INDIA

December

1984

universal
NiCad charger

10W valve
amplifier

TV receiver
as monitor

mini printer

Volume 2 - Number 12

EDITOR: SURENDRA IYER
PUBLISHER: C R. CHANOARANA
PRODUCTION: C. N MITHAGARI

ADVERTISING & SUBSCRIPTIONS
ElEkTOR ElECTRONiCS pVT lid.
Chotani Building

52 C. Proctor Road Grant Road (E)
Bombay- 400 007.

DISTRIBUTORS: INDIA BOOK HOUSE

radership survey results 12-14

news, views, people 12-16

selektor 12-18

New materials for optical memories.

the story of valves 12-20

sential for some applications. Here we review what used to be common knowl-
edge among electronics hobbyists and see what valves are, where they are used,
and*how to look for a fault if they do not work.

circuit boards and soldering 12-24

How to make printed circuit boards at home and techniques (or good soldering

PRINTED AT:

TRUPTI OFFSET

103. Vasan Udyog Bhavan.

Off Tulsi Pipe Road,
j Lower Parel, BOMBAY- 400 013.

Elektor India is published monthly under
agreement with Elektuur B.V. Holland.
August/ September is a double issue.

SUBSCRIPTION

INLAND

mini printer 12-28

A full Centronics interface connects this thermal printer to almost any computer.

It prints 40 characters per line at a speed of 80 c.p.s. Vesatility combined with low
cost make this printer a must for any personal computer user.

burglar deterrent 12-36

A LED to alarm would-be housebreakers. It protects your valuables without
waking half the neighbourhood and without annyoing the Police by falsely crying
'wolf.

RS232/V24: the signals 12-39

The signals recommended by this standard are dealt with in this article. Particular
modems.

1 Yr Rs. 75/- 2YrsRs. 140/- 3YrsRs.200/-

FOREIGN
One year Only

Surface mail Rs. 125/- Air mail Rs. 210/-

use your TV receiver as a monitor

The special video amplifier featured here will enable most TV sets to be converted

12-42

INTERNATIONAL EDITIONS EDITOR: P HOLMES

COPYRIGHT i ELEKTUUR B.V. -
•HE NETHERLANDS 1984

switchboard 12-63

missing link 12-70

index of advertisers 12-70

1984 12.03

National

semiconductor

manufacturers

For an ever expanding spectrum of
microelectronic applications —

We can offer co-fired substrates and interconnects
with precise tolerances for high-lead-count
packages, complex motherboards and a wide
variety of intricate modules.

For technical and commercial details, write to:

SUDHIR SURT!

2B, Parekh Mahal. Vir Nariman Road,
Churchgate, Bombay 400 020.

Phone: 297895, 250352

COLOUR TV
MRNUFflCTURCRS

FOR YOUR REQUIREMENT OF

We are manufacturing electronic
components and are interested in
diversifying our activities.

Are you interested in developing or
suggesting microprocessor based
commercially viable projects?

Suitable ideas will be taken
mutual benefit.

CONTACT:

DANNIES ELECTRONICS ENTERPRISE

I m porters and Fxporters
77, High Street # 10-12, High Street Plaza.
Singapore 061 7.

Phone: 3372297, 3397696 Telex: RS 28612 ESQIPE

Please rush with your ideas to

M/s R.P. Associates,

3/35 Lokmanya Nagar, (June - 411

12.04

ALL YOUR
COMPONENT
REQUIREMENTS

CAN BE MET
FROM A SINGLE SOURCE

IN SINGAPORE

© danha Elaalicaias Fla. lid. I

we stock:

eletet©? I ^ izumiyaicinc T teledyne

Magazines P.C.B. Drafting Aids relays

I1CS1

KOIMTAKT

(SHERMS

^Raytheon]

Semiconductors

Cleaners

Semiconductors

F=/=\l RCM 1 L_CD

Semiconductors

Texas Instruments

Semiconductors

vun Trimmin g

Potentiometers

Bread Boards

as

Semiconductors

Transistors

Metal Film Resistors

Zener Diodes

Bulk importers/end users in India may
kindly forward their specific enquiries

TO

Device Electronics Pte. Ltd.

101 Kitchner Road # 02-04, Singapore Electrical,
Electronics and Hardware Centre, Singapore 0820.

TELEX: DEVICE RS 33250
PHONE: 2986455

We also invite enquiries for items not mentioned above.

12.05

394, Lamington Road,
BOMBAY-400 004.

FOR EVERY APPLICATION

Manufactured and Marketed by:

PADMA ELECTRONICS (P) LTD

PRN Building, 24, Theradikadai Street,
Tiruchirapalli— 620 002, Tamil Nadu.

DOT MATRIX

I PROGRAMMABLE GRAPHIC PRINTERS
WITH SERIAL & PARALLEL INTERFACES.

FLOPPY DRIVE

SLIM TYPE 5V." FOR APPLE II

MONITORS

CRT 12" — Green — P31 phosphor.

Small Quantity available, Exstock

tML&IIDQte)©

15 New Queen's Road. Opera House. Bombay-400 004
Phones 350747 382968 CABLE DOMIRA

Aplab 3030 Aplab 3131

15 MHz compact single trace oscilloscope * ' 15 MHz dual trace triggered oscilloscope

DC-15 MHz bandwidth ‘ ' DC-15 MHz bandwidth

5mV/div. sensitivity * * 5mV/div. sensitivity on both channels

Built-in component tester ' * Built-in component tester

One x 10 probe free * * Two x 10 probes free

Other models available at short deliveries

3033-15MHz, Battery operated • 3034-15MHz, Dual, Battery operated • 3035-10MHz, Wide screen
» 3337-30MHz. Dual • 3339-30MHz. Dual, VDU scope • 3536-50MHz, Delayed sweep, Dual • 3537-50MHz,
Dual • 3538-50MHz. Dual Storage scope

Applied Electronics Limited

Aplab House, A-5 Wagle Industrial Estate, Thane 400 604. Phone: 591861 (3 lines) Telex: 011-71979 APEL IN.

8/A Candhi Nagar, Secunderabad 500 003. Phone: 73351.

22C. Manohar Pukur Road. Calcutta 700 029.

Nos. 44 & 45 Residency Road. Bangalore 560 025. Phone: 578977 Telex: 0845-8125 APLB IN.

MF-3 Stutee Building. Bank Street. Karol Bagh. New Delhi 110 005. Phone: 578842 Telex: 031-5133 APLB IN

dplab — Leadership through technology

12.13

New materials for optical

The storage of information by optica!

| methods has many advantages over
I the conventional method of magnetic
recording. Philips research
laboratories are currently studying
tellurium-selenium alloys, organic
compounds, and magneto-optical
| materials that can function as optical
memories. Depending on the
j material used, digital data
(alphanumeric data and digital audio)
and video information can be stored.

| The advantages are rapid access to
the information and a very large
' storage capacity. It is becoming
apparent that the scope for the
I application of optical recording is
j very varied. It will be possible to
meet the specific requirements of
new categories of users.

Magnetic materials for use as a
memory for storing information have
been studied for many years at
| Philips research laboratories. One
I result of the fundamental studies of
| iron oxides is magnetic tape for
many applications, including storage
of large quantities of alphanumeric
data and audio and video recording.
As the use of magnetic tape
increases and user requirements
become more specific, various fail-
I ings of this medium become
j apparent. The storage capacity is
limited and the information is only
reliable for a certain time owing to
| demagnetization. But sometimes the
law requires that information should
be stored for a long time. It then
becomes necessary to copy the
information every few years to
| guarantee its reliability. A further
disadvantage of magnetic tape is
that it may take a long time to locate
| a particular item.

Philips research have long been seek-
ing new methods of storage. The
electro-optical techniques originally
developed for LaserVision and the
| Compact Disc have provided a good
| starting point, since they are used
for the storage of images and sound
and are centrally produced. However,
it is also possible for the user himself
to store and retrieve information. In
some cases, this information stored
locally can be erased and replaced by
new information. The major advan-
tages of the new optical techniques
are the larger storage capacity and
more rapid access to the information.
In brief, an electro-optical recording
system consists of a disc the size of
an LP covered with a sensitive layer

in which a laser makes microscopi-
cally small pits. Depending on the
basic material, a particular physical
effect occurs during read-out by the
laser so that the information
becomes available in coded form.

The nature of the material deter-
mines whether digital data (alpha-
numeric information and digital
audio) or video information can be
stored. This depends on the required
signal-to-noise ratio. The require-
ments for video in this respect are
more difficult because of the large
number of grey levels. For digital
data (only two levels) things are
much easier. The material also deter-
mines whether the information can
be erased.

As optical recording obviously had
much to offer, an intensive search
began for materials on which infor-
mation could be stored with the aid
of a laser. Philips research labora-
tories are currently studying three
classes of material that seem suitable
for the optical recording of informa-
tion: tellurium-selenium alloys,
organic compounds, and magneto-
optical materials. The last two
groups are still almost completely at
the research stage. Much more is
known about tellurium alloys and,
indeed, these have already been
used in, for instance, the data disc
for the digital optical recorder used
in the Megadoc system that Philips
introduced earlier this year.

Despite great differences between
the new media, there are a number

of characteristic similarities in the
recording and reproduction systems.
Whichever disc is used, the system
works best with a diode laser that
operates in the infrared (about
800 nmlregion. This laser creates a
physical change in the storage
material (hole formation or a phase
change in a tellurium-selenium alloy,
pit formation in an organic com-
pound. and magnetization direction
in a magneto-optical material I . All
such areas have a cross-section of
about 1 micron, as the photographs
show. The power of the laser for
writing in information is about
10 mW at a pulse length of 50 ns.

The read-out power is about 0.5 mW j
for all materials.

One of the new materials for the
storage of information is a
polycrystalline tellurium-selenium
alloy to which small quantities of
other elements have been added,
e.g. arsenic to give better control of
the melting point and the stability of
the material. A thin layer of the alloy
is applied to a substrate. A narrow
laser beam is used to melt this
material locally so that holes are
created with the same depth as the
layer. During the read-out process,
with a less intense laser beam, the
presence or absence of holes pro-
duces differences in the reflection of
the laser light. These differences in
reflection represent the information in
coded form.

12.18 elekla

Research has been concentrated on
determining the composition of the
alloy and on finding an efficient
technique for applying a very thin
layer of the alloy to a disc. The 'shelf
life' of the discs is extremely good.
Life tests have shown that the stored
information can be guaranteed for at
least ten years without any need for
special environmental conditions.
Shelf life will be greatly increased in
a controlled environment.

The signal-to-noise ratio that can be
achieved is so high that the disc
with a tellurium-selenium alloy is
ideally suitable for use as a storage
medium for both digital data
(alphanumeric information or digital
audio) and video recording. The data
disc for the digital optical recorder
uses this technology. A compatible
player is made by Van der Heem
Electronics to a design by Philips
Research Laboratories in Eindhoven
(the Netherlands). Disc and player
form one section of the Megadoc
electronic data-storage system made
by Philips Data Systems. A second
type of player is currently being
developed by Optical Peripherals
Laboratory (U.S.A.), a joint venture
of Control Data Corporation and
Philips.

The use of tellurium alloys also
makes it possible to record infor-
mation on a disc, erase it, and then
use the disc again to record new
information. By choosing the energy
output of the laser appropriately
(compared with the level necessary
for the 'hole' disc) the polycrystalline
material is melted locally, but no
holes are formed. After the laser
pulse the molten areas cool down so
quickly that they solidify in a
metastable amorphous phase. These
amorphous domains reflect differ-
ently from the crystalline surround-
ings on read-out. Erasure takes place
when a laser with a sufficiently high
energy level transforms the amorph
ous domains into the crystalline

In most applications the disc can be
used and erased many times. In prin-
ciple, storage of both digital data
and video recording is possible
because of the high signal-to-noise
ratio. These materials for erasable
storage are now at the transition
stage between research and
development.

Organic compounds
Organic dyes exist that absorb a
great deal of light and have a high
reflectance even when applied in
very thin layers. These thin layers of
organic compounds seem to be a
promising alternative to tellurium-

selenium alloys. The memory effect
is again obtained by melting the
material locally with a laser to create
small pits. The difference from the
tellurium-selenium alloy is that these
pits do not normally penetrate
through to the substrate. The reflec-
tance varies with the depth of the
pit. The difference in reflection
created by the pattern of pits is used
when the information is being read.
This melting process is irreversible,
so the disc can only be written once.
The shelf life is good: it has been
found that these organic compounds
retain the information just as well as
the 'hole' discs with tellurium-
selenium alloys. A great deal of
research has been done on the 'light-
fastness' of the material, which
ensures that its characteristic proper-
ties remain unchanged. These com-
pounds have also been found to be
very resistant to heat and moisture.
One attractive feature is the simple
spin-coating process for applying the
organic compound to the disc.

This type of disc has many appli-
cations. The signal-to-noise ratio
obtained experimentally is high
enough for the storage of both
digital and video information.
Magneto-optical materials
Amorphous magnetic gadolinium-
iron-cobalt compounds have been
known for a long time. A laser can
be used to heat the material locally,
reverse the polarity of small areas
and freeze it in this state. This
technique makes it possible to 'write'
on a magnetised layer in a pattern of

areas of opposite magnetization
directions. This type of pattern can
then be read out with polarized laser
light. The direction of polarization of
the reflected light is rotated slightly
with respect to the polarization of
the original laser beam as a result of
the Kerr effect. The 'written' areas
on the disc can therefore be
distinguished from the unwritten
ones, and information can be read |
out. The information can be erased
just as easily as it is written. The
areas to be erased are heated by the
laser, while an external magnetic
field is applied with the same direc- j
tion as the original magnetization of
the layer; the magnetization of the |
heated area reverts to its original
direction after cooling. The infor-
mation can be written, erased, and
rewritten as often as required.

The present research is much con-
cerned with the operational life of
the stored information. The stability |
of the material is very important
here.

At present the signal-to-noise ratio is
only moderate, so this storage
method is suitable for digital data
only (alphanumeric information and
digital audio signals). It could very
well be possible to improve the
signal-to-noise ratio sufficiently for
the recording of video signals.

(944 S)

A Philips Research press report

1984 12.19

Once upon a
time...

the story of valves

Before the arrival of the transistor all amplifiers, transmitters,
receivers, etc., were made with valves. In the eyes of many modern'
people valves were (and are) fragile and unreliable and had a short
lifespan. Not all that long ago, however, there was no alternative.
Before the valve there were simply no amplifiers, and the transistor
was only invented in 1948. But think about this: without FM
transmitters (which contain valves) would there be any point in
having a compact transistorised FM receiver?

[ What exactly is a valve? Many of our more
] mature readers know, of course, but some
I of the youngsters may think something
along the lines of: "Oh yes, one of those
old-fashioned fragile glass things with all
sorts of complicated-looking bits and
i pieces inside". This definition is not
| strictly wrong but it leaves out rather -a lot.
! True, a valve is made of glass but, in spite
[ of its appearance, it is not all that fragile,

[ nor is it necessarily 'old-fashioned'. Valves
are actually indispensable for certain ap-
plications (even today) and in others —
such as hi-fi for example — they are on
I the way back ‘in’. (To see this you need
look no further than the valve amplifier
I elsewhere in this issue).

