do with developments in electronic
technology. To be fair, the latest
telephones have caught up with the
times to a large extent.

Our home telephone system here does
not profess to break any boundaries of
technology, but it is very much in
keeping with what our readers expect
an electronic system to be. In fact,
no 'high technology' components are
used, they are all very much of the
'common or garden' variety — and easily
obtainable!

It must be stated here and now that this
project is for a self contained intercom
system for use in any desired manner -
but it must not be used with the British
Telecom network. When that saga is
sorted out we can do something about
it, but until then — no tampering with
the 'company lines'!

What are the basic requirements for a
telephone system using the available
sets? The first problem that arises is

require a telephone exchange. Each telephone set requires a small
printed circuit board and interconnections are made via a four-way
cable.

Home telephone systems are fast
| growing in popularity with the
availability of telephone sets at
a reasonable cost on the surplus
market. Even many small busi-
nesses are catching on to the fact
that a do-it-yourself intercom
system is far cheaper than a
professional installation. A further
advantage is the ease with which
the system can be modified to
cater for changing con-
ditions.

The design de-
scribed here
is a self-
contained
system
and Ji

does jl

not

that the circuit must carry out a part of^
the task that is usually performed by
the exchange in the national telephone
network. Obviously it must be possible
to address the extension you require.
This is performed by a pulse train
produced by the dial mechanism when aJ

number is dialled. Fortunately, this is
no great problem since, of course, the
telephone set already has this facility
built in. However, we have to make sure
that the dial pulse train corresponds to
the number of that particular extension.
If the answer happens to be 'yes' - then
the bell must ring. In the event that
somebody answers, — that is, the
television is switched off - then the
handsets of the two extensions must be
interconnected. It should also be
impossible for others to 'listen in' to
the conversation. It all adds up to a fair
amount of complications that must be
ironed out before the network can be
called fully operational.

No switchboard!

The Elektor telephone system meets all
the requirements expected of a home
(or small business) intercom plus a few
added features. While the number of
extensions is limited to nine, it should
be quite enough for the majority of
users. This number would probably even
be sufficient for private functions, such
as exhibitions, fetes and the like.

The main requisite of a private network
of this type is that it should not need a
master 'switchboard' or exchange. An
advantage of this is that installation is
very much simpler. Instead of the 'star'
network (all extensions are wired
directly to the master), the extensions
are simply interconnected by means of
a four-way cable as shown in figure 1 .
This removes the necessity of those

mega-way cables that our B.T. friends
are so fond of. The cable carries the
following:

• The ringing line (U u )

• The power supply line (+12 V)

• The ground line (0)

• The speech line (L)

The power consumption for the whole
network is very low and the single
power supply is quite sufficient. Wiring
between individual extensions is made
simple by the connections provided on
the printed circuit board.

Features of the system

The number of each extension (1 ... 9)

is determined by means of a wire link

on the printed circuit board of that
extension. When a specific number is
dialled, it will cause the bell of that
extension to ring and an intermittent
tone will be put on the L line. A LED
will light on all the other extensions to
indicate that the line is busy. The inter-
connection between the two receivers
is made as soon as the handset of the
extension called is lifted. It is not poss-
ible for any other extension to 'listen
in' to the conversation. The connection
is broken when both receivers are
replaced.

In spite of having said that any conver-
sation remains private (no other exten-
sion can listen in) we can, if required,
invite a third party to join by means of
a little 'glitch' in the system. Supposing
the extension being talked to was
number 3 and it was considered that the
topic of conversation might be enjoyed
by extension 7 also. To obtain number
7 (without losing number 3) simply dial
the difference between the two num-
bers, in this case 7 — 3 = 4. Both exten-
sions, 3 and 7, will now be in circuit.
Similarly, when talking to extension 3
and extension 8 is required to join, the
number to dial is 5. A 6 is dialled for
extension 9 and 8 is dialled for exten-
sion 1 and so on.

As if this was not enough, it is possible
for any number of parties to join in. In
this case the number to dial will be the
difference between the number of the
last extension to join and that of the
potential newcomer. This has all the
overtones of impending chaos (es-
pecially if junior discovers it), but the
facility is there.

The inside story

The circuit diagram in figure 2 is fairly
complicated but construction is helped
over this problem by the use of the
printed circuit board. The complete

iber 1982 - 9-49

circuit is fitted into the telephone set
of each extension required.

A voltage level of 25 to 30 V exists on
the speech line (L) but no current is
drawn from this until the handset is
lifted and the dial is operated. When this
happens, line 'b' is switched to earth via
the dialling switch (once for every
number dialled - plus one extra). When
line 'b' is first taken low, pin 10 of A3
follows suit. The output of A3 then sets
FF2 (via N2 and N6) which causes T1
to switch on with the aid of A2. Now
the speech line L is directly connected
to 'b' of the telephone, and the dial
pulses will be sent along the speech line
to all other extensions. The pulses on
the speech line produced by the dialling
mechanism are approximately 40 ms in
length with a pulse interval of about
60 ms.

The first pulse received by all the exten-
sions will reset the counter IC1, FF1 1
and FF2. The following pulses are I
then counted by IC1. Comparator A1,
together with D2, R3 and C3, forms a
retriggerable monostable which will pro-
vide a clock pulse to FF1 0.2 seconds
after the last dialling pulse arrives. At
the required extension the linked out-
put (between one output of IC1,
Q1 . . . Q9, to m) will be high thus
setting FF1. The Q output of this flip-
flop will now go high and connect the
speech line (L) to the handset by means
of A2 and T1. At the same time, the Q
output of FF3 (now high) will start the
oscillator around N5 and ring the bell
via T4 . . . T6. When the handset is
lifted, FF1 is reset and the bell stops
ringing. The person at the extension is
now connected to the caller.

All other extensions are disconnected.
The Q output of FF1 will be low with
the result that T1 will be switched off.
LED D5 will be lit, denoting that the
line is busy.

If the handset of the telephone being
called is not lifted (there is a good
programme on television), FF3 will be
reset and the bell will stop ringing.

Power supply

Power consumption of the whole
telephone system will be very low and
the power supply shown in figure 3 will
be quite sufficient. The supply must
fulfil three requirements. The 12 V is
used to supply the power for all elec-
tronics and the 7812 regulator 1C takes
care of this easily. The U line provides
the power for ringing the bells and is
provided for by the two 18 V secondary
windings of the transformer in series.

The purpose of the L line may not be
quite as obvious. As many readers may
know, the microphone inserts in the
handsets are carbon granule capsules
and these require a voltage across them
to allow them to function.

All the four output lines of the power
supply; 0, 12 V, L and U are connected
to the printed circuit boards of all the
extensions.

The printed circuit boards
The track patterns and layouts of the
printed circuit boards for the extension
circuits and power supply are shown in
figures 4 and 5. It is strongly rec-
ommended that printed circuit boards
are used, since one complete circuit of
figure 2 is required for each extension
telephone set. As previously mentioned,
only one power supply is required for
the whole system.

The extension board is small enough to
be mounted in a small box under or
near to the telephone of each extension
or even in the telephone itself. However,
this will depend on the telephone, since
not all of them have enough internal
free space to accommodate the extra
board. The 'line busy' LED must
obviously be mounted so that it is
visible to the user.

Two points to note regarding the
printed circuit board. It will be seen in
figure 4 that there are a row of points
on the board, marked 1 ... 9 and m.
A link connected between m and one
of the numbers will give that extension
that particular number (link m to 6 =
extension 6). It should be remembered
to provide the regulator IC7 on the
power supply with a heatsink, especially
important if a lot of extensions are
being considered.

The wiring

The interconnections between the tele-
phone sets are made by means of a four
way cable and, if figure 1 is followed,
will create no problems. Bear in mind
that the U connection will not be found
on the power supply board. This line
carries the bell ringing voltage and
must be connected in the power
supply directly to one end of the 18 V
secondary winding of the mains trans-
former as shown in figure 1 .

Parts list for figures 2 and 4

Resistors:

R1,R23 = 1 M

R2,R13,R24,R25,R30 = 100 k
R3.R21 = 4M7

R4.R6.R7.R1 1 ,R17.R20,R22,R29 = 1 k

R5 = 22 k

R8.R9 = 47 k

R10 = 220 n

R12 = 4k7

R14 = 12k

R15.R16 = 2k2

R18 = 330 k

R19 = 220k

R26 = 2M2

R27.R28 = 3k9

R31 = 18 k

Capacitors:

C1,C5 = 100 n
C2 = 100 p
C3 = 68 n
C4 = 1 ill 50 V
C6= 1 p/16 V
C7 = 1p5/16 V
C8.C9 = 1 n
CIO, Cl 2 ■ 470 n
C11 -33n

C13 • 1 p/16 V Tantalum
C14 * 10 n

Semiconductors:

T1,T6= BD242

T2 = BC640

T3.T5 = BC 547B

T4.T7 = BC 557B

D1 . . . D4,D6,D7,D8 = 1N4148

D5= LED

D9 = 1N4001

IC1 =4017

IC2.IC6 = 4013

IC3.IC5 = 4093

IC4 = LM 339

alephone syster

iber 1982 -9-51

How to connect the telephone set itself
will probably be the most popular
question. Here is where the major
difficulty may present itself due to the
fact that the telephone sets that are
available on the surplus market may

Final remarks

A few pointers are included here to
provide readers with a 'ring of con-
fidence' in the final stages.

The reference levels of the comparators
A1 . . . A4 are set at a level of 3.75 V.
This is verified if LED D5 lights as soon
as one handset is lifted (A4 operating
correctly). If this is not the case then
the value of R17 can be modified. It
may also be necessary to change the
value of R18 if the LED still does not
behave correctly.

The bell may be caused to switch off
inadvertently by the current sensor T1
although this is unlikely as the majority
of telephones have a power consump-

have originated anywhere. However, it is
relatively safe to assume that most, if
not all, telephone networks will have
been based on the G.P.O. system.
Having said that, we must advise readers
that this is an area where Murphy's law
is most active. It is strongly hoped that
the four rather nice telephones you have
just picked up at the right price were
not designed solely for use as replace-
ments for the unique Klutterbang
exchange that existed in the middle of
the Burmian jungle in 1934!