What, then, is a better definition for a
valve? The transistor's predecessor is seen
as a device in which electrons are fed
into one side and come out on the other
side. Between the two electrodes is a con-
trol electrode that can pass or inhibit the
flow of electrons as desired. A major dif-
ference between this and the transistor is
that no current flows through the control
electrode. In this respect valves are more
similar to (MOS)FETs than to bipolar tran-
sistors.

Are there other notable differences be-
tween valves and transistors? Plenty! It is
quite normal for a valve to become warm
even in its quiescent state: its innards must
glow, actually, in order to generate the

cloud of electrons needed. Although
mechanically it is vulnerable, the valve is
very robust in an electrical sense: it is
almost indestructible! If something does
go wrong then the impending failure can
almost always be predicted beforehand
simply by looking carefully at the tube. It
does not just suddenly kick the bucket
like a transistor!

That, basically, is a sum-up of the most
important points about valves. Up to now
Elektor has had very little to do with
valves but none the less some of our old
hands are very knowledgeable on the
subject. When we picked their brains this
is the story that came to light.

Under the magnifying glass

An essential part in the operation of any
valve is the movement of charge carriers
(electrons) in a virtual vacuum. A valve
consists of a glass tube containing a
simple or complex electrode system. The
electrodes must include at least a cathode
and an anode.

The cathode often has the shape of a
nickel tube covered with a layer of barium
strontium oxide. It is warmed to a
temperature of about 700. . .800°C by a
filament in the tube. The surface then
attains a dark red colour. The filament is
electrically isolated from the cathode by
means of a layer of aluminium oxide but
the heat conduction is very good.

The heat increases the motion of the elec-
trons in the cathode. As a result of this
some of the electrons will reach a speed
greater than the so-called 'emission vel-
ocity’ and will leave the surface (this is
thermal emission, also known as the
Edison effect). An electron cloud (known
as the space charge) then forms around

the cathode. This cloud has a negative
charge so the cathode is positively
charged. A balance between cathode and
electron cloud is reached, depending on
the cathode temperature and material.

If a metal plate which has a positive
potential with respect to the cathode (an
anode in other words) is now placed at a
certain distance from the cathode it
attracts some of the electrons. The
cathode then redresses the balance by
releasing more electrons into the space
charge. (From now on we will forget the
interaction between cathode and electron
cloud and simply refer to ‘the cathode').
From the previous paragraph we see that
electrons flow from cathode to anode (this
is the anode current). Even if the anode is
not positive with respect to the cathode a
(small) current will still flow because the
electron cloud is negative with respect to
the anode. This valve, called a diode, has
no threshold voltage. As the anode is not
heated no current will flow in the vacuum
if the anode is negative with respect to
the cathode. Current flows in one direc-
tion only so this diode can act as a rec-
tifier.

Triode, pentode, and other valves

A three-electrode valve (triode) is made
by placing a third electrode at a certain
position between cathode and anode. This
third electrode is normally in the form of a
spiral with a fairly large pitch and is
called the grid or control grid. If the
voltage presented to this control grid is
negative with respect to the cathode then
the electric field between cathode and
control grid will oppose and possibly
even completely suppress the field
between cathode and anode. The voltage

* Ug2 = 140V U B 3-0V

U 8 1 = 0V

u 9l * V
*7 'V - -2 V

r ... . U9l ’~ 3 V

anode voltage must be
slightly negative in order
to suppress the anode
current completely. At
zero volts there is already

i grid voltage
a much larger
tage change

node voltage is lower

Hilarity to a transis

12.21

2

on the control grid thus affects the anode
current. If the magnitude of the negative
control grid voltage is increased it can
shut off the valve completely. By applying
an alternating voltage to the control grid
the anode current is made to vary in time
with the alternating signal.

The grid is much closer to the cathode
than the anode so the anode voltage
(which gives an attractive force) must
change more than the grid voltage
(repulsive force) in order to compensate
for any fluctuations in the grid voltage and
thus keep the anode current, la, constant.
The ratio between these two changes is
called the amplification factor and is given
by the letters ji or g. The ratio between a
small change in grid voltage and the
resultant change in anode current (if the
anode voltage remains constant) is called
the mutual conductance or slope (S) of the
valve. The valve can be used as an ampli-
fier if a d.c. or a.c. resistance is connected
in series with the anode line.

The anode and control grid of a triode
form a capacitor. The anode circuit and
control grid circuit are therefore
capacitively coupled. The capacitive reac-
tance decreases as the frequency
increases. At high frequencies this can
result in transfer from the anode circuit to
the grid circuit so the whole circuit may
oscillate. An extra grid, whose voltage
remains constant with respect to the
cathode, can be included between the
anode and the control grid. This fourth
electrode, called the screen grid, reduces
the transfer characteristic drastically. The
screen grid must not inhibit the anode

current so it is fed a suitably high positive
voltage.

Electrons that manage to pass the screen
grid are speeded up by the attraction to
the anode. In some cases the speed may
become so great that the impact energy is
too much for the anode. The impact of a
single electron can then cause the anode
to release a number of electrons. The
electrons thus released (secondary elec-
trons) can either return to the anode or go
to the screen grid. In the latter case the
anode current characteristic displays a
marked ‘dip’ at which point the circuit
displays negative resistance properties
and a tendency to oscillate.

A further electrode may be introduced
between anode and screen grid to
oppose the flow of electrons from the
former to the latter. This so-called sup-
pressor grid is generally connected to the
cathode. Its purpose is to reduce the
speed of secondary electrons so that they
reverse direction and return to the anode.
This sort of valve, with five electrodes, is
called a pentode.

Other types of valves were also common,
such as: the hexode (6 electrodes), the
heptode (7 electrodes) and the octode (8
electrodes, six of which were grids).
Numerous combinations were also made,
producing the duodiode pentode, triode
hexode, triode heptode, and so on.

Pros and cons

Normal radio valves were, of course,
inferior to transistors in some respects.
Transistors, for example, require no power

to drive a filament but valves can handle
much higher voltages and temperatures.
Breakability of valves was not really such a
problem as transistors cannot survive too
much rough handling either. Just as with
filament lamps, the anticipated lifespan
was a compromise. Where long life was
necessary (and more important than low
cost) special types of valves could be
specified, such as SO (Special Quality),

LL (Long Life), and telephony valves, all of
which had an expected lifespan of at least
10,000 hours. Apart from the ability to
handle more power, the most important
difference between transistors and valves
is size. Valves are much larger so the case
housing a valve amplifier, for instance,
must be larger than its transistorised
counterpart and it must have plenty of
holes or slits to allow cooling air to enter.
For the most part valves have been re-
placed by transistors. They are generally
only used now in high-power transmitters
and for high frequency heating in industry.
Valves still appear in other forms as
magnetrons in radar transmitters and
microwave ovens, as klystrons in TV
transmitters and, of course, as cathode ray
tubes in TV receivers.

Practical tips

Compared to transistors, some problems
in equipment containing valves are fairly

4

dissipation is much too great. There are
many possible reasons for this, such as:

■ the design is bad so the valve is
overloaded;

■ incorrect circuitry at the anode side
with not enough energy dissipation;

■ the valve does not have enough nega-
tive grid voltage, with the result that the

anode current is too large (caused by, for
example, a short circuit in the cathode
decoupling capacitor, far too much
resistance in the grid line, an internal
short between cathode and control grid,
and so on).

A violet glow within the anode shows that
the valve is 'soft', which means that there
is some gas inside the tube, its vacuum is
not correct so the valve is approaching
the end of its life. In some valves,
however, this glow is normal, especially at
high voltages. A violet glow may also be
noted outside the anode, particularly
along the length of the electrode system.
This phenomenon is generally harmless,
lb finish, here are two practical obser-
vations. First of all, a valve should be
mounted in a good-quality socket. Do not
use a cheap one and do not solder it
directly to a printed circuit board. Sec-
ondly, valves must have enough venti-
lation. They can take a lot of punishment
but long-term overheating will kill even
the best of valves.

easy to track down.

After power is applied it is easy to look at
all valves and see if they are glowing. If
so this means that the filament is intact
and that it is being fed a voltage.

In a tetrode or pentode the screen grid
(the second out from the middle) must
never glow. A red glow from inside, which
may only be visible from some viewpoint
underneath, signifies an overload of the
screen grid. The power must be switched
off straight away. The likelihood in this
case is that there is no voltage at the
anode of the tube, probably due to a
break in the wire to this electrode.

If the anode also starts to glow the power
must be removed instantly because
something is really wrong. The anode

The set-up shown here (figure 4) was
sometimes used when studying the oper-
ation of a valve, ft consisted of a fairly taut
sheet of rubber, in which the changes of
potential at the different electrodes are
represented by rises and falls in the sur-
face. As the middle was higher than the
outside rim the effect of gravity helped
simulate the various forces on the
electrons.

Steel balls were released from the middle |
(the cathode) and rolled outwards. The
braking effect of a grid was mirrored
using metal rings to raise the rubber sur- I
face at places. A ball that had to roll up j
an incline was slowed down, only to
speed up again when rolling down the |
other side of this ‘grid'. M

Printed circuit boards

Whal exactly is a printed circuit board?
Well, basically it is an insulating sub-
stratcon which components are mounted,
and to which are bonded copper con-
ductors in the required circuit inter- I
connection pattern. A typical printed
circuit board starts life as a piece of
'copper laminate board'. This is a sheet
of synthetic-resin-bonded paper (SRBP)
or epoxy-bonded fibre glass, to which
a continuous coating of thin copper foil
is fixed by adhesive. Once the required
circuit connection pattern has been
designed it is transferred to the copper
surface in the form of an acid-resistant
ink. The board is then immersed in an
etchant solution that dissolves away the
areas of copper not protected by the
resist, leaving only the circuit track
pattern. The resist is then cleaned from
the board, holes are drilled to mount
the components, the component leads
are inserted through the holes and
soldered to the copper tracks.
Professionally produced printed circuit
boards can, of course, be considerably
more sophisticated. As an aid to inserting
components in the correct locations
a component layout is frequently prin-
ted on the top face (non-copper side)
of the board. The track pattern may
also be printed, in ink. on the top of the
board as an aid to circuit tracing. The
copper side of the board may be com-
pletely covered with a ‘solder mask',
exept for small areas around the holes
through which the component leads
protrude. This means that the copper
track can only be soldered in the area
of these 'pads', and the solder mask
prevents accidental’ solder splashes from
adhering to other areas of the board.

The pads themselves are frequently
covered with a thin plating of tin, which
aids soldering and prevents oxidation of
the copper if the board is stored for
some length of time before use. Alterna-
tively a thin coating of a special lacquer
may perform a similar function.

If a circuit is particularly complicated it
may be impossible to make all the
required interconnections on one side
of the board, in which case a ‘double-
sided’ board may be used, which has
copper tracks on both sides of the
board. To avoid the necessity of wire

In terms of ease of constructing
electronics projects, enthusiasts
have never had it so good as they
have it today. In the bad old days
of twenty or thirty years ago,
circuits were constructed on
laboriously-manufactured metal
chassis using valveholders,
tagstrips and wire. Nowadays the
functions of support for and
interconnection of components
are frequently f ullfilled at one
fell swoop by the indispensable
printed circuit board.

links to make connections between the
top and bottom of the board, ‘plated-
through holes’ are often employed. This
means that metal is electroplated
through a hole from a pad on one side
of the board to a pad on the other side.
An interesting possibility offered by
double-sided printed circuit boards is
that components can be mounted on
both sides of the board.

Boards available from the Elektor Printed
circuit board Service (EPS) are typical
examples of current p.c.b. practice (see
figure 1 ).

Home-made p.c.boards

Home production of all but the simplest
p.c.boards involves considerable outlay
and a fair amount of skill, which is why
Elektor offers ready-made boards for
many projects. However, it is apprecia-
ted that some readers will wish to 'have
a go’ themselves.

By far the most difficult aspect of
printed circuit board production is the
design, i.e. transforming a theoretical
circuit into a practical p.c.b. layout.
Unfortunately there are no hard and
fast rules for this, and skill only comes
with practice. The best plan is probably
to study professionally produced lay-
outs such as those in Elektor, and to
build up one’s skill gradually starting
with simple circuits.

If a p.c.b. layout is already given then
no design problem exists, and the design
can be transferred to the copper lamin-
ate board.

First of all the board must be cut to the
correct size. -Then the copper surface
must be scrupulously cleaned to ensure
even etching. This can be done using a
Brillo pad, wire wool and soap or an
abrasive cleaner such as Vim or Ajax.
After cleaning the board should be
washed thoroughly to remove any traces
of the cleaner and dried using a lint-free
cloth.

To make a ‘one-off board for a not-too-
complicated circuit the simplest method
is to draw the layout directly onto the
copper using an etch-resist pen or an
acrylic marker pen For complicated
shapes such as ICs.etch-resist transfers
are available. These are simply rubbed
off the backing sheet onto the

I

Figure 1. Boards from the Elektor Printed
circuit board Service are typical examples of
modern p.c.b. practice.

Etching

Once the layout is complete the board is
immersed in an etchant solution. Various
exotic chemicals are used in industry,
but for the home constructor ferric
chloride remains the standard etchant.
This is available in solution, either con-
centrated or ready for use, and the sup-
plier’s instructions should be followed.
Ferric chloride is also available in crys-
talline form, in which case a solution
must be made up. A suitable solution
for etching is 500g of ferric chloride
crystals to one litre of water. When
making up the solution the crystals
should always be added to the water,
never the other way round. One litre of
etchant is sufficent to etch 3000 to
4000 sq cm of board.

Ferric chloride is extremely corrosive
and it is advisable to wear protective
clothing such as rubber gloves and a
plastic apron when using it. If ferric
chloride comes into contact with the
skin it should be washed off immedi-
ately. If it contacts the eyes these
should be washed with copious amounts
of cold water and medical assistance
sought immediately.

All utensils used to contain ferric
chloride should be of glass or plastic,
never use a metal container. If it is to
be stored for any length of time, the
container must be air-tight. Ferric
chloride is hygroscopic, which means
that if given half a chance it will capture
moisture out of the air until it over-
flows a normal container!

Etching can be speeded up by warming
the solution. The easiest way to achieve
this is to place the dish containing the
etchant in a bowl of warm water. Whilst
the board is in the solution it should
frequently be agitated to bring fresh
solution into contact with the copper
and to dislodge the ‘sludge’ of iron that
is displaced from the solution as the
copper dissolves.

The board should be checked period-
ically to see how the etching is proceed-
ing. It should not be left in the solution
once etching is complete as the etchant
will begin to undercut the edges of the
copper track where the resist does not
protect it.