Whatever sets you may have found,
figure 6 should help. Coupled to this,
our spies in B.T. (planted in 1953 for
just such an occasion) have sent us the
following information. In most ex-
G.P.O. telephones, the BE line (in
figure 6) will be labelled connection 16.
The 'a' connection will be 9 and the 'b'
will be 19. The circuit in figure 6a is
what you can expect to find in the
average telephone set, but our system
here requires the circuit of figure 6b.
Our agent informs us from the inscru-
table B.T. that the simple removal of a
wire strap between connections 17 and
18 will be all that is necessary for the
change to be made. But we don't believe
a word of that, do we? So ... we in-
vented a simple test procedure to find
the truth of the matter.

Use one of the 1 8 V transformer
windings as a test voltage and connect a
1 00 p/35 V electrolytic capacitor in
series with one lead, for self-preser-
vation and protection against any
damage to the telephone. Connect this
test voltage across any two terminals on
the telephone set (normally only 3 or 4
in use . . . hopefully). If the bell rings
you have the bell connection, 'a' and
BE. If the result is a loud hum in the
earpiece, then the 'test voltage' is con-
nected across 'a' and 'b'.

Hands up those readers who got it right
first time ... I Who believes in Murphy's
law now?

Figure 6. The fi
telephon

tion of about 20 to 25 mA.

The voltage level at pin 10 of A3 must
become practically 0 V when the
handset is lifted in order to connect the
telephone to the speech line.

It is also possible to use a two wire
interconnection between the telephones
if a small power supply is provided in
each set. This will supply each tele-
phone with its own power for the
speech line. This may be very useful for
some installations.

The discerning reader may i
realism of crossed lines and wrong
numbers but a steady diet of B.T. each
day should help!

6a

9-52 - elektor September 1982

The recognised method used by syn-
thesiser builders is, the one already
mentioned: several VCOs in parallel.
Readers wishing to know more about
this particular technique should refer to
the Elektor Formant books. We are
certainly not going into it in this article
simply because, the alternative intro-
duced here, is a more viable proposition.
And, anyway, apart from being expens-
ive the results achieved by the conven-
tional method can sometimes be com-
pletely unpredictable.

In our opinion the best solution is to
use a single VCO and still produce the
same end result. First of all we need a
single VCO producing a sawtooth wave-

synthesised

sound

animation

an inexpensive way of phase-shifting a
sawtooth waveform

Sound animation or to put it in another way: the changing, delaying or
phase shifting of any periodic waveform, enhances any final result,
sometimes quite dramatically.

One way of achieving this is by using several VCOs in parallel, each one
having a tiny tuning deviation. The result is a rich ensemble type sound,
but, unfortunately it is expensive, and time consuming to build.

This article introduces an effective solution which although being
relatively inexpensive produces the same results.

form. The animation circuit is based
upon an algebraic phase shift of this
waveform. The input to the circuit is
the sawtooth signal from the VCO.
while the output is a sawtooth that is
shifted in time from the original by an
amount depending on the control volt-
age. Thus the shift is already under
voltage control. If a number of these
units (typically eight), are used in paral-
lel, a rich animated bright sound results.
As a matter of interest A1 in figure 1,
can be used to provide the inverted
sawtooth signal for all the phase shift
circuits used. This means that all the
additional circuits will not require A1,
saving components!

Figure 1 illustrates one way of achieving
the necessary shift. The circuit has three
main stages:

• An inverter A1.

• An adder or summing up stage A2.

• A comparator and rectifier A3 and
D1 .

The reference voltage level (Ur) will
have a direct influence on the time shift
of the output compared to the input.
Figure 2 clearly indicates this.

Figure 2c illustrates the result of adding
the following constituents:

• A pulse form positive signal which
has an amplitude, equal to the peak-

to-peak value of the sawtooth.

• a negative going edge which corre-
sponds to the positive edge of the

sawtooth, that is as far as the time fac-
tor is concerned.

• The sawtooth voltages depicted as
the dotted signal line shown in fig-
ure 2a.

Turn figure 2c upside down and add the
D.C. voltage value to the result as shown
in figure 2c. The final result is a time/
phase shifted sawtooth as depicted by
the solid black lines in figure 2a. The
phase shift basically equals the pulse
width of the signal illustrated in fig-
ure 2b.

ithesised sound ;

iber 1982 - 9 53

In practice it is not the voltage but the
currents which are added (see figure 1).
The dotted lines in figure 2a, correlate
to the current passing through R3.
Assuming PI has been correctly cali-
brated, the current passing through PI
and D1 is depicted by the shape and
value of the curve in figure 2b. There
are two further additional D.C. factors
represented by the currents through R5
and R4. All of this is necessary in order
to ensure that the shifted sawtooth
swings around the OV potential, just
like the input signal.

Looking at figure 1, you will notice
that the circuit contains three presets,
so don't expect to carry out the cali-
bration procedure in just a few minutes.

0.10 V higher than the negative supply
voltage of the LM 324 (typically
-15 V). The output signal of A2 should
be the same as the inverted version of
the one shown in figure 2c, which inci-
dentally will only occur when PI has
been correctly set. In this case PI is
turned until the positive going edges of
the sawtooth are no longer interrupted
by any short positive or negative 'jumps'.
P3 is used to add sufficient D.C. com-
ponents in order for the output to have
a D.C. potential of 0 V. The phase/time
shift is adjusted with P2. The range is
from zero to one a full sawtooth cycle.
As already mentioned at the beginning
of the article, for each additional cir-
cuit, A1, resistors R1 and R2 are not

Crystal oscillator

(Elektor 87/88 Summer Circuits 19821

An error on the drawing may have given

readers a little trouble. The gates G1 and G2

of T3 should be the other way around.