Once the board has been etched the
resist can be scrubbed off and holes for

the components can be drilled. The
components should be mounted and
soldered before the copper has time to
tarnish, and the copper should be pro-
tected by a coat of lacquer immediately
after the circuit has been tested. If the
board is to be stored for some time
before mounting components then it
should be given a coating of special
printed circuit lacquer, available from
Doram. This is somewhat more expens-
ive than ordinary decorative lacquers,
but the board can be soldered through
the lacquer, whereas ordinary lacquers
inhibit soldering.

Photographic methods

If several boards of the same design are
to be made, or a complicated layout
is to be copied from a magazine, then
it is worth considering photographic
methods. There are several ways of
transferring a layout onto copper
laminate board photographically.

The method for making a board from
one’s own layout design is to draft the
layout onto transparent or translucent
film (available from shops selling artists'
materials) using black, self-adhesive
draughting tapes and pads This is
known as a positive master

The cleaned copper laminate board is
then coated with a positive photo-resist
such as Fotolak, according to the manu-
facturer’s instructions. The master art-
work is placed in contact with the resist
and the resist is exposed to light (which
may be ultraviolet or visible light de-
pending on the type of resist) through
the master artwork.

The exposed board is then placed in a
"developer bath (or sprayed with devel-
oper depending on the type of resist)
when the exposed portions of the resist
(those not covered by the black track of
the artwork) are developed away.

The board is then washed and etched in
the normal way.

Negative photoresists are also available;
if these are used then the unexposed
portions of the resist are developed away.
Of course, a negative photoresist entails
the use of a negative master, i.e. a black
background with transparent areas for
the track pattern. This must be produced
by making a contact print of the positive I

master onto photographic film. Only
readers who do their own photographic
processing will have the necessary equip-
ment, and it is not intended to discuss
this method further.

Layouts printed in magazines may also
be photographed, and the photographic
negative can be enlarged to the correct
master. Here again, readers who carry
out photographic processing will know
how to do this. Alternatively, any local
photographer should be able to carry
out this work for a modest charge.
Soldering

Having purchased or made a printed
circuit board, there is then the problem
of making a reliable electrical (and
mechanical) connection between the
component leads and the copper tracks
on the board.

Soldering involves the use of a metal
that will melt at a relatively low temp-
erature (usually about 200°C), which
will form a molecular bond with the
component leads and the copper track.
The temperature must be fairly low
since components are susceptible to
damage by excessive heat, as is the
adhesive used to bond the copper to the
printed circuit board.

Electrical solder is an alloy of lead and
tin. Pure lead melts at 232°C and tin
melts at 327°C, but an alloy of the two
metals, paradoxically, melts at a lower
temperature than either of the consti-
tuents. The temperature at which the
alloy melts depends on the proportions
of the two constituents. The lowest
melring point for a tin/lead alloy is
1 83 C, and is obtained when the propor-
tions are 63% tin to. 37% lead. An alloy
with the lowest possible melting point is
known as a eutectic mixture (Greek:
eutektos - easily melted). A eutectic
alloy of tin and lead changes from a
solid to a liquid at exactly 183°C. If the
mixture is not eutectic then the alloy
will not melt at exactly this temperature
but will exhibit a range of temperatures
where it has a ‘plastic’ consistency. This
is shown in figure 2.

It is not a good idea to have solder with
too large a plastic range. If the soldered
joint is moved whilst it is cooling from
the liquid state, through the plastic state
to the solid state, this can result in the

alloy solidifying with an extremely
crystalline structure which has poor
mechanical strength and high electrical
resistance. The actual proportions of
electrical solder are normally 60% tin to
40% lead. Small quantities of other
metals are also added, such as antimony
to improve mechanical strength.

Even this is not the whole story of
solder, however. The component lead
and p.c.b. track are covered with a layer
of oxide that prevents the solder from
‘wetting’ the metal and forming a
molecular bond. Even scrupulous clean-
ing of the board and component leads
will not help, because an oxide layer
only a few molecules thick will form
instantaneously on a clean metal surface.
To enable soldering to be carried out,
flux is required. This consists of an
organic resin that improves the wetting
properties of the solder and an activator
that dissolves oxide. Electrical solder for
general use is produced in the form of a
wire of circular cross-section. The flux
is an integral part of this wire in the
form of three or more cylindrical cores
of flux running down the centre of the
solder, as shown in figure 3.

To make a soldered joint the components
to be joined (e.g. a component lead and
circuit board pad) are heated simul-
taneously with a soldering iron to a
temperature higher than the melting
point of the solder. The solder wire is
then fed into the joint, not to the
soldering iron as the excessive heat will
vapourise the flux too quickly and will
cause the solder to oxidise.

At about 160°C the flux becomes active
and cleans the surface of the com-
ponents. At around 200°C the molten
solder displaces the flux from the metal
surfaces and wets them, forming a
molecular bond. The soldering iron is
then removed and the joint is allowed
to cool without moving it.

A good soldered joint should have a
smooth, shiny appearance and a concave
surface, and the solder should flow
smoothly into the surface of the two
components. Excessive amounts of

12.26 eleklot india december 1984

solder and large blobs with convex
surfaces are signs of a poor joint. A
cross-section of a good soldered joint is
illustrated in figure 4.

When making electrical soldered joints,
no flux is required other than that
contained in the solder, and the use of.’
acid-based fluxes, such as those used in
plumbing and metalwork, should be
avoided since they are corrosive and'
electrically conductive.

Soldering irons

Soldering irons have come a long way
since the days when they had to be
heated up on a gas ring, and a large and
bewildering range of types is now avail-
able.

The cheapest type of soldering iron,
which will be perfectly adequate for the
home constructor’s purposes, is the
continuous heat type. This typically
consists of a thermally and electrically
insulated handle, from which protrudes a
stainless steel shaft containing a ceramic
encapsulated electrical heating element.
The business end of the iron - the ‘bit’ -
is a hollow copper cylinder that slides
over the shaft and is secured by a spring
clip. The tip of the bit may be a variety
of different shapes depending on the
intended application, and a selection of
different bits are shown in figure 5.
Large bits are obviously used for heavy-
duty work and small bits for fine work.
The element of a continuous heat iron is
connected permanently to the supply,
and there is no control over the bit
temperature. This means that the iron
will cool down whilst actually making
joints, since heat is drawn from it to
heat up the joint and melt the solder,
but it will become very hot when not
being used. This can mean that the first
joint made after the iron has been stand-
ing idle may be overheated. The problem
can be reduced by using a metal stand
for the soldering iron, which will act as
a heat sink and will ensure that the iron
does not become too hot whilst idle.
Continuous heat irons are available in
various wattage ratings, but for general

Figure 2. Illustrating the malting points of
different alloys of tin and lead and the plastic
region exhibited by non-eutectic mixtures.

Figure 3. Electrical solder has cores of flux
running down the centre of it. No additional
flux is required with this type of solder.

Figure 4. Illustrating the principal points of a
good soldered joint.

1. p.c.b. substrate

2. copper layer

3. alloy of solder and copper track (only a
few molecules thick)

5. alloy of solder and component lead

6. component lead

7. the maximum angle the solder makes with
the track should be less than 30°.

Figure 5. A selection of soldering iron bits.

Figure 6. When making a joint the component
and pad should be heated with the soldering
iron and the solder run into the joint, not

5 V

use a 20-25 W model, together with a
selection of different bits, should prove
adequate. If a great deal of fine work is
to be undertaken then it may also be
worth considering a 1 0-1 5 W model, and
if any metal work is to be undertaken
(e.g. screening enclosures for r.f. cir-
cuits) a 60 W iron will be useful.

The smaller wattage rating irons are
often available in different voltage
ratings. For general use a mains powered
iron is probably the best bet, but if the
enthusiast’s interest involves outdoor
work such as car electronics, mobile
radio or field servicing then a 12 V iron
may prove useful.

Temperature-controlled irons

The use of a soldering iron whose bit
temperature is controlled allows more
precise control over joint quality, and
helps prevent delicate components from
being damaged by overheating. There
are two principal types of temperature-
controlled irons. The first type uses a
thermistor to sense bit temperature and
an electronic control circuit to switch
the power on and off. The temperature
of this type of iron may be continu-
ously varied by a potentiometer that
alters the switching temperature of the
control circuit.

The second type of temperature-con-
trolled iron is made by the Weller
company and utilises an unusual
property of magnetic materials. Above a
certain temperature known as the Curie
point, ferromagnetic materials cease to
be magnetic. The bit of a Weller iron
contains a small slug of ferromagnetic
material. When the iron is cold this
attracts a magnet, which closes a switch
and applies power to the element. When
the Curie point is reached the slug
becomes non-magnetic and the magnet
s released, opening the switch.

To alter the bit temperature of a Weller
-on it is necessary to change the bit for
one which has a slug of material with
the required Curie point.

Soldering iron bits

Soldering iron bits almost invariably
used to be made of copper, since this is
a good conductor of heat. However,
each time a soldered joint is made a
little copper dissolves in the solder, and
eventually a copper bit becomes pitted
and has to be filed down. Modern bits
are generally made of copper, plated
with some harder metal such as iron or
nickel, which does not dissolve. These
bits should never be filed, but should
periodically be wiped on a damp sponge,
while hot, to remove excess solder and

Before using any bit for the first time, it
must be tinned - coated' with a fine
layer of solder - to prevent oxidation
and improve thermal contact with the
joint when in use. The iron should be
switched on and the solder held in
contact with it. As soon as the solder
melts it should be run over the entire tip
of the bit. Any excess solder may then
be wiped off.

Soldering techniques

Having chosen a suitable soldering iron
and the correct bit for the job, it is
important to use solder of the correct
diameter. If the solder is too thick it will
be difficult to control the feed rate into
the joint and the joint may become
flooded with solder. On the other hand,
if the solder is too thin then a much
greater length must be fed into the joint
and it will take longer to make each
joint. Fine solder is also more expensive
(per unit weight) than thick solder.

For general purpose use 1 8 SWG solder
should prove adequate, and for fine
work such as soldering ICs 22 SWG
solder should be used.

When soldering components into a
printed circuit board the following se-
quence should be adhered to:

1 . Any terminal pins should first be in-
serted into the board.

2. Small, horizontally mounted com-
ponents such as resistors and diodes
should then be inserted into the
board. During the soldering operation

the board can be laid, component
side down, on a piece of plastic
foam, which will hold the com-
ponents in place. Alternatively, the
leads can be bent outwards at an
angle of about 45° to hold the com-
ponents in place.

3. When the components have been in-
serted into the board the leads can be
cropped off fairly close to the board,
using wire cutters.

4. To solder components, apply the tip
of the iron to the component lead
and the pad simultaneously and run
solder onto both (see figure 6). When
sufficient solder has run onto the
joint remove the solder and the iron
and allow the joint to cool.

5. The procedure can then be repeated
for ICs or IC sockets, transistors and
large or vertically mounted com-
ponents.

6. To improve the appearance of the
board any excess flux can then be
removed with methylated spirit.

If components have to be removed from
the board for any reason, this should be
done with great care to avoid damaging
the copper track. Grip one lead of the
component to be removed with a pair of
pliers, reheat the joint until the solder
melts then pull the lead clear. Repeat
for the other lead(s). To remove ICs it is
best to use a ‘solder sucker’ to remove
solder from every pin of the IC, thus
leaving the IC free to be removed.

Before inserting a new component it is
essential that all the holes should be free
of solder. This can be ensured by using a
solder sucker, or by heating up the pad
and inserting a pencil point into the
hole. The board should be allowed to
cool completely before inserting the
new component, as otherwise there is a
danger of lifting away the copper track
from around the hole due to weakening
of the adhesive by heat.

If all the preceding recommendations
are followed there is no reason why the
constructor should not enjoy a high
success rate when using printed circuit
boards. M

1984 12.27

pnnter

p •=! r s t u u w
xy z i ! > " v

with Centronics
interface

J \

4 *1? X P 3. 3 *y

',Mbb09:

Many home computer
enthusiasts dream of the day
they will be able to get
themselves a printer.
Programming and editing on the
monitor can be very tiring — to
say the least. In many cases the
Centronics interface is already
available or, as in the
Commodore 64, programmable.
All that is needed, therefore, is a
printer.

As the article describes the
principle of a dot matrix printer
in some detail, it is also of
interest to those who have no
intention of building the printer.

u

1 2.28 elektor in

For most listings, the eighty or even 126
characters per line as provided on most
dot matrix printers are not really
necessary. For disassembler listings, forty
characters per line are ample. And
therefore, the only real limitation of our
Mini Printer compared with its bigger,
commercial brothers is that it prints only
forty characters per line. This is adequate
for most BASIC programs, too, but if you
want to print a BASIC program from a
commercial cassette or diskette that has
more than forty characters per line, simply
format it by adding line numbers. Table 1
gives an example of how this is done.

Price and specification

We obviously do not want to compare the
Mini Printer with an Epson or NEC printer
which may cost up to eight or ten times as
much. What is important is the perfor-
mance of the Mini Printer, and, as can be
seen from the technical data in table 2,
that stands up very well.

At this stage, some of you will ask: "That’s
all well and good, but what about the
mechanism and the processor? Are they
included in the price, or where do I get
them from?" Not to worry — both are
catered for. Actually, they caused this proj-
ect to take off, for some time ago we were
provided by Seiko with one of their basic,
low-cost mini printers. Once our designers
had assessed and modified it, and Seiko
had expressed their willingness to supply
the processor and hardware on a one-off
basis, we had the nucleus of the design
presented in this article.

Hardware and chemistry

As shown in the photograph in figure 2,
the mechanism of the printer is an
ingenious piece of precision engineering.
The motor drives not only the spiral guide
roller for the print head but also the paper
feed, which is made possible by the
special construction of the guide roller.
The motor speed is monitored constantly
by a tacho-generator built into the motor
housing.

The print head contains seven thermo
needles (miniature heating elements) one
above the other. During printing those
needles that are to place a dot onto the
paper are actuated simultaneously. The
thermo paper is continually pressed
against the print head by the paper guide.
A thermo-chemical reaction, which
discolours the paper, takes place at the
position of those needles that are heated.
As the paper is white with a dark
background, dark dots are caused on the
paper. Note that this thermo-active paper
may be obtained from stationers and
department stores: it is not metallized
paper!

Block schematic

The print mechanism must be driven so
that an ASCII unit at the input of the Cen-
tronics interface is converted into a
character on the paper. This cannot be

achieved by a simple conversion, because
there are also intervals of various lengths
between the characters to be considered,
as well as the control of the return
mechanism and the shifting of the paper
feed once a line has been completed. All
this is taken care of by the single-chip
central processing unit (CPU) type 8049:
when this is programmed for our pur-
poses it is type-coded 8049C289.

As can be seen from the block schematic
in figure 3, the CPU is at the centre of a
number of additional stages which are
actually contained in only a few com-
ponents. The Centronics interface matches
the CPU input to the Centronics standard.
The 'print format' determines the number
of characters per line. ’Control' enables
the manual control of the paper feed and
reset. The clock for the printer is
generated separate from that for the CPU.

Table 1

2111 Mal£nt(M.t>!lMSC<M>:IF DMXCft') OR DMSCCO'I TKX2MI

or OMScrtn necHi

Technical Data

■ Centronics interface with StI. READY, ACK,
D0...D7

■ CPU: single-chip microcomputer 8049C289

■ dot matrix print head, 7 needles (styiil

■ 5 x 7 dot matrix

■ characters separated by two spaces

■ contents: 159 characters

■ speed: 80 characters per second (c.p.s.)

■ 13, 16, 17, 20, 24. 25. 32. or 40 characters per
line (presettable or programmable)

■ printing direction: left to right

■ width of thermo paper: 79 mm

■ switches for paper feed and reset

■ power supply requirements: 5 V + 5%, current
consumption 3 A maximum during printing.

130 mA on standby: power supply on printed

Figure 3. The block

wire bridgels).

16

, 7

20

24

25

32

40

P20

no

yes

no

yes

no

yes

no

yes

P21

no

no

yes

yes

no

no

yes

yes

P22

no

no

no

no

yes

yes

yes

yes

By adjusting the printer clock, the con-
trast, that is, the darkness with which the
dot is printed on the paper, is changed.
Moreover, the supply voltage and the
ambient temperature also affect the cir-
cuit, so that the contrast ensures an even
print quality.

The 'power down reset' stage will be
discussed in detail in the circuit descrip-
tion. The ‘power supply’ is shown con-
nected to the print head interface, the
thermo print head, and the motor only,
because these elements between them
consume by far the larger part of the cur-
rent. but it powers the other parts of the
circuit as well, of course. The 'print head
interface' transforms the logic level of the
CPU output into a sufficiently large cur-
rent for the individual thermo needles,
and also controls the motor and the print
head. Finally, the 'pulse shaper’ converts
the sinusoidal output of the tacho-
generator into rectangular signals at TTL

Circuit description

The various blocks of figure 3 are easily
recognized in the circuit diagram in
figure 4 which again is dominated by the
CPU. The Centronics interface consists of
pull-up resistors R24 . . . R31 and R37, as

well as the two monostable multivibrators,
MMV1 and MMV2. The strobe signals pro-
vided by different computers vary be-
tween a half and several microseconds. As
the 8049 requires a signal of about
SO milliseconds, the strobe signal, STB, is
stretched appropriately in MMV1. In the
Centronics standard, at the READY signal
the level of the sign al is determinant,
whereas at the ACK signal, the trailing
edge of the pulse is. Most computers,
including the Junior, and interface
elements such as the PIA8255, need the
trailing edge and therefore use the ACK
signal for acknowledgment. Here, i t is
derived by MMV2 from the READY signal
generated by the CPU.

The print format, that is, the number of
characters per line, is determined by wire
bridges P20. . .P22 as shown in table 3. If
you want, a DIL switch may be used
instead of the bridges, or the port lines
may be controlled by TTL levels so that
the number of characters can be changed
every line. The fewer characters per line
are chosen, the wider and bolder they
become.

The reset and paper feed signals, con-
trolled by push button switches S2 and SI
respectively, are actuated on negative
logic levels. Both these networks need a
current limiting resistor, R4 and R5, but

12.30

the paper feed circuit also needs a also by the supply voltage and the mini?

decoupling capacitor, C3. The decoupling ambient temperature. In this way the

is not necessary for the CPU but as a effects of voltage and temperature vari-

‘kindness’ to the motor and print ations are kept within tight limits to ensure

mechanism. Port P23 of the CPU is not even quality of print.

scanned during the printing process so To understand this, you have to take the

that switch S2 is then inoperative. print head drive bus and the print head

The clock oscillator for the printer con- interface into consideration as well. The

sists of gates Nl, N2, N4, resistor R9, and head drive bus consists of the data bus of

capacitor C9. A presettable current source the CPU, DB0. . . DB7, and port line P27.

comprising transistor T3, resistors R6 . . . R9, The dot information, which the CPU has

and preset PI loads the oscillator circuit built up from the ASCII units, is available

and can therefore affect the frequency. at DB0. . . DB6. The motor is controlled

This arrangement makes diode D1 from DB7. Integrated circuit IC2 is an

necessary. The output of the oscillator is 8-transistor array which is used here as a

buffered by N3. The frequency of the non-inverting line driver. The common

clock is nominally 16 kHz but can swing connection of the heating elements, as

over quite a wide range. It should be well as the positive terminal of the motor, Fj uf

noted here that the current source is is at + 5 V. The motor and the appropriate posit

affected not only by the setting of PI but heating elements are actuated by connec- also

4

I. Tha dominant
i of the CPU is
ident in the circuit

ting the relevant outputs of IC2 to earth.
The pulse width of the needle point out-
puts is determined by the frequency of
the printer clock and the CPU The pro-
cessor checks whether the corresponding
heating element has been in operation
recently. If so. the element is still warm
and should not be supplied with heating
current for too long, otherwise it may bum
out. Therefore, the CPU holds the relevant
output active (ie. at logic 0) for only six-
teen clock pulses, starting with the fifth of
the dot cycle. If the element has not been
heated recently, the CPU output remains
at logic 0 for twenty clock pulses, starting
with the first of the dot cycle.

When the data bus is in the high-im-
pedance state, pull-up resistors R16. . .R23
are connected to it via port P27 and T2.
This is essential as the 8049 does not have
internal pull-up resistors. Always make
sure, therefore that the inputs of IC2 are
unambiguously at logic 1 when the data
bus of the CPU is inactive.

Terminal R of the print head interface con-
trols the print head in position ‘home’.
Capacitor C13 is necessary to decouple
the motor power line from that to the
heating elements, which has the added
benefit of contributing to even quality of
printing.

The pulse shaper for the tacho signal is
formed by D2, Tl, R12, R13, and C8. Ignor-
ing the threshold voltage of the diode, the
shaper functions as follows. During the
positive half cycle, D2 blocks, whereupon
Tl gets a sufficiently high base current via
R13 to start conducting. During the
negative half cycle, D2 conducts, so that
the base of Tl is negative, and the tran-
sistor is cut off. A rectangular signal at
TTL level, the frequency of which is equal
to that of the sine wave at pin 1 of the
8049, therefore exists at the collector of
Tl.

Capacitors C6 and C7, and inductor LI,
are the externally required components
for the internal clock of the CPU. The

2.32

clock frequency is around 6 MHz: its exact
value depends on the tolerances of the
external components. The precise value
is, however, not important as the 8049 is
on ‘hold’ for most of the time.

The power down reset circuit is based on
the precision voltage comparator ICL8211.
The circuit ensures that during short
breaks in the supply voltage the program
of the CPU is not confused which might
conceivably give rise to the heating
elements being actuated inadvertently and
so cause the print head to bum out.

To do so, the circuit generates a reset
pulse during supply breaks: a printing
error is better than a bumt-out print head!
Strictly speaking, however, the circuit is
not necessary because in most cases
mains power failure is completely taken
care of in the power supply. In any case,
in the absence of mains voltage, the
power on reset is actuated when the
printer is switched on. And if the worst
comes to the worst, a print head costs

only a few pounds. It is, however, import-
ant that if the power down reset circuit is
not used, the RESET terminal, pin 4, of the
CPU is taken to earth via C5: there should
be no other connections to this pin!

The power supply is a conventional circuit
with fixed voltage regulator, for which in
this case a 78H0S (with aluminium TO-3
housing!) is used to cope with the current
requirement of the printer.

12.33

and exit slits. The position of these slits is
shown in figure 10. If the holder is posi-
tioned accurately, the beginning of a new
paper roll (cut straight beforehand) is
simply inserted into the entry slit, the
paper feed picks it up (press SI!), and it
then emerges from the exit slit. It is thus
not necessary for the case to be opened
to change the paper roll.

Either touch-type or normal push button
switches may be used for SI and S2. In
either case the relevant part of the PC
board (clearly marked on figure 5) must
be cut off and fitted behind an
appropriate cut-out in the front panel. Cut-
ting part off the board enables the use of
a variety of mains transformers.

It is, of course, perfectly feasible to
assemble the printer to your own design:
the only thing you have to be careful with

fore best to mount this component last
and preset it immediately prior to solder-
ing it in position.

Before commencing work on the printed
circuit, have a look at the photographs in
figures 6 and 7 which show how we
assembled our prototypes. The printing
mechanism is fitted on a metal plate above
the printed circuit and is fastened to the
rear panel of the case. This arrangement
saves a lot of space and the case can
therefore be smaller and less expensive.
The connection between the output ter-
minals on the printed circuit and the
socket on the side panel (for the flexible
print head cable) is best made in flat rib-
bon cable.

We have designed a simple holder for the
paper roll and this is fitted at the rear
panel of the case behind the paper entry

1 2.34 elekla

8

is to ensure that the paper does not pass
across the heat sink of the voltage
regulator or the mains transformer.

Finally, it is recommended that in spite of
the temperature-compensated oscillator
circuit some air vents are provided in the

Table 4 shows the pinout of the Cen-
tronics socket on the printed circuit, while
figure 8 shows the connectors to the prin-
ting mechanism and the print head.

Calibration

IMPORTANT: before the printer is con-
nected to the mains, make certain that PI
has been preset as instructed under 'con-
struction'; failure to do so may result in a
bumt-out print head! Also, before the
calibration is commenced, the printer
must be connected to the Centronics out-
put of a computer. The computer is then
programmed to give forty ietter characters
in a line. Switch on the printer and let the
computer pass the line of characters to
the printer: the print head should now
move across the paper. In most cases
there will also be a print-out on the paper,
most probably too bold or too faint, and
as likely as not there will not be forty
characters across the paper width.

There will either be forty characters
across part of the width of the paper, or
there will be fewer than forty printed in
too wide a fount. Careful adjustment of PI
and repeated test printouts will result in
optimum setting of the potentiometer and
this is evidenced by the printing of forty
clean characters in line across the width
of the paper. During this calibration it will
become quite clear how the printer clock
affects both the number of characters per
line (or rather, their width on the paper)
and the contrast (ie. how bold or faint the
characters are printed).

If you have incorporated the power down
reset circuit, this should next be
calibrated. First, pull out the mains plug
and that of the print head! Next, connect a
regulated power supply across Cll and
adjust P2 so that pin 6 of IC6 becomes
logic 0 as soon as the output of the power
supply drops below 4.S V. Take care that
the voltage does not exceed 5 V during
the calibration.

Finally, test the paper feed switch,
whereupon the lid can be closed onto the
case: the printer is ready for use. H

l

/

01234567
89 s ; < =>?
3ABCDEFG
HI JKLMNO
PQRSTUUW
XVZC ¥]-•_
' a b c d e f 3
h i ■ j k 1 m n o
p 3 r s t u u w
x y z £ ! I:-""

D r _, -. ■ T>?

4 ■S x 5t P j. 3 *y

o l> 7 . 1 z y

k tlfl

*

I Li l> □ n n

Bure 9. The sum total of
e characters available In
e mini printer. Above

Figure 10. Rear panel:
positions In our prototype

12.35

Every week the local papers carry tales of burglaries and break-ins,
sometimes in our own street or neighbourhood. Most of these crimes
are the work of the 'amateurs' or opportunists of the criminal world
and they, unlike their 'professional' counterparts, should be quite
susceptible to some sort of deterrent, even if it is very simple. A
popular ploy has been to mount an empty burglar alarm box on the
side of the house but as the number of affordable burglar alarms has
increased recently most people are now more likely to fit the full
system. The problem then is the large number of alarms falsely
crying 'wolf', with the result that real burglaries often go unnoticed.
The circuit proposed in this article does not give false alarms; in fact
I it gives no alarm at all. Instead it produces a light signal that will
never call out the Police unnecessarily, which in itself is a distinct
| advantage.

deterrent

the window and sees a nondescript case
with an enatically flashing LED in the side
he will be forced to think about all the
technological secrets that LED could be
hiding. It could be an infra-red sensor, or
maybe it indicates ultrasonic waves bounc-
ing around the room, or maybe. . . (Mental
gear-wheels engage in a criminal mind.)
Soon an unwished-for thought shouts for
attention: ‘But why is it flashing at
all?. . .Does it know I’m here?. . .Has it
told anybody?’ At this stage your average
criminal will (hopefully) let instinct take
over and leave while the going is good. If
he does the circuit has performed its pur-
pose at least as well as an alarm; if not,
the chances are that no alarm would have
deterred him anyway.

a pseudo
burglar alarm
with a 'noisy'
LED

j A clever defence lawyer may call a
] burglar caught in the act "the victim of an
| imperfect (or unjust, or whatever) society”
and try to prove that society is the real
cause of all crime. Be that as it may, com-
ing home to find your house burgled and
ransacked is an experience most people
| will gladly forego. Often the worst thing is
knowing that somebody has literally in-
| vaded your privacy. To prevent this sort of
occurence many homeowners decide to
instal (or, more likely, pay somebody to
instal) a burglar alarm. The trouble is that
it is generally not at all easy to fit a really
effective alarm, and you invariably have to
pay dearly for this sort of security.

| If we go right back to the basic problem
| it is clear that although detecting a
burglary while it is in progress is very
laudable it is certainly not to be preferred
to preventing the criminal from even
beginning in the first case. Nowadays a
crook (‘amateur’ or ’professional’) who
sees a burglar alarm box tacked to the
side of a house knows that he will just
have to work quickly and make a getaway
before the neighbours are properly
awake. If, on the other hand, he looks in

The basic circuit

This circuit is a burglar deterrent unlike
the run-of-the-mill alarms, as the block
diagram of figure 1 shows. Connected
directly, to the mains is the power supply
section, consisting of two parts: a voltage
dropper and rectifier, and a regulator.

This is directly followed by a clock, which
feeds a noise generator formed from a
shift register. The resultant noise signal is
applied to the last stage, the ‘display’ via a
control section.

A noisy LED

Unusually, for a mains-powered circuit,
there is no transformer to be seen on the
circuit diagram of figure 2. This means
that certain tracks on the printed circuit
board carry 240 V a.c. so be careful about
working on the circuit, or trouble-shooting
it, while the power is switched on.
Whenever the mains power is removed
capacitor Cl is discharged through
resistor R8. If this were not done there
would be a chance that the voltage across
this capacitor could give somebody a

Numbers, simple arithmetic and variables
are the main elements in a BASIC program.
They are all dealt with extensively in this
second part of the series. We will also
discuss the memory, commands, error
detection, editing, spacing, comparisons
and the LET and PRINT statements.

Quite a 'program'!

The first part in this series introduced BASIC. The
difference between compilers and interpreters was
explained and the importance of flow charts was
stressed. A simple example illustrated the use of
numbered program lines, the statements END and
PRINT were introduced, and the RUN command
was explained.