Slave flash

(Elektor 87/88 Summer Circuits 19821
An error occurred on the printed circuit
board (EPS 82549) for this project. Capacitor
Cl is placed in parallel to R6 which is in-
correct. To solve the problem break the
track between the base of T3 and the negative
end of Cl. Now just link this end of Cl to the
•0' point on the board.

Output unit and keysoft for the
Polyformant

(Elektor 87/88 Summer Circuits 1982)

In a 10 channel version the values of R7 and
R8 must be increased to 1 k5.

T eletext decoder
(Elektor 80 December 1981)

is not shown in the drawing of figure 9. The
C49 in the drawing is in fact C48. Values are

RAM/EPROM card for the Z80
(Elektor 85 May 1982)

When the RAM/EPROM card is modified
for Z80 systems, it is then not protected
against programming errors. To overcome
this 2C of IC7 is linked not to earth, but to
the RD line. Incidentally, gate N3 is drawn
in figure 3 as a NAND. It is, of course, an
AND gate.

Figure 2. The function of the shifter visualised.

Calibration

A two channel oscilloscope is an absol-
ute must. Constructors without one will
have to beg, steal, or borrow one,
because it is practically impossible to do
without it.

First check whether the output of A1 is
an inverted sawtooth, when compared
with the input. Now check that the out-
put of A3 is a signal similar in shape to
the one shown in figure 2b. The lowest
point of the trough should be around

required.

The results of using up to eight parallel
circuits is outstanding. We don't suggest
using more than eight, as after this point
the law of diminishing returns comes
into effect.

Literature:

B.A. Hutchins, Analogue circuits for

sound animation, JAES

November 1981. H

The Elektor Artist
(Elektor 85 May 1982)

An incorrect wiring in figure 5 may have
caused some problems to the constructors.

Obviously, point A of socket Bal must not be

soldered to connection Ba4, but to Bal on
the printed circuit board. The connections 1
and B of Bal must be linked to SI. Similarly,
point A of socket Ba4 must be attached to
connection Ba4 on the board. Points 1 and B
of Ba4 must be linked to S4.

9-54 - 1

)r September 1982

siver for the Rugby MSF

time receiver
for the
Rugby

MSF

the right time
all the time . . .

This circuit was designed
as an addition to the '6502
housekeeper' published in
the May 1982 edition. The
two together provide an ex-
tremely accurate time clock
controlled by transmissions
from the Rugby MSF transmit-
ter. The timing is derived from
a caesium atomic clock that
boasts an accuracy of 2 x 10’ 3 .
The receiver circuit described in
this article is designed to oper-
ate from the 60 kHz trans-
missions (VLF) from Rugby that
provide good reception through-
out the U.K.

the third stroke,
it will be . . . '. How often
have we all heard that, without
realising that our entire life style is ruled
by time - a concept that we haven't
got a hold on, yet! We at Elektor are
working on it though. Our dual made
time machine will be published last
month, if all goes according to plan. For
the time being however, we have to be
content with providing a 'time machine'
that can maintain an accuracy of better
than one second - at any time!

A little background into the real sub-
stance of time for those readers who
haven't had any to think about it, may
be appropriate.

The second became the standard unit
of time in 1968. In that year the
second was defined by the Conference
Generate des Poids et de Mesures as
being 9,192,631,770 periods of the
radiation that corresponds to the tran-
sition between two hyperfine levels of
the ground state of the atom caesium
133! A suitable ground definition —
coming from the Poids et de Mesures
— but roughly speaking it means that
(give or take a transition or two) there
are about sixty to every minute!
Anyway, the facts of the matter stand
that you do not have to write away for
your box of caesium atoms (complete
with hyperfine levels), the circuits here
can provide you with an accuracy to
within 9,192,631,770 of all those things
right in your own house.

well be suggested
that we do not really need
a clock with anything like that kind of
accuracy and for the vast majority of us
this is quite true. However, for various
institutions (like laboratories) it is
vitally important and it is these people
that the Rugby MSF transmissions are
aimed at. Since the service exists (and
for nothing) why don't we make use of
it? The fact that it is highly accurate is
almost beside the point for us.

The time transmitter
The power output of the Rugby trans-
mitter is approximately 50 kW and,
thanks to its location, its transmissions
can be received throughout the land.
The transmission frequency is 60 kHz
which is itself derived from the atomic
standard at the National Physical
Laboratory. The deviation of the atomic
clock, is less than 2 x 10‘ 3 over a period
of 100 days, so except for the phase
shifts that are typical of long wave
transmissions, the carrier wave is excep-
tionally stable.

The Elektor clock is equipped with its
own time base, in case the MSF trans-
mission fails or is lost. Very short inter-
vals, due to interference, will probably
not be noticed as the clock continuously
compares the indicated time with the
signal transmitted from Rugby.