The next step is to find out what to write on the
program lines: what numbers, arithmetical oper-
ations, variables etc. will the BASIC interpreter
understand? Furthermore, it is not possible to
write good programs without some understanding
of computer memory capabilities. These 'basics'
will be dealt with here.

Since programs are inevitably associated with
errors, error detection must also be discussed; at
the same time, a discussion of spacing and editing
will help to enter programs correctly in the first
place, and correct them if necessary at a later date.
Finally, two useful statements will be discussed:
LET and PRINT. The latter was already intro-
duced in part 1, but some further uses for this
statement remain to be explained.

Computer Memory

In a BASIC computer, part of the available mem-
ory space is used for storing 'control programs'
- the BASIC interpreter, for instance. This section
of memory cannot normally be erased: it uses
so-called 'Read Only Memories', or ROMs (see
figure 1). Storing information in a ROM is a once-
only process, usually taken care of by the manu-
facturer, From then on, this information can be
read out as often as described, but it cannot be
altered or erased.

The rest of the memory will normally consist of
'RAMs' (Random Access Memories). These offer

the possibility of storing, reading out, altering and
erasing information at will. However, the informa-
tion will also be lost if the supply voltage fails, so
a more permanent form of storage is useful: mag-
netic tape (reel-to-reel or cassette) or 'floppy disc'.
Although these are extremely useful for storing
complete programs, they are not much use when
running programs: the information is not readily
available - in other words, ‘Random Access' in
the full sense of the word is not possible.

All in all, when it comes to writing and running
programs the RAMs are the section of memory
that is of primary importance. Only part of this
section will normally be used for storing the
current program (with any further information
required for it); this subsection is called the
program memory. It will normally be possible
to erase the program memory while retaining
information in other subsections (other programs,
for instance).

When using NIBL, the total RAM area is divided
into so-called 'pages'. Programs can be stored on
one or more of the pages 1 to 7. A more detailed
description of the NIBL memory is contained in
a separate article.

Control commands

When control commands are keyed in to the
computer, they will be carried out immediately
- unlike 'statements', which are keyed in as part
of a program and only become operative when
the program is being executed. (Note, however,
that when statements are keyed in without a
(program) line number, they are treated as
described in part 1).

When the computer prints a 'prompt' symbol, it
expects further information from the user (via
the keyboard). This information can be either a
command or a new program line; after keying
it in, the user operates the CR key (carriage
return), whereupon the command is carried out
(or the program line stored).

This much we knew, from part 1 . It is now time
to find out what commands the (BASIC) com-
puter will recognise.

RUN

This command was introduced in part 1 of this
series (page B7). As explained, once a complete
program has been stored in the memory the
command RUN can be given. The computer will
then start to execute the current program,
starting at the first line (i.e. the lowest line
number).

In some cases (NIBL and the Motorola M6800
BASIC, for instance) some further 'ground-work'
is done by the computer before it actually
starts on the program proper. After receiving the
RUN command, it first changes all 'variables' to
zero and resets all 'program parameters'

( variables' and 'program parameters' will both be
discussed later).

LIST

This command is similar to 'PRINT' or, more
accurately, the non-existent command 'PRINT
PROGRAM'. When the computer receives the
LIST command, it will respond by printing out
the entire current program as stored in its pro-
gram memory.

Let us take the first program on page B7 (part 1)
as an example. We will assume '.at, having typed
in the program and giving the command 'RUN',
we discovered that there was a mistake in the
program: the intention was to add 5 + 7, so the
answer should be 12. We therefore request the
computer to print out the program: LIST.
Discovering the error in program line 10, we can
correct it by simply typing in the correct infor-
tion; to make sure, we can repeat the LIST
command; finally, a RUN command will result in
the desired answer appearing. The total print-out,
starting with the keying in of the incorrect
program, will be as follows:

tions of the LIST command. These vary, however,
from one dialect to another. In NIBL, for instance.
'LIST n' (where n is a line number) means: list the

program from line number n on — even if line
number n itself is not used. An example:

r

> 10 PRINT 1+8 1 ,

>20 PRINT 1 +9
>30 PRINT 1 + 10
>40 END

I > LIST 25

30 PRINT 1 + 10
40 END

In the Motorola BASIC dialect for the 6800,
however, the same command has a different
meaning: LIST 30, for example, merely causes the
contents of program line 30 to be printed out.
Several dialects recognise a variation that is
unknown to NIBL (or, for that matter, the DCE
Tiny BASIC for the 8080): 'LIST n, m’, where n
and m are both line numbers. In this case, the
print-out of the program starts at line n and ter-
minates at line m.

PAGE

As stated earlier, the NIBL computer memory is
subdivided into 'pages'. When a NIBL computer is
first switched on, it automatically turns to page 2
and starts to execute the program that is stored
there. This can be useful when using the computer
as a 'process controller' that must get to work as
soon as it is switched on. Of course, this presup-
poses that the information on page 2 is stored in
ROMs. If it was stored in RAMs it would be lost
when the computer was switched off! If the NIBL
computer is not being used in this type of applica-
tion, it will discover that page 2 is blank. It then
automatically turns back to page 1 and prints a
'prompt' symbol.

If a program is now typed in, it will be stored on
page 1. However, this may be undesirable (for
instance, page 1 may be required for some other
program), in which case the command 'PAGE c n'
can be given". This causes the computer to turn to
page n (n = 1 ... 7) so that the program can be
stored there.

In general, we can jump from any page to (the top
of) any other by giving the command PAGE = n.
Alternatively, a minor variation of the same
command can be used: 'PAGE = PAGE + n', or
'PAGE = PAGE - n'. This is best clarified by an

■ Note that the PAGE command on a NIBL computer
should not be confused with the 'page' key on the Elek-
terminal: the latter refers to 'pages’ in the memory of the
terminal, not in program memory.

example. Assume that the computer is presently
working on page 3. 'PAGE = PAGE - V is then
interpreted as PAGE = 3 — 1; in other words, as
PAGE = 2. The computer will therefore turn to
page 2. Note that, as always, the new page number
must be between 1 and 7 — no other page numbers
exist!

SCRATCH, DELETE, PURGE, NEW

Different dialects, different words - but the same

command.

SCRATCH (sometimes abbreviated to SCR)
causes the computer to erase the current program
and the display. Some BASIC dialects also recognise
the command 'SCRATCH ALL': in this case the
entire user-programmable memory is wiped clean.
'DELETE', 'PURGE' and 'NEW' are other words
used for the same command in different dialects.
NIBL, for instance, only recognises the word NEW.

No matter what page is actually in use when the
command NEW is given, the computer will always
go to page 1 and erase this page in preparation for
storing a new program. If a different page is
required, this must be specified by giving the
command 'NEW n'. This will cause the specified
page to be erased instead, in readiness for a new
program.

CLEAR

A program may require the use of 'variables' and
'stacks', as will be described later. After running
the program, these may contain all sorts of infor
mation that is no longer required (intermediate
results, etc.). Before running the program a second
time, this information can be erased by means of
the command 'CLEAR'.

SYNTAX ERROR

'There's many a slip . . . ' - certainly when it
comes to writing programs. Even when the pro-
gram itself is perfect, there is always the possibility
of typing errors when keying it in. The slightest
mistake of this kind - printing RAN instead of
RUN, say, or PRANT instead of PRINT - will
make the command or statement completely
unintelligible to the computer. Fortunately, it
will usually recognise typing errors and print out a
warning to the operator. One example of an
error indicator is the phrase 'SYNTAX ERROR’

(although computers are sometimes programmed
to use less polite language . . .).

SYNTAX ERROR' (sometimes abbreviated, as in
NIBL, toSNTX ERROR) indicates 'bad language':
the phrase keyed in does not exist in that particu-
lar BASIC dialect. Other error indicators also exist,
as we will see; for instance, any attempt to divide
by zero usually results in a very explicit warning
on the screen.

, M?" cfeQi

I

-5Z3. _

q

. . completely unintelligible to the computer . . .

Editing

If (typing) errors are noticed while keying in a
program, these will normally be corrected by
operating the 'back space' key (<-). For instance,
after keying in 'PRINK', this error can be cor-
rected by operating the back space key and then
typing T. The K will be replaced by the T, and the
correct phrase (PRINT) will be stored in the
memory. Similarly, if one forgets to use capitals,
this can be corrected by operating the back space
key as often as necessary to return to the
beginning of the word: pri <- «- <- PRINT.
Alternatively, as described earlier, a complete
program line can be corrected by again typing the
same line number, followed by the correct infor-
mation. Similarly, a complete line can be deleted
by typing in the line number followed by CR
(carriage return).

Most BASIC dialects include other editing facili-
ties. but the exact details are usually dictated by
the type of keyboard used. NIBL, for instance,
includes the command 'CONTROL/U' - i.e. the
Control and U keys are depressed simultaneously.
In this case, the 'current line’ (the line being typed
at that moment) is erased from the display - but
not from the program memory. This facility is
particularly useful if the wrong line number has
been keyed in: the error can be corrected without
losing instructions already stored under that line
number in the memory.

Spacing

When typing in programs, it is often useful to add
spaces here and there — between statements, for
instance. Although the computer hasn't the
faintest idea what they mean and will normally

ignore them, it does store them in its memory and
will print them out when requested to 'LIST' the
program.

The main reason for adding spaces, therefore, is
simply to make the program 'legible' for the
operator. However, there are a few places where
spacing is forbidden:

- within words that are part of statements or
commands. For instance, PR I NT is wrong, it
must be PRINT. However. PRINT5 and
PRINT 5 are both permitted.

— within numbers (including line numbers). The
program line '150 PRINT 2500' is correct, but
■1 50 PRINT 2500' and '150 PRINT 2 500-
are both wrong. It should be noted, however,
that '150 PRINT "2 500"' is correct: the
computer isn’t interested in the text between
quotation marks, and simply prints out what
it finds there.

Some other cases where spaces are forbidden will
be dealt with as we come to them.

On the other hand, some BASIC dialects demand
correct spacing in one or two places; for example,
before and/or after statements or commands.
This will not normally be a problem: one will
normally add spaces at these points in the interest
of legibility! For example (and using a few state-
ments that will be described in part 3, just to add
to the confusion!), the following would be almost
unintelligible:

10 I FA = BLETA = B — C

However, adding a few spaces turns it into what is
almost 'plain English':

10 IF A = B LET A = B = C
As a final note: if one space is permitted, more
than one are also allowed. In the above example,
say:

10 IF A=B LET A=B— C

Numbers

In BASIC, numbers can be written in the usual
way. There are, however, a few points that should
be noted.

Whole numbers (integers)

Numbers that do not include fractions are referred
to, quite logically, as 'whole' numbers (or
integers). 23 is a whole number, 23.1 is not.
Numbers can be either positive or negative. If they
are positive, they may be preceded by a '+' sign,
negative numbers must be preceded by a sign.
As mentioned earlier, no spaces are permitted
within numbers. A few examples:

Correct: 3. >-3, +1 23456789; -3, -567.

Wrong: 123 456.

More properly, decimal fractions: numbers that
include a decimal point. As before, '+' and '-'
signs may or must be included, respectively.
Several BASIC dialects get confused by a leading
zero before the decimal point: .38 is alright, but
0.38 is not. Once again, a few examples:

Correct: 2.2. +1.23, -55.5, - 44.

Wrong: 2%, — %.

NIBL doesn't recognise the decimal point, so that
only whole numbers can be used. Any attempt to
include a decimal point will be rewarded by the
print-out 'CHAR ERROR' (from 'character error').

Number range

In most 8ASIC dialects, the maximum number of
digits in a number is nine — not including the '+'
or sign and/or decimal point. For instance:
-123456.789. The largest number that can be
written in this way is 999999999; the smallest
(positive) number is .000000001 ,

The number range in NIBL is rather more limited:
only (whole) numbers between —32767 and
+32767 are permitted. These limits are not as
arbitrary as they may appear at first sight: they
correspond to the largest number that can be
written in a 16-bit binary system. If a larger
number is keyed in, the computer will respond
with the warning 'VALU ERROR' (from 'value
error'). For instance:

> PRINT 44253
VALU ERROR

Scientific notation

In some cases, the range of numbers outlined
above may prove too limited. For this reason,
many BASIC dialects also include an extension
facility: 'scientific notation', also known as 'E
numbers'. Basically, this consists of a normal
number, followed by the letter E and then two

These last two digits determine how many places
the decimal point is shifted to the right (positive
number after the E) or to the left (negative num-
ber after the E). In other (mathematical) words:
the number is multiplied by a power of ten, as
defined by the two-digit number. A few examples
may seive to clarify this:

4.35E5 = 435000
1234.5E 3 = 1.2345

Regrettably, this type of notation is not possible
in NIBL.

The accuracy with which a computer deals with
numbers (e.g. when storing them in memory or
when performing calculations) depends both on

the interpreter and on the computer itself.
Normally speaking, the accuracy will be some-
where between 5 and 7 digits; any larger number
of digits will be 'rounded off'. In other words,
the number 123456789 may be rounded off to
123450000, 123456000 or 123456700.

Arithmetic

Five arithmetical operations are defined in BASIC:
+, — , *, /, and t. Their meaning is listed in table 1,
together with some examples.

Table 1.

example result explanation

15

3

8

involution (raising
to the n ,h power)

When several operations are included in the same
formula, they are performed in the well-known
order: first involution, then multiplication and/or
division, and finally addition and/or subtraction.
For example, 6 + 4/2 would be calculated as
follows:

4/2 = 2; 6 + 2 = 8.

If the addition (or subtraction) is to be performed
first, this must be indicated by including this oper-
ation in brackets. (6 + 4)/2 is calculated as fol-
lows:

6 + 4= 10; 10/2 = 5.

Comparisons

Numerical comparisons, such as 'is A greater than
or equal to B', are often required in programs. The
various comparison symbols used in BASIC are
listed in table 2, together with a few examples. As
can be seen from this table, the result of a com-
parison can be only one of two things:true (T) or
false ('0').

In those cases where two possible symbols are
shown (e.g. < > or > < for 'does not equal') Nl BL
uses the first of these alternatives, as shown in the
examples.

In most BASIC dialects, including a space between
the two parts of one symbol is not permitted. For
instance, > = should not be typed as > =.

Variables

A variable is quite simply a name, or 'token' to

which a numerical value can be attached. POWER,
say, or A5. The use of variables can be illustrated
with an example. Let us assume that we want to
calculate the maximum output current (I) of some
circuit, and that this current depends on the
supply voltage (U) and some load resistance (R) as
follows:

It is not difficult to write a suitable program:
> 10 PRINT U/2-R
>20 END

In this program, U and R are variables. After
giving them numerical values — for instance,
U = 10 (volts) and R = 5 (ohms) — the program can
be started by giving the RUN command and the
correct result will be typed out. The complete
print-out, including a minor sophistication added
on program line 15, will be as follows:

The advantage of using variables is that new
calculations can be performed for different values
of the variables, without having to rewrite whole
sections of the program. The print-out given above
could be continued, from the line immediately
following 'BRK AT 20', as follows:

At this point it should be noted that the program
example given above will not run correctly on all
computers. The reason is that, in some BASIC
dialects, the RUN command causes all variables to
be reset to zero before the program is started. This
is useful, in that it eliminates the risk of acciden-
tally running a program with 'old' values for the

variables; however, it also means that variables can
only be assigned values within the program — not
beforehand. This can be accomplished quite easily
by adding as many program lines as required
before the first program line of the program
proper; in the above example, for instance, and
adding a LIST command to check the program
before running it:

> 5 U = 2000

> 6 U = 2

> LIST

5 U = 2000

6 R =2

10 PRINT U/R-2;

15 PRINT "AMPERE"
20 END

> RUN

500 AMPERE
BRK AT 20

>

previous progr
keying
BASIC dialed

In these program examples, the letters U and R
were chosen as 'tokens' for the variables. A few
BASIC dialects permit the use of several letters
for a token: POWER, say, or ALPHA. In most
cases, however, only one letter is permitted,
followed (if required) by a single digit. In other
words, variables can be correctly named A, D, D1,
Z9 — but AZ, G12 etc. are not allowed. In this
way, up to 286 different variables can be 'named'.
In larger programs, so many may in fact be used
that one can easily forget what each 'name' stands
for. This can be extremely awkward: the computer
won't give any warning (it doesn't know that its
operator is getting confused . . .), so the program
will run normally — the only trouble being that
the results are all wrong!

To avoid this type of problem, it is advisable to
make a list of all the variables used, with their
'token' and true meaning. A systematic list like
that shown in figure 2 is usually the best.

When using NIBL, this type of confusion is less
likely to occur: the only 'names' permitted are
the 26 letters of the alphabet.

The type of variables discussed so far are some-
times referred to as 'simple numerical variables'.
A second ’ype also exists: so-called 'string vari-
ables', 'i i .-re the variable does not represent a
number, instead, it represents a 'string' of charac-
ters (letters and numbers). This group will be
discussed later.

LET

LET is a so-called assignment statement: it is used
to 'assign' a certain <

s variable. In the

i examples, this was done by
= 2000', for instance. Although most
s tolerate this (mis-)use of the '='
t the 'official' way to give a numeri-
cal value to a variable. It is more correct to use the
LET statement. The complete instruction is then
keyed in as follows: first 'LET'; then the 'name' of
variable; then the '=' sign; and, finally, the 'ex-
pression' — i.e. the (numerical) value or operation
that the variable must be made equal to. A few
examples:

LET A= 15
LET A = B
LET A = 3 + 4
LET A = B + C

As illustrated, one variable may be made equal to
another - or to some mathematical operation in
which other variables are included. Even more
surprisingly, perhaps, the same variable may
appear on both sides of the '=' sign! For example:
LET A = A + 1

In this case, the new value for the variable (A) is
derived from the previous one. If the value was 4,
say, this instruction will change it to 4 + 1 = 5.
Some BASIC dialects (not including NIBL) offer
the possibility to assign a value to several variables
simultaneously. The instruction
LET A = B = C = 15

will cause all three variables (A, B and C) to
assume the value 15.

In most BASIC dialects, use of the word LET is
optional; in other words, it is 'unofficially' per-
missible to write 'R = 5' instead of 'LET R = 5',
This abbreviated form is also recognised in NIBL.

More about PRINT

The PRINT statement was introduced in part 1.
Let us briefly sum up the possibilities discussed so

PRINT "5 + 6 ="

In this case, the text included in quotation marks
is printed exactly as it stands: 5 + 6 =.

PRINT 5 + 6

The 'expression', i.e. the mathematical operation,
that follows the PRINT statement is first carried
out and the result is then printed: 11.

PRINT

Since no text or operation follows the PRINT
statement, nothing is printed on the corresponding
line. Effectively, therefore, a one-line gap is left in
the print-out.

Normally, a PRINT statement is automatically
followed by CR and LF (Carriage Return and Line
Feed). If required, these can be suppressed by
adding a semi-colon between PRINT statements:
10 PRINT "TOM"; " DICK"; " HARRY"

20 PRINT "TOM";

21 PRINT "DICK";

22 PRINT " HARRY"

The print-out obtained from program line 10 is the
same as that from the other three lines taken
together: TOM, DICK and HARRY are printed on
the same line.

At this point, one further possibility of the PRINT
statement can be explained: use of a comma
between PRINT statements:

PRINT 121, 122

The result is that the various 'texts' are printed in
so-called zones — equivalent to tabulation on a
typewriter. A standard zone contains 15 characters,
so that in the example given above '121' is printed
at the beginning of the line and '122' in the 16th,
17th and 18th positions. The length of a line is
72 characters, so it contains just less than five
zones (more accurately, the fifth zone consists of
only 12 characters). Use of zones can be extremely
useful when printing tables.

Use of the comma for printing in zones is not
possible in NIBL.

72 positions

ZONE 1

ZONE 2

ZONE 3

ZONE 4

ZONE 5

■

ill

■

II 1

15 pos.

1 2 pos.

1 15

16 30

31------ 45

46 60

61—72

79084 3

Figure 3. Within a PRINT statement, commas can be used
to divide the print-out into so-called zones.

Questions

1. Why is an interpreter program rarely stored in
RAM?

2. What is the effekt of the SCRATCH command?

3. When is a CLEAR command used?

4. What errors are contained in the following
program lines?

a) 150 LI ST 5

b) 1 0 PRINT 18

c) 160 PRINT CHAIR

d) 170 PRINT 1253 14

e) 190 LET A = 0.31

f) 200 PRINT 4.35E1.2

5. What are the results of the following calcu-
lations:

a) 3*2+8+15/3

b) 17-24/3/2

6. What error is contained in the following state-
LET A15= 12

Answers to questions in part 1 .

1. Tiny BASIC is a simplified version of 'stan-
dard' BASIC; for this reason it is less versatile.
Tiny BASIC is intended primarily for micro-
processors; however, the modern tendency is
toward 'standard' BASIC for all applications.

2. Tiny BASIC is often used for microprocessors
since the necessary interpreter program
requires less memory space.

3. The main difference between a compiler and
an interpreter is that the latter translates
programs line for line and causes the instruc-
tion to be carried out immediately, whereas a
compiler first translates the entire program.

4. The advantages of an interpreter are that it
requires less memory space (since the transla-
tion does not have to be stored in memory)
and that certain programming errors are
indicated immediately. The disadvantage is
that parts of the program that are used
several times within the program have to be
re translated every time. This takes up more
computer-time.

5. The various BASIC 'dialects' are tailored to
suit particular microprocessors, in an attempt
to shorten the corresponding interpreter
programs as much as possible.

6. A flow chart is an important aid when devel-
oping a program; furthermore, at a later date
it helps to gain rapid insight into the program.

7. A prompt is a character, printed out by the
computer to signify that it is waiting for
further information from the keyboard.

8. The program lines are numbered to indicate
(to the computer) in what order they must
be carried out.

9. Operating the CR key indicates the com-
pletion of the preceding instruction or com-
mand; simultaneously, it initiates 'Carriage
Return' in the print out.

10. The computer will print the result of the
operation, i.e. 12.

SfiSEC

<F«T2)

Summary of symbols, statements and commands used in

RUN
LIST
LISTn
LIST n,m
PAGEn

SCRATCH. DELETE.
PURGE, NEW
NEWn

CLEAR

SYNTAX ERROR
SNTX ERROR

This command causes the com-
puter to execute the program.
This command initiates a print-

The program is printed out from
The program is printed out from

This command (in NIBL) causes
the computer to jump to page n,
readying the computer to store
(or modify) a program there.
These commands cause the
program memory to be erased.
This command (in NIBL) erases
page n in the memory, prepara-
tory to storing a new program.
This command can be given
before re-running a program.
Indicates that an error has been
made in the BASIC language

CHARACTER ERROR 1 Indicates that a character is
CHAR ERROR /being used in a place where it is

GLOSSARY
(PART 2).

assignment statement

Instruction to give a variable some (numerical)
value; for example: LET A.= 1.

control command

Instruction that is carried out immediately, not
as part of a program.

current line (program)

The program line (or program) that is being keyed
in at that moment.

editing

Entering and correcting programs (via the key-
board).

error indication

Some programming errors can be detected by the
computer, in which case it will print out a warning.

VALUE ERROR
VALU ERROR

/

t

} large, or consists of too many
digits.

Backspace. This key is used
when correcting keying errors.
Symbol used in scientific no-
tation. The number following
the E defines the number of
places over which the decimal
point must be shifted,
addition 1

mathematical

symbols

floppy disc

Flexible magnetic disc, used to store large amounts
of information.

integer

A whole number, without (decimal) fractions,
listing

After receiving a LIST command, the computer
will print out the current program. This print out
is called a listing.

page

Subsection of memory in a NIBL computer.

RAM

Random access memory: a section of memory in
which data can be stored as required.

A1 . . . Z9
LET

'names' of variables

Statement, by means of which
a value can be assigned to a

ROM

Read only memory: a section of memory in
which data has been stored during the manufac-
turing process. This data cannot be erased.

PRINT "TEXT"

PRINT 2A + 3

This statement causes the text
included in quotation marks to

The expression following
'PRINT' is carried out and the

simple numerical variable

A variable to which a numerical value can be
assigned; this value can be altered, as required,
while the program is running.

PRINT.

The semi-colon can be used to string variable

separate groups of symbols A variable to which any group of characters can be
and/or expressions to be printed. assigned - a word, for instance.

A semi-colon at the end of the

PRINT.

statement causes a
PRINT statement to

following
be carried

By using commas, the print-out
can be divided into 'zones'.

variable

A 'name' or 'token', to which a number or group
of letters can be aasigned. See: simple numerical
variable and string variable.

nasty shock if they touched the pins on
the plug-top

A 10 V zener diode limits the amplitude of
the half-wave rectified power signal and
this is then buffered by smoothing ca-
pacitor C2 before being fed into voltage
regulator IC1. The output of this 78L05
provides the 5 V needed to supply the cir-
cuit. Two EXOR gates. N1 and N2, make
up the clock oscillator we mentioned
when describing the block diagram. The
frequency of oscillation, with the values
stated here for R2 and C7, is about 2 Hz
but this may vary depending on the
source of IC2. Changing the frequency is
quite simple as its magnitude is roughly
1/(2RC). The clock signal is applied to
pins 1 and 9 of a double four-bit static
shift register. These two registers are
cascaded to form a single eight-bit shift
register, with the eighth bit supplying the
control signal for the LEDi The purpose of
this is to form a noise generator. Two bits
from the second shift register are fed
back to the D input of the first register via
an EXOR gate so that the final effect is
that the signal output at Q7 has the form
of a pseudo mains noise. When the power
is first applied to the circuit the section
based on N3 automatically resets the
second shift register to zero by taking its
D input high. Initially N3 functions as an
inverter but after a delay introduced by
the RC time of R3 and CS it becomes a

Figure 1. This circuit is
totally unlike any normal
burglar alarm, but this la
hardly surprising as it is
intended as a deterrent.

non-inverting buffer. From then on the
information from IC3a's highest output
(03) is passed straight via pin 7 to the first
register of IC3b. With each successive ris-
ing edge of the clock signal the data is
shifted one place to the right. The same
effect is seen with the information trans-
mitted via N4 to pin 15 of IC3. Every 128
clock periods the pseudo mains noise
generation cycle repeats itself. This cycle

12.37

burglar deter

Btf

should be used for IC2 and IC3. Note that
all resistors and diodes are mounted ver-
tically and that capacitor Cl should be fit-
ted last. As part of the board carries mains
voltages the case used must be made of
plastic. Before locating the integrated cir-
cuits in their sockets check that the
supply of + 5 V is present on pins 14 and
16 of IC2 and IC3 respectively. If this is
correct all that remains is to fit the ICs
and close the case. Plug the circuit into a
mains socket and the LED should light to
show that the deterrent is working.

time is slightly longer than a minute,
which should be quite sufficient as we
( would not expect any would-be house-
i breaker to stay around long enough to
i notice the repetition.

| The pseudo noise signal is applied to
j integrator R4/C6 where it is 'smoothed'
somewhat. In this way the transistor (Tl)
conducts and the LED’s light waxes and
wanes rather than flashing violently.

Using the burglar deterrent

The burglar deterrent is put into service
by plugging it into the mains. Probably
the most important point in this respect is
where it is placed. A suitable location
must be chosen so that any would-be
housebreaker will see the LED flashing
and assume that he has been detected.

The impression can be heightened by
making a suitable front panel for the case.
Let your imagination run wild — that is
what the burglar is supposed to do. If the
LED does not seem to be striking enough
it can also be replaced (together with
resistor R5) by a small 6 V/SO mA bulb
painted red. Possibly the idea of having
240 V present on the printed circuit board
does not appeal to you. If this is the case
the section consisting of R7, R8, Cl, D1
and D2 may be replaced by a mains trans-
former rated at 8 V/100 mA and a bridge
rectifier (or four 1N4001 diodes). K

191 \

Construction

It is a simple matter to assemble the
burglar deterrent on the printed circuit
board shown in figure 3. As usual we
recommend that good quality sockets

The direct-coupled modem featured in last month's issue of Elektor
provides a new (for us) and very important application for the
RS232/V24 norm. It is unlike any demands we have made of this
protocol before as it makes use of a number of auxiliary control
signals that have rarely been needed in Elektor circuits up to now.
This means that they are less familiar to us than the normal signals.
For that reason we decided to have a close look at the CCITT
recommendation and along the way we will see why, when this is a
serial 'interface', so many lines are needed.

The RS232/V24 standard is seen as the
original serial interface. It was introduced
to define a specific connection, namely
that between terminals and modems. In
the words of the CCITT (Consultative
Committee for International Telegraph and
Telephone) it is intended “for interchange
between data-terminal equipment and data
circuit-terminating equipment”. To avoid
becoming too ‘wordy’ a number of
abbreviations will be used. The most com-
mon are: DTE (Data Terminal Equipment)
for the two interfaced ‘machines’ that pro-
duce and/or process the information
(computer, terminal, etc.); DCE (Data
Circuit-terminating Equipment) for the
interfaced equipment that transmits or
receives information but does not process
it. This latter is the modem (MOdulator/
DEModulator), which is sometimes re-
ferred to as the data set.

It is clear that using the RS232/V24 be-
tween two computers, or between a com-
puter and a printer, if that is possible, is a
departure from its original purpose. The
specific signals needed for communi-
cation between a terminal and a modem
will, of course, be completely different to
those required when a computer wants to
drive a printer.

The electrical characteristics of the
RS232/V24 and the pin-out of its 25-pin
connector are not repeated here. They
have already been dealt with in detail in
Elektor, most recently in the May 1984
issue (page 6-59). and on infocard 64.

RS232/V24 is a standard for inter-
facing a modem to data process-
ing equipment

An RS232/V24 interface will be found at
each end of a telephone line used for
communication between two DTEs. At one
side it is between the data transmitting
computer (or terminal) and its modem and
at the other end it is located between a
second modem and the data receiving
computer or terminal.