The time signal

The MSF carrier wave is modulated by
reducing the amplitude to 25% at the
beginning of each second, with the

• the Rugby MSF

elektor September 1982 - 9-55

exception of the 59th second. This
'mark' is missing altogether to denote
the impending arrival of each new
minute.

The time code

The complete time clock, including the
exact time and full date (day of the
week, month and year) is transmitted
before each approaching minute. The
second 'markers' are used to transmit
the data in BCD format. The second
marker itself consists of a break in the
carrier with the timing for the second
being accurate at the beginning of this
break. Each second marker also doubles
as the data for one bit of the trans-
mitted information. A carrier break, if
100 ms long, indicates a logic '0' while a
break of 200 ms is a logic '1'. This is
illustrated in figure 1 .

The first 1 6 seconds of each new minute
are used to correct discrepancies which
occur between the international atomic
time scale (used by Rugby MSF) and
the universal time scale that relates to
the rotation of the earth as a reference
source.

As mentioned, the time code infor-
mation is transmitted during the minute
prior to that data becoming accurate.
The drawing in figure 2 shows what data
bit arrives when in the minute cycle.

The time signal receiver
The drawing of figure 3 shows the block
diagram of the receiver section. It was
designed with three main parameters in
mind:

• high sensitivity

• narrow band width

• no 'problem' coils

■High sensitivity is achieved by the use of
an active aerial together with a low
noise preamp and a bandpass filter to
keep the bandwidth narrow. Calibration
is kept to a minimum by the use of a
crystal oscillator and fixed coils for the

1

specific data arrives during each minute.

bandpass filter. It was decided that a
superhet design presented the best
possibility of meeting all the required
parameters.

The receiver circuit is based on the
TCA440 1C. This contains an RF
preamp, a balanced mixer, a separate
internal oscillator and an internal IF
amplifier. Furthermore, all character-
istics of the TCA440 are virtually
independent of the supply voltage due
to an internal stabiliser.

The preamp and the triple IF stage
require a feedback input from the
automatic gain control. This AGC is
obtained by rectifying and filtering the
IF output signal and feeding it back via
pin 9. The AGC is passed to the preamp
via an emitter follower in the control
amplifier and an external link between
pin 3 and 10.

2

Figure 2. The MSF time information is
transmitted by modulating the carrier as

The MSF output at pin 7 is rectified and
filtered before being passed to a Schmitt
trigger. The 'clean' MSF signal then
appears at TTL level at the 'DATA' out-
put. The Schmitt trigger also switches
an LED on and off at one second
(effectively) intervals.

The circuit for the time receiver is a
combination of figure 4 and 5. The
active aerial circuit is that of figure 4.
The aerial itself consists of a coil (L8)
wound on a ferrite rod. With the ad-
dition of C43, this forms a tuned circuit
that is resonant of 60 kHz. The coil can
be moved on the ferrite rod if any
tuning is necessary. The FETT5and its
associated components forms an am-
plifier with a gain of 20.

The aerial circuit is connected to the
main receiver circuit by means of a
screened cable. Power supply decoupling
is provided by R1 , Cl and C4, while R4

) -56 - i

iber 1982

jgby MSF

4

L8:

The rectified MSF signal appears at
TP3 and an oscilloscope connected to
this point can be used to examine the
waveform.

The remaining calibration required
concerns R25 and R27 in the Schmitt
trigger circuit. Set R27 to its mid.
position and rotate R25 until LED D8
flashes. R27 can be treated as the
'coarse' adjustment and R25 as the

It will be clear that the output of the
receiver cannot be connected to any
simple digital circuit that drives some
display. Some sophisticated hardware is
required before we can tell the time.

Figure 4. This active aerial circuit will provide good reception of the Rugby transmitter
throughout the country.

ensures that the RF signal is not short
circuited by the supply.

In the circuit of figure 5, a bandpass
filter network with a centre frequency
of 700 Hz and a bandwidth of 100 Hz,
is connected between pins 12and 16of
IC1 . It would be easy to make the band-
width narrower, but then calibration
would be necessary and that is some-
thing that we want to avoid if possible.
IC2 divides the crystal oscillator out-
put by 32 to provide a frequency of
59.3 kHz. This is fed to pin 4 of IC1
and is mixed internally with the carrier
frequency of 60 kHz to provide a centre
frequency of 60 kHz (60,000 Hz —
59,300 Hz = 700 Hz). This appears at
pin 7 and is then filtered and amplified.
The AGC voltage is obtained by rec-
tifying the 700 Hz output by D3. The
MSF signal reaches the DATA output
via D4 and the Schmitt trigger formed
, by IC4. The output is tailored to TTL
I level by means of D7 and T3. The LED
is flashed at the data frequency by
means of T4.

If required, the TX output (viaT5) will
provide an indication when reception
| is lost, due possibly to reception con-
ditions or in the event of the trans-
mitter going off.

The power supply for the circuit is
shown in figure 6.

Construction

If the printed circuit board (figure 7)
is used no problems should be en-
countered during construction. The
active aerial section must be separated
from the main receiver and fitted,
together with the ferrite aerial, in a
small plastic box. This will allow the
aerial (which is mounted horizontally)
to be rotated for the best reception.
The connection between the two boards
must be made via screened cable and
small BNC connectors.