Data connections to a modem are bidirec-
tional and much more complex than the
one way lines needed with printers or
VDUs. Furthermore the data is fed into the
telephone network so there is a whole
battery of protocol signals with clearly
defined functions. This makes it possible
to automate the processes of accepting a
call, making a call, replying to requests

RS232/V24
the signals

and even choosing a transmission rate.
The format of the signals and the number
used depends on the options chosen.
Possible choices are: one-way or two-way
communication, with or without verifi-
cation, synchronous or asynchronous,
automatic call or answer, and so on.

A look at all
the signals
recommended
by this standard

| CCITT 1 Function | DTE I

CE

102 1 Signal ground or common return

102a 1 DTE common return

102b DCE common return

zz

ground

I 103 | Transmitted data

104 | Received data

j 118 1 Transmitted backward channel data
] 1 19 Received backward channel data

i !

-

105

106
107

108 1
108/2

109

110

is

n?

120

121

122

124

125

126
127

129

130

132

133

134

140

141

142

191

192

Request to send

Ready for sending

Data set ready

Connect data set to line

Data terminal ready

Data channel received line signal detector
Data signal quality detector

Data signalling rate selector (DTEI

Data signalling rate selector (DCE)

Select standby

Standby indicator

Transmit backward channel line signal
Backward channel ready

Backward channel received line signal
Backward channel signal quality detector

Calling indicator

Select transmit frequency

Select receive frequency

Request to receive

Transmit backward tone

Ready for receiving

Received data present
Loopback/Maintenance Test

Local loopback

Test indicator

control

(condition)

113 j Transmitter signal element timing (DTEI •]-»

114 Transmitter signal element timing (DCEI «-L«

115 I Receiver signal element timing (DTE) 1 <;-«

128 1 Receiver signal element timing (DTE) I •!-»

131 | Received character timing j <4-e

1

12.39

• •

RS232/V24:

*

7

EfE

Hf

*

♦

-unmm ■

-j K

rt

1

CDSL signal. It then connects to the
telephone line and indicates that it is
ready to transmit by activating the DSR
line. For its part the terminal must indicate
that it is primed for action by activating
DTR. The DTE + DCE unit that initiated
the call is then ready and simply waits for

If the unit that is called has a bell detector
its modem activates the CIN line and its
DTE reacts with the CDSL signal. When
the call is accepted this modem activates
its DSR line to let its DTE know that the
connection has been made. This pro-
cedure is summarised in figure 1. The
automatic call unit (ACU) should conform
to V2S, which recommends a specific pro-
tocol. As that is a different norm we will
not deal with it here.

When the physical connection between
the two modems has been made the data
transmission procedure can begin. No
matter how the connection was made the
DTR and DSR signals at both ends of the
line should be active. One unit is then
ready to transmit, the other to receive.

Figure 1. This is the pro-
cedure for a call, whether
it is manual (when the
user dials the number! or
automatic (ACU). The
ringing is detected by the
receiving modem, which
then signals the DTE by
activating the CIN line.

Figure 2. The DTE in the

data (TMD). The modem

All the RS232/V24 signals are listed in
table 1. The numbers indicated represent
what the CCITT refers to as circuits (by
which they mean lines or signals). Data
and ground lines require no explanation
so we will not deal with them here. Cir-
cuits 118 and 119 (back channel) are effec-
tively dealt with by the two modem
articles, in the last two months' issues. The
other signals (for control, state and clock)
are regrouped here according to their
functions.

Unit ON, incoming call,
automatic answering ON

The signals used are:

DSR (Data Set Ready)

CDSL (Connect Data Set to Line)

DTR (Data Terminal Ready)

CIN (Calling INdicator)

In the case of communications via the
telephone network the unit that makes the
call must first get a line: this can be done
manually (by the operator or user) or
automatically (Automatic Calling Unit), as
can the answer. If the call is not made
automatically the modem must receive a

Transmitting the data

While data is being transferred, during
which time we assume that CDSL, DTR
and DSR are active, the following signals
are the ones that interest us:

TMD (Transmitted Data)

RCD Received Data)

RTS (Request TO Send)

RFS (Ready For Sending)

DCD (Data Carrier Detector)

The actual serial data travels on RCD and
TMD between DTE and DCE at each end
of the line (see figure 2). Between the two
units, on the actual telephone line in other
words, data can only travel in one direc-
tion at a time. Two different modes of
communication are possible: duplex and
half-duplex (or simplex, as the CCITT call
it). Half-duplex communication is strictly
one-way. When a modem has finished
transmitting data it must immediately
remove its carrier from the line to give the
second modem a chance to transmit its

The transmitting modem is started by
means of the RTS signal given by its DTE.
In half-duplex mode this signal automati-
cally blocks the modulator in the DCE at
the other end of the line. When the car-
rier is present the transmitting modem
activates the RFS line (also called Clear To
Send) to let its DTE know that it is ready
to send data. When the carrier is detected
by the DCE demodulator this is immedi-
ately signalled to the receiving DTE by
making the DCD line high.

Transmission of data can start (TMD) as
soon as the RFS line is active. Tha data
appears on the RCD line and is demodu-
lated by the receiver modem.

In duplex mode the carrier is not re-
moved after the data has been sent. The
difference between duplex and half-

duplex modes is more them simply a mat-
ter of protocol between modems. The
mode used must be agreed, either ver-
bally or by means of a program, before
transmission of data can start.

Synchronization and time bases

The signals mentioned up to now can only
be used for communication between asyn-
chronous modems. Each has its own clock
and synchronization is achieved by means
of start and stop bits that precede and
follow each character. Synchronous
modems, on the other hand, use the
following signals:

TSET (Transmitter Signal Element Timing)
RSET (Receiver Signal Element Timing).
These signals allow the modulator and
demodulator clocks to be synchronized.
Also present is a circuit to change the
baud rate (DSRS). This is generally used if
the transmission is very noisy, whereby a
lower baud rate may be temporarily
selected.

The STF (Select Transmission Frequency)
and SRF (Select Reception Frequency)
signals are used by duplex modems to
decide the frequencies used by main and
back channels. If one of these uses the
upper frequency band the other
automatically uses the lower band.

This leads us to the signals that relate to
the back channel. Their functions are
identical to those of their main channel
counterparts. Apart from the data trans-
mission and reception lines (Transmitted
Backward Channel Data and Received
Backward Channel Data) there is TBCS
(Transmit Backward Channel line Signal) to
start back channel transmission, the cor-
responding reply when the DCE is ready
(BCR - Backward Channel Ready) and the
carrier detector on the back channel,

BCRS (Backward Channel Received
Signal). These three signals are seen in
figure 3.

The other circuits

In addition to the signals already men-
tioned there are some that are less fre-
quently used. Both the main and the back
channels have a signal to indicate the
quality of the modem's transmission when
no distortion is noted. There is a mode
changer and indicator (standby), a selector
for the frequency groups, a Request To
Receive signal, a back carrier selector
and some test signals whose use is
obvious. These latter are circuits 140...142,
which allow the quality of the transmission
to be tested by looping together either
the local unit (DTE + DCE) or the two
units via the telephone line
(DTE + transmitter DCE and receiver
DCE). The three connection possibilities
are indicated in figure 4.

Many of the signals we have been talking
about are for control or indication of a
condition so it is worth noting that a con-
trol circuit must have a voltage of at least
3 V if it is active (on). Anything less than
this and the circuit is inactive. In the case
of data lines, on the other hand, a logic 1
is indicated by a voltage lower than —3 V
and a logic 0 by a voltage greater than
+3 V. These are the V24 recommen-
dations so it might be wise to check that
all the equipment used conforms to these
norms before placing too much trust in
them. M

Figure 3. The back chan-
nel is brought into oper-
ation by the TBCS and
BCR signals. The modem

signals the presence of
the back channel carrier
to its DTE and sends it
the data received on this
channel (RBCD).

Figure 4. Certain specific
signals recommended by
the V24 norm allow

are available, namely the
line, and the telephone

modem.

12.41

If you have a modern television receiver that is not only suitable for
remote control, but is also fitted with a video input, there is no need
to read this article! However, if you want to convert a second or
perhaps your portable TV receiver into a monitor, the versatile
amplifier described here could be what you have been waiting for!

use your TV receiver as a
monitor. . .

. . .with this

versatile

amplifier

Most television receivers cannot be con-
trolled by an external video signal: only
those fitted with an A/V (audio/video) or
SCART connector (see Elektor India,
October 1984 issue) can.

Right from the outset we must tell you that
if your television receiver is not fitted with
an isolating mains transformer (the most
likely case), it should be as a first priority.
An isolating transformer can be installed
in most sets immediately after the mains
on/off switch (see figure 1).

A second prerequisite is a full circuit
diagram of the TV receiver without that
you cannot proceed. Many sets are pro-

vided with one nowadays, but if you have
none, you should be able to get it from
your local dealer or the manufacturer's
service/spares department.

Video output

Although monitors normally only have a
video input, an output may prove very
useful, as may be seen from figure 2.

Here, the output of the demodulator in the
TV receiver is taken via the video output
to the video colour inverter featured in
our November 1984 issue in an experimen-
tal set-up. If you use this set-up for your
own experimental work, note that the
100-ohm preset is imperative to attenuate
the signals into the video inverter; further-
more, it is advantageous to use an
amplifier between the output of the
inverter and the video input of the TV set.
This amplifier may either be the present
one, or that described on page 1-30 of our
January 1984 issue.

Before you can start experimenting, it is
necessary to make a small modification in
the TV set as shown in figure 3. This con-
sists of breaking the connection between

1 2.42 elekta

the demodulator and the video amp/sync applied to the BNC (or jack) socket via C3
separator. The modular construction found and R5. If its amplitude is greater than
in most modem TV receivers makes it 3 V pp , it must be attenuated to that level

easy to find this connection. by P4. This preset is normally so adjusted

The signal level at this point should be that the emitter of T1 is connected direct

2 ... 3 Vp p . The break in the video signal to C3. The signal at the base of T1 should

path will affect the AGC (automatic gain be not greater than 6 V pp . The power
control) setting in the u.h.f. tuner. This supply for T1 must be token from

may be noticeable from either a deterio- somewhere inside the TV set: the u.h.f.

ration in the picture quality or even a tuner normaUy works from 12 V so that a

complete absence of signal. It is therefore suitable supply can almost certainly be
necessary to establish for certain whether taken from this unit,
the AGC is affected or not. If it is not. the
AGC connection to the u.h.f. tuner and i.f.
amplifier can remain, but if it is. the AGC Video input
connection should be broken as shown We have now come to the heart of the

and replaced by Manual Gain Control PI matter. A TV receiver can only be used as

via switch S. The gain must then be set a monitor if it is fitted with a video input,

separately for each transmitter owing to A basic circuit for this is given in figure 3:

differences in field strength. Fortunately, if this is adequate for your purposes, all

this situation is likely to arise only in older well and good, but most of you will prob-

black-and-white seto. ably have set your sights a little higher.

It is quite easy to fit the video output First, let’s have a look at the basic circuit

socket onto the inner back cover of the of the amplifier in figure 3. The transistor

TV set. The d.c. voltage onto which the amplifier has two tasks: (a) to raise the

video signal is superimposed serves to set video signal to the required level, (which

the operating point of emitter follower T1 is set with P3), and (b) to superimpose the

(see figure 3). The video signal proper is video signal onto the appropriate d.c.

2.45

Preset PI allows attenuation of too large
input signals, while too small input signals
are amplified by T1/T2 (gain presettable
with P2). The maximum output level this
i amplifier can provide is about 8 Vp p .

The amplitude of the ac. output signed is
determined by the setting of presets PI
and P2, while the level of the d.c. voltage
depends on the setting of P3 and clamp-
ing diode D2. Values obtained in our pro-
totype are given in table 1: the setting
i limits are dependent upon the output
signal.

Transistor T3 is a buffer which provides
either a normal or an inverted signal to
! the output socket. This stage can also be
connected as an emitter follower.

I The versatility of the circuit in figure 4
j may be demonstrated by a few examples.

Figure 6. A modified cir-
cuit of a video amplifier 0
for type TX television
sets. The BF869 (or

Figure 5 shows sections of circuit
diagrams of a number of black-and-white
and colour television receivers.

Integrated circuit TBA900 is often found in
mains-operated black-and-white receivers.
The video signal is applied to pin 9 of this
1C (see figure 5a). The connection to this
pin should be broken and taken to the
pole of a change-over switch. One of the
contacts of this switch is connected to the
video signal line in the TV set (for
instance, to "M8”), and the other to the
output of the monitor amplifier. In this
case T3, Rll, and R12 (figure 4) may be
omitted and the output signal taken from
C5+. Set the a.c. level to 3 V p n with P2,
and the d.c. level to 2 V with P3.

HAVE YOU REMEMBERED THE MAINS
ISOLATING TRANSFORMER?

A second example may be seen from
figure 5b which shows part of a portable
B/W receiver fitted with a mains isolating
transformer. The circuit of figure 4 is con-
nected as shown for version A (that is,

T3 = BC 547B, collector of T3 to the
positive line, emitter of T3 to the negative
supply line via R12). As the level of the
video signal should not exceed 1.3 V pp ,
preset P2 must be replaced by wire
bridge D-E and the level of the output
signal (for an input signal of 1 V„ p into
75 ohms) preset by PI. The d.c. ravel is set
to 6.8 V with P3.

A third example concerns an older B/W
set partly using valves (see figure 5c). The
video input here should be located
somewhere in the vicinity of TS412. Again,
as in example 1, version A of figure 4
should be used. The change-over switch
is connected as in example 1 but with the
first contact connected to terminal 6 in
figure 5c instead of to “M8”. The d.c. level
should be set to 2 V with P3.

1 2.46 elekto

Our fourth example concerns the quite
common Philips type TX B/W portable,
although the following considerations
apply equally to the portable B/W sets of
most other manufacturers. The relevant
part of the circuit is shown in figure Sd. In
this case, the circuit of figure 4 is con-
nected in version B (that is, T3 = BC 557B,
collector of T3 to earth or negative supply
line, and emitter of T3 direct to output
socket 2 because R350 (figure Sd) serves
here as the emitter resistor. The pole of
the change-over switch is connected to
R350, and the two contacts to the emitter
of T3 (that is, output 2 in figure 4) and
TS350 (a test point in the TV set) re-
spectively.

A further modification, to improve the pic-
ture quality in type TX sets, is shown in
figure 6. As you will see, the TS560 stage
in the original circuit has been replaced
by a cascode stage which increases the
bandwidth of the set up to IS MHz. This
will be especially appreciated by com-
puter owners who want to read 24/25 lines
with up to 80 characters. The modified cir-
cuit is easily built onto a small prototyping
(Vero) board. Transistor BF869 (or
equivalent) must be mounted on a heat
sink.

A fifth example is given in figure Se and
this time it concerns a portable colour TV
which needs special treatment! First, it
needs an isolating transformer and,
second, the video output is designed to
figure 7. Again, the circuit is easily con-
structed on prototyping (Vero) board and
then connected to point B on the monitor
amplifier. Note here that the sync pulse
for the line time base must be generated
separately otherwise the picture quality
will suffer.

Construction and calibration

If your desired setup is covered by one of
the examples given, the completion of the
printed circuit shown in figure 8 should
be straightforward. It becomes a little
more difficult when your equipment is
dissimilar to any discussed so far. In such
a case, it is best to start with the simple
arrangement of example 3 and then attack
the problem with the monitor amplifier.

DO NOT FORGET THE ISOLATING
TRANSFORMER!

The completed printed circuit can in most
sets be mounted onto the inside of the
back cover together with the video input
socket and change-over switch. All signal
lines should be in coaxial cable, while
simple stranded wire may be used for the
supply lines. The wiring between the
television receiver and the printed circuit
may give rise to high-frequency (greater
than 20 MHz) oscillations in some cases.
These oscillations do not affect the picture
but if they are present, a 470 ohm resistor
may be put in series with the output line.

It goes without saying, of course, that
whenever a connection in the set is cut
this should only be done once you are

4 ii

•0 0 - -
nr*

ii

■ffl W "' k

absolutely certain that it is the right one.
Calibration and presetting instructions
have already been given under the
various examples. If your case falls out-
side these examples, follow the circuit
diagram of your particular TV receiver.

The calibration is fairly simple and can be
carried out with a good multimeter and by
keeping a sharp eye on the screen. An
oscilloscope with which the various
waveforms can be checked is, of course,
an advantage. M

Resistors:

R1* = 330 B or 68 B
R2 = 680 B
R3.R4.R10 = 10 k
R5 = 1 k
R6.R8 = 180 B
R7 = 3k3

R9.Rir.R12’ = 470 B
PI’ = 100 B preset
P2 = 2k5 preset
P3 - 10 k preset

Capacitors:

C1.C7.C8 = 100 n

C2.C3 = 10 p/1*5 V
C4.C5 = 100 p/16 V
C6 - 47 p/16 V

Semiconductors:

D1.D2 = 1N4148
T1.T3* = BC 5478
T2.T3* = BC 5576

Miscellaneous:

BNC sockets as required’
SPST switch’

Single-pole change-over

Printed circuit board 84101
’see text

8

2.47

teiephase At last, here is a small unit that provides a quick, safe, and simple

way of checking the presence or absence of a voltage in an electrical
line without the necessity of physical access to the conductor. And,
what's more, it's really cheap to build.

Telephase will enable the detection of a break in any normal, non-
shielded, 'live' electric cable or wire. It is suitable for use with a.c.
voltages from about 60 V to 250 000 V. With a little practice, it should
be possible to gauge the voltage level from the distance between the
detector and the wire at which the indicator LED extinguishes.

F. Pipitone

teiephase

a simple
detector

voltage Circuit description

The circuit is based on a type 4049UB hex
inverter. The sensor is formed by a small
piece of thin (about 0.2 mm) tin plate. The
electro-magnetic field surrounding the live
! conductor or source induces a very small
voltage in the sensor. This voltage is suf-
ficient to start a low-frequency oscillator
formed by inverters N1/N2 and associated
components. The onset of oscillations may
be preset, within a narrow range, by PI.
i The oscillator signal is applied to
; N4 . . . N6 via N3. Inverters N4 N6 are
i connected in parallel to allow sufficient
current for LED D1 to light.

Power is provided by two 1.5 V size N bat-
I teries. Current consumption is determined
primarily by the type of LED used. As the
unit will normally not be used for long
periods at a time, the batteries should last
I 6. . .12 months.

Construction

The unit is constructed on a printed cir-
cuit board: if our EPS84100 is used, no
special problems should arise. Note that
the sensor and the batteries are con-
nected to soldering pins.

The sensor is made simply from a piece
of tin plate about 0.2 mm thick and
40 xl5 mm in area to make it fit neatly in
the proposed case.

The on/off switch is mounted in the side
of the upper part of the case: make sure
that it is clear of the battery and adjacent
to the SI terminals on the PC board after
assembly.

The completed unit is then assembled in
a case of 100 x 50 x 25 mm; a Vero case
202-21027E is ideal.

Operation

Switch the unit on when the LED should

2.48 elefctc

light briefly to indicate that the Telephase
is ready for use.

Test the unit by pointing the sensor end at
a known live source, such as a mains
outlet socket or cable. Switch the
Telephase on and bring it in close prox-
imity of the outlet or cable: the LED
should now remain lighted.

The Telephase is now ready for checking
whether a cable or appliance is live or
not. Always point the sensor end of the
unit at the source being checked.
Approximate distances at which various

voltages may de checked are given in
tabel 1.

Not^ that it may happen that the LED sud- j
denly extinguishes, although the cable
being checked is alive and well! This may
be caused, for instance, by the live and
neutral conductors being twisted v'.’hich
gives rise to zero nodes in the electro-
magnetic field. If the LED therefore sud-
denly goes out, check the immediate
vicinity to make certain that the Telephase
is not situated at such a node. M

12.49

valve amplifier

A 10 Watt hi-fi
amplifier with
just four valves

Of late there seems to be a renewed interest among audio
[ enthusiasts for valves. Valve amplifiers are 'in'. Those in the know
now say, as. indeed, they always have, that valves sound better than
j transistors. The fact that we have designed a valve amplifier does not
| necessarily mean that this is our opinion. Appreciation of sounds is,
j in any case, purely a personal matter so everybody simply has to
l decide what he personally prefers. That is now very easy, at least for
anybody who builds this 'good-old-fashioned' amplifier it is.

With the invention of the transistor, valves
lost their 'monopoly' as the active element
in electronics. They have never com-
pletely disappeared, however, and for
many applications, especially where a lot
of power has to be handled, they are

Specifications

nominal output power: 10 watt into 4, 8
or 16 ohm

maximum output power: 12 watt
harmonic distortion: 0.5%

(SO Hz... 20 kHz)
signal/noise ratio: depends on
individual circumstances
input sensitivity: 200 mVrms
input impedance: 1 MQ
damping factor 25
frequency characteristic:

20 Hz. . .40 kHz + 1 dB (at 1 watt)
feedback: about 26 dB

indispensable. Even in cases where where
the transistor might seem to be the logical
choice, however, valves are still to be
found. Some audiophiles, as we have
already mentioned, prefer tubes but ham
radio users have also refused to let the
‘new-fangled’ transistor supercede their
beloved valves.

The HF people favour tubes for their
indestructibility and power handling
characteristics, the audio enthusiasts for
other reasons. They consider that valves
have a different (and better) sound than
transistors. Whether this is so or not there
is certainly a resurgence in interest for
valves. Just one of the indications of this is
seen in the increasing number of valve
power stages in the high-end sector.

We have also been bitten by the valve
bug, as witnessed by this amplifier. The
power output is quite small (10 watt) but
this could be just the beginning. We may
even come up with a heavier version at
some stage in the future (but that is not a

1 2,50

promise). The valves themselves are still
readily available so that will not be a prob-
lem; the ones we use are actually adver-
tised in this issue.

A classic circuit
Those who grew up with valves will
recognise the 'classic' layout of the circuit
diagram. It is shown in a modernised form
in figure 1: this is how it would look if it
could be built with semiconductors. Our
showing it like this is, of course, a com-
plete reversal of the situation of a quarter
century ago when designers convened
the new-fashioned transistor circuit
diagrams back to valves in order to
understand what was going on.

Compared to modern circuits, the layout
of figure 1 looks extremely simple. All it
has, basically, is a preamplifier stage (Tl),
a differential stage (T2) and two power
transistors. Such a layout would be
impossible to achieve with normal bipolar
transistors; at very least a few drivers
would have to be included. This is one
obvious advantage that valves have. As far
as modem semiconductors are con-
cerned, only the MOSFET can be con-
sidered in any way comparable to tubes.
Having seen how simple the circuit
design is, we can now move on to the
actual circuit diagram, as shown in figure
2. If we ignore compensation networks,
decoupling capacitors and so on, we see
that the circuit is essentially the same as
that of figure 1. An EF86 pentode (VI) acts
as a preamplifier, a double-triode ECC83

(V2) forms a differential amplifier, and
finally two EL84 pentodes make up a
push-pull stage, that drives the
loudspeaker via an output amplifier.

The EF86 is connected as a triode and has
a gain of about twenty times. Filter R6/C3,
which is in parallel with anode resistor RS,
ensures that the gain is reduced at high
frequencies. This is necessary in order to
achieve stability. The phase shifting
needed for driving the power elements
(V3 and V4) is provided by the ECC83
double-triode with cathode coupling. This

Figure 2. The circuit
diagram for this amplifier
is very unusual for
Elektor. The reason for
this is. of course, the fact
that it contains four
valves. Note that the
values of C2 and R4
depend on the impedance
of the loudspeaker used.

2

12.51

amplifier

3

•differential stage’ is used because it The signal from the secondary side of the

keeps distortion to a minimum and output transformer is fed back to the non-
enables a direct coupling to be made to decoupled pan of the cathode resistor of

the preamplifier tube. The reasoning is VI. The values given to the feedback net-

easy to understand knowing that the grids work (C2/R4) depend on the impedance
in the double-triode must have a positive of the loudspeaker used. The relevant

potential due to the large voltage drop values are given in the table at the top

across cathode resistor R7. right-hand comer of figure 2.

The power stage consists of a conven- The power supply is straightforward and

tional push-pull circuit with two EL84s set follows the well-known transformer, bridge
to an anode voltage of 310 V. There is no rectifier, electrolytic, formula. In this case

need for V3 and V4 to be paired as each we have used a supply transformer in-

has its own cathode resistor (R17, R18). The tended for use with valves. This has two

improvement here would, in any case, be secondary windings to provide the anode

very small. The resistors in series with the voltage of 250 V at a minimum of 75 mA

grids (R15, R16) and the screen grids (R19, and the filament current of 2 A at 6.3 V.

R20) improve the stability. Some output
transformers have special screen grid tap-
off points on the primary side. If these are Construction

available points A and C should be con- Although this sort of project would have

nected to them and the power stage will been constructed differently before, there

then be ‘ultra-linear’. If the transformer is now no reason why it cannot be

used does not have this facility A and C mounted, as a transistor amplifier would
should simply be connected to the be, on a printed circuit board. The valve

positive supply at points R mounting sockets for insertion into printed

12.52

here (2 x50 ij/ 450 V in a single case) but a
single 100 jj/ 450 V type may be used
instead. When fitting the components to
the printed circuit board simply follow the
sequence normally used. The valves are
delicate, of course, so fit them last.

We have already mentioned the trans-
formers briefly. The mains transformer
must have at least two different secondary
windings as we need 250 V at 75 mA and
6.3 V at 2 A. The output transformer must
have an impedance of 2 x 4 kQ at the
primary side, preferably with screen-grid
tap-off points. The impedance at the
secondary side depends on the loud-
speaker that is to be used. Well-informed
suppliers will know what you want if you
just ask for a 10 watt valve output trans-
former, or a transformer for a 2 x EL84
push-pull stage. If you were in the habit of
‘salvaging’ parts from radios when valves
were in fashion there may be a suitable
transformer lying at the bottom of your
junkbox. Don’t reject it simply because of
its vintage; it may be just what is needed.

circuit boards have been available for a
long time and the other components are
the same as a modem amplifier would

The printed circuit board that we have
designed for this project is seen in figure
3. In spite of its compactness everything
fits on the board, except for the two
transformers and R21 (which is soldered
across the loudspeaker terminals). In
general, construction is just the same as
for any other Elektor project but there are
a few points to note. The board does not
contain any tracks to feed the valve
filaments so these must be wired by hand.
Make sure the cable used for this is
capable of handling the filament current
of 2 A. It is also wise to twist these two
wires together. The filament connections
are pins 4 and 5 for VI, V3 and V4, but
pins 4, 5 (already joined on the board) and
9 for V2.

Plenty of room has been left on the board
for mounting smoothing capacitors Cll
and C12. We used a double capacitor

12.53

Case and wiring

In a mechanical sense it is very easy to
■finish’ this amplifier and make it into a
very attractive project. Unlike power tran-
sistors, valves do not have to be mounted
on heatsinks. This makes the choice of a
case easier. As long as everything fits
inside, any sturdy metal case will be
suitable. It is important to have enough
ventilation slots in the case as the valves
dissipate a lot of heat and this must be
dispersed. If the case is just big enough
to fit all the components it is a good idea
to mount the printed circuit board on its
side. The valves will then be horizontal
and can pick up as much cooling air as
possible.

A very important part of building any
amplifier is the wiring. If this is not done
carefully the chances are that a lot of hum
will be generated and that can be very
difficult to get rid of. In principle the
same rules apply when wiring any
amplifier, whether it has transistors or
valves. The most important points are:
Always use a single central ground point

and wire all the amplifier's ground con-
nections directly to this. The ground
should be connected to the metal case
either from the central point or directly at
the input; try both and use whichever
gives less hum. Lines from the input
socket to the board must be made with
screened cable. Finally, keep all the wir-
ing as short as possible so as to minimise

Make sure that the correct polarity is used
for the feedback connection from the out-
put amplifier. If the loudspeaker connec-
tions are reversed the amplifier will be
heard to oscillate.

Before applying power check that the
anodes of V3 and V4 are connected to ‘ + '
(via Trl if applicable). Failure to do this
will mean that the screen grid will take on
the job of the anode and this is something
that is not recommended (not even by
'Murphy’s handbook of self-destructive
valves and other associated phenomena’!).

Final points

By now the printed circuit board should
be completely assembled and all the
appropriate ’bits and pieces’ should be
correctly wired up. It is time for the ‘acid
test'. When power is applied to the
amplifier it should work properly. No
calibration or adjustment is needed.

Before use, however, check that the test
voltages shown in figure 2 agree with
those measured on the printed circuit
board. If they do not, then recheck
everything on the board and all the wiring
because there is undoubtedly a mistake in
it somewhere. If you want a stereo valve
amplifier remember that all the com-
ponents must be duplicated. This means
that not only do you need two printed cir-
cuit boards but also two mains
transformers and two output transformers.

I

12.54 elekto

liversal NiCad charger

universal
XiCad Hiiinjif

one charger for all NiCad cells

It is not possible to connect NiCad cells
in parallel in order to charge them from
one power source simultaneously be-
cause of the tolerance in the charge
characteristics and the various initial
charge conditions of the cells. The
charge current can only be determined
exactly if the cells are connected in
series. The current depends on the
capacity (number of mA) of the cells.
Most of them can be charged in 1 4 hours
With a current that is 0.1 x number of
mAh. This current will ensure that the
cells won't be damaged if they are left
on charge for too long, and for a charge
of at least 14 hours, it doesn't matter
whether the cell is completely ex-
hausted or not. It will be obvious