Calibration

The need for calibration has been
reduced wherever possible in the circuit,
but there are a few areas where it is

necessary. Connect the aerial circuit and
power supply to the receiver. After
checking the supply voltage turn the
preset R25 . . . R27 and C29 to their
mid. position.

Guesswork will have to be applied to
the trimmer (C29) but 'about right'
will be close enough. An oscilloscope
must now be connected to test point
TP2 in order to check the amplitude of
the 700 Hz signal. Adjust the amplitude
for a maximum by moving the coil LI
on the ferrite rod. C29 can now be ad-
justed to improve the amplitude still
further. The oscillator frequency can be
set precisely if a frequency counter is
available. In practice, the setting of C29
is not that critical and the mid. position
will be fine.

If an oscilloscope is not at hand the
calibration can be carried out with the
aid of a multimeter switched to the
500 /iA range. This is connected to test
point TP1 and the above adjustments
are made for a maximum reading on the
meter. At the very least, a crystal
earphone or small AF amplifier with a
loudspeaker can be used. The reception
is at its best when the 700 Hz tone is
clearly audible.

\

MSF + 6502 housekeeper

It was stated in the summary that the
MSF receiver was intended for use with
the 6502 housekeeper published in the
May 1982 issue of Elektor. The combi-
nation of the two circuits will provide
a clock that will remain very accurate
Furthermore, this clock has the added
facility of four switched outputs that
can be programmed over the period of

Very briefly the 6502 housekeeper is a
microprocessor-based circuit that is able
to accept the MSF output of the re-
ceiver section and decode it, in order to
provide a visual display of the time.

It will be remembered that the MSF
information is transmitted throughout
the period of each minute and there-
fore a simple decoder will not be
enough. The incoming data will have to
be put into memory and processed to
arrive at the display at precisely the
correct time. The 6502 microprocessor
was chosen for this task and for a
complete and detailed description of the
housekeeper we refer the reader to the
May issue, since, if the construction of
the complete clock is contemplated,
this article will be required.

The MSF time clock

The MSF receiver is connected to the
PA7 input (shown in figure 1 of the
6502 housekeeper) with screened lead.
(Switch Sa and resistor R 1 2 will not be
required). The clock will start to count
and the MSF signal stored and decoded
as soon as the power is switched on.
After approximately two minutes, the
correct time in hours, minutes and
seconds will be displayed. Thereafter,
the clock will be adjusted by the MSF
transmission once every minute. The
seconds shown on the display are
derived from the internal crystal os-
cillator. This ensures that the clock
will continue to run in the event of a
reception failure. Should this occur, the
calender program will continue to
provide the correct date. If the internal
oscillator has been correctly calibrated,
the error will not exceed half a second
per day if reception is lost for a long
period of time. This is, hopefully,
unlikely to happen.

The data will be displayed for as long as
the 'date' key is held. The days of the
week are indicated by seven LEDs. One
LED is also used to indicate good
reception of the Rugby MSF signal. This
LED will flash if reception fails for any
reason. It will cease to flash two full
minutes after reception has been re-
stored. This will indicate that the clock
is once again synchronised with the
MSF signal.

It is worthwhile remembering that the
NiCads in the housekeeper will maintain
the internal time if the mains supply
fails. The time will of course automati-
cally be corrected when the power
supply returns. The only noticeable
difference will be the display becoming
considerably dimmer.

The software

There are easier things to explain than
a complete program. However, we will
describe what it actually accomplishes.
The MSF time code enters the house-
keeper via line PA7. The logic 'V and
'0' are decoded from the pulse lengths
and the data obtained are then stored.
Furthermore, the beginning of each
minute is detected and the processor
'bears in mind' how many pulses are to
be received in one minute's time. The
data is only used when the correct
number of pulses has been received and
the parity check of the input data is
correct. If an error occurs, the processor
will automatically start counting and
decoding afresh. The data that arrives
during the minute will be compared to

that of the previous minute. The pro-
cessor decides that everything is all
right when the two lots of data differ
by exactly one minute. The data re-
ceived last will then be indicated on
the displays. The display multiplexing
is controlled by the program as is the
keyboard scanning. All switching times
are held in memory and continuously
checked against the displayed infor-
mation. When the two coincide, the
corresponding output (TO . . . T3) is
switched high.

The complete hex dump for the
EPROM is given in table 1.

The ASCI I data outputs

The PBO . . . PB6 output of the 6502
housekeeper will now provide the MSF

three phase

three

phase

tester

When connecting three phase motors to the power
source, confusion can arise if the cable markings are
incorrect, illegible or non-ex istant. How do you deal
with this problem? The simple answer lies in the circuit
described here, where an optical indication of power on each
of the three phases is provided together with an indication of
the direction of motor rotation. The completed circuit is self-
powered and compact and can be fitted as a permanent indicator if
desired.

Those of our readers who are familiar
with three phase mains power supplies
will no douot be fullly aware of the
confusion that can arise if, for one
reason or another, the supply cables
lose their identification. The problem
is increased by the fact that either the
motor being connected will not run at
all ( in which case the fault will in-
variably oe blamed on the motor), or
the motor will run the reverse direction.
The latter case brings with it the atten-
dant possibility of damage to equipment
being driven. This is as good a case as
any for building a phase detector.

The circuit derives its power from the
three phase supply under test and
therefore requires no batteries or
power supply. Three LEDs will indicate
that power actually exists on the
cables of the three phase supply, when
connected. The output of the circuit
consists of three further LEDs arranged
in the form of a triangle. These are
a 'running light' display and show the
direction of rotation of the motor when
it is connected to the three phase
supply in a similar manner.

The circuit

As has already been mentioned, the
circuit derives its power from the three
phase supply. This results in a very
unfamiliar power supply in the circuit
diagram of figure 1 and readers will
be forgiven for not having spotted it
immediately. It basically consists of
capacitors Cl . . . C3, diodes D1, D4
and D7 and the reservoir capacitor,
C7. The resistors R1 ... R3 are in-
cluded to limit the initial capacitor
charging current. The diodes form a

electronic rotating field
direction indicator

three phase half-wave rectifier which
provides a d.c. voltage across C7. This
voltage level is then stabilised to 10 V
by the zener diode DIO.

The LEDs D3, D6 and D9 are the
indicators that check the three phase
connections. If one phase is connected
incorrectly or power does not exist on

that phase then the corresponding LED
will not light. Furthermore, if a connec-
tion is made to the neutral line instead
of a phase, the LED will light at about
half its normal brightness. These checks
take care of the incoming supply.
The remainder of the circuit provides
the rotation indicator for a three phase
motor. The phases of a three phase
supply (normally indicated by the
colours red, yellow and green) provide
sine wave voltages with a frequency of
50 Hz that are phase shifted apart by
120°. The voltage is 220 V with respect
to the neutral line.

Phase detection is essential for the
circuit to be able to display the direc-
tion of rotation. In the case of the red
phase (the top one in the circuit dia-
gram) this is carried out by C4, Dll,
D12 and R7. These components to-
gether provide a pulse output at point 1
in the circuit that corresponds to a
particular point in the sine wave of
that phase input. It follows then that
points 1, 2 and 3 in the circuit are
relative to the phase angles of the
three phases.

One other parameter essential to the
circuit is of course the pulse sequence
from the three phases. This is 'decoded'
by means of the flip-flops FF1 and FF2
and the two gates N3 and N4. The R
phase (red LED) is used as a reference.
Rotation of the field will be clockwise

Direction detection

How the logic circuit for direction
detection makes sense of the pulse
sequence of the three phases is quite
straightforward, if we start with an
input from the red phase (input R). A
pulse at point 1 will set flip-flop FF1.

three pha

elektor September 1982 - 9-61

Figure 1. The circuit diagram of the three phase tester. Three of the LEDs provide an indication that power exists on the three phases. The
remaining three LEDs form a running light display to indicate the direction of rotation of the motor.

Its Q output will now be logic 1 (high)
which enables gate N3. With clockwise
rotation a pulse from point 2 will now
follow and this will be passed by N3 to
the clock input of FF2 via N4. The
Q output of this flip-flop will now also
be at logic 1. The final pulse to arrive
in this chain of events will be that of
phase T (green) at point 3 and, when it
occurs, this will reset both flip-flops.
Since this all happens at 50 times a
second it is not a lot of good to us as it
stands. The simple answer would be to
have a LED that lights when rotation is
clockwise and this is exactly what we
can have at point B in the circuit. The
train of logic 1 pulses from the Q out-
put of FF2 is 'stored' by capacitor C9
to provide a constant 'high' at the out-
put of the inverter N10. A LED at this
point would light to indicate that
direction of rotation is clockwise.
That takes care of the clockwise indi-
cator but what about anti-clockwise?
Figure 2 tells us that, in this case, phase
is phase R is followed by phase S and
then T. It will be anti-clockwise if
phase R is followed by T and then S.
This is clearly illustrated in figure 2.

Running lights

We were tempted to call the rotation
indicator display a Progression of
R must be followed by T and S in that
order. This means that the pulse at
point 3 will be continually resetting
FF1 before the S phase pulse at point 2
can reach the input of gate N3, with the
result that FF2 will never be set. Its Q
output will therefore remain low. A
LED at point A will now be lit to advise
the world that the direction of rotation

o

<3> TO-^~j-

is anti-clockwise! This is all very fine for
a simple yes/no indication, but the
silicon chip must be capable of better
things!

Horizontal Illuminated Lights In Se-
quence (PHILIS?!) but the LEDs are
formed in a triangle. This led us to
Progressive Illuminated Lights Equally
Sequenced which is something else to
sit down and think about...! So running
lights it is, with one LED at each corner
of a triangle to give a very clear indi-

o

cation of motor rotation. The basis of
this section of the circuit is the CMOS
decade decoder IC1, together with the
clock oscillator formed by the gates N1
and N2. Two outputs of the decoder are
fed to the N5 . . . N8, which are in turn
controlled by the yes/no signals at point
A and B. If A is high the lights run in an
anticlockwise direction round the tri-
angle and if B is high in a clockwise di-
rection. It will be obvious that since N9
and N10 are inverters, A and B can

Figure 2. This illustrates the six possible combinations of phase sequence with respect to points
1,2 and 3 in the circuit diagram.

« l “ H > l»— h|£ H .

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i n/630

never be at the same logic level.. .a fairly
convenient state of affairs! LED D23 is
controlled directly by the Q1 output of
IC1 without interference from either of
the A and B signals. Being the centre
LED at all times, it does not need to
concern itself about direction!

Construction

A point to note regarding the three
LEDs D22 . . . D24. Care must be taken
when wiring these as one placed in the

wrong position will play havoc and
provide a 100% incorrect display!
While not wishing to go on at great
length about the safety aspect of a
project of this nature, it must be borne
in mind that any three phase supply
carries a totally unhealthy respect for
human feelings and may be backed up
by far larger fuses than are normally
found in the domestic tranquility of
the home! Be reminded, a 150 amp fuse
will make a lot of pretty colours with

your screwdriver before you can say
999! The completed circuit must be
housed in a good quality plastic or resin
based case with three, preferably
locking type, sockets for connecting the
inputs from the phases. The LEDs can
be positioned anywhere on the case that
is convenient provided that they can be
easily read. It is strongly advised that all
testing of the circuit should be carried
out with the printed circuit board
mounted in the case. K

prr.-rr

Illll&lllllfiO

Video colour pattern generator

The RGB-1 1 is aimed at the market for both
commercial and hobby VDU's including video

held pattern generator and delivers red, green
and blue TTL or lower level signals compat-

This DMM weighs only 330 grams including
battery and is carefully designed to give long
and reliable performance for all applications.
Priced at £46.00 complete with free battery.

adaptor. This unique hand held unit House of Instrumen
; complete with rechargeable battery, Clifton Chambers,
cting cable, adaptor/rechargeable and 62 High Street,
ng case. It is fully guaranteed for 12 Saffron Walden,

>2 High Street,

Saffron Walden,

F ssex CBIO 1EE.
Telephone: 0799.24922

Audible digital multimeter

This hand held instrument, the MIC-6000Z %

AUTO M P ROGR
TIME^H^TEST

The FD-55E and FD-55F are half height
drives which give 0.5 Megabyte and 1 Mega-
byte storage capacity respectively. The two
devices satisfy single sided and double sided
requirements and the track to track access
time is reduced to less than 3 ms. Recording
density is 96 TPI and FM or MFM recording
methods can be employed. 2 Mbytes of
information can be stored in the space pre-
viously needed to store 1 Mbyte. Side by side,
two drives can be neatly packaged beneath a
9 inch monitor or above a standard QWERTY
keyboard, taking up only 41.3 mm in height.
The compact drives are ideally suited for
use in intelligent terminals or as back-up for
5 Vt" Winchester drives.

at 5.08 mm, the panel is easily compatible
with printed circuit boards. Overall dimensions
are as follows - package size 22.9 x 53.1 mm,
lit size 193 x47 mm, length 30.2 mm and lit
area 907.1 mm sq. The panel costs £6 (plus
P&P at £0.45 & VAT).

Electronic Hobbies Ltd..

17 Roxwell Road.

Chelmsford,

Essex CM 1 2LY.

Telephone: 0245.62149

(2449 Ml

Single trace oscilloscope with
built-in component tester

measuring instrument for the hobbyist. P
at £ 1 45 (plus P&P at £ 1 2.00 and VAT) .
Electronic Hobbies L td..

17 Roxwell Road.

Chelmsford,

Essex CM 12LY,

Telephone: 0245-62149

50 series solar cells from Solarti
ranged in series or parallel and h,
s, including charging NiCad batter

high-speed data transfer
and the junior-computer
junior computer book 3

Book 3 describes a number of steps that need to be
taken to transform the single-board, basic Junior
Computer into a complete personal computer
system. This involves adding an interface board
to allow the machine to communicate with the
outside world (its operator) in 'adult'
manner. The interface board provides
additional I/O, a cassette interface, an
RS 232 interface and an internal connec-
1^. tion with the buffered bus board.

The hardware extensions complete
the physical development of the
computer and the sophisticated
software in Book 4 enables the
| S machine to fully utilise its additional

memory capacity by extending its com-
munication skills. As a result, a number of periph-
eral devices, such as a printer or a video terminal can be
'hooked up' to the computer.

£4.75

ISBN 0905705 09 2

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Broadcasting Authority).

We’d like you to help us keep advertising
up to standard. /

The Advertising

Standards Authority.*

If an advertisements wrong,

we’re here to put it right.

ASA Ltd., Brook House, Torrington Place,
London WC1E7HN.

Please mention

elektor

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elektor

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copy servic<

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THE ELEKTOR
CASSETTE BINDER

This cassette style binder will help tc
your copies of Elektor clean and i
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le and time again. The cham-
fered corner of the cassette allows
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PRICE £2.60
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available from —
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practical proposals for input ;

JUNIOR COMPUTER BOOK 3

single-board Junior Computer into

300 CIRCUITS

DIGIBOOK - provi,
theory and applicatior
of the fundamentals of <

ides a simple step-by-step introduction

sc/mpuler

formant

popular

Wjs'ic synfepeisf:

published in Elektc
Price - UK

TV GAMES COMPUTER

sc/mputer w

ikki

JUNIOR COMPUTER BOOK 1 - for anyon
with (micro)computers, this book gives the oppoi
a personal computer at a very reasonable cost.
Price -UK £4.50 Oversea

JUNIOR COMPUTER BOOK 2 - follows ii
Book 1. and contains a detailed appraisal of the

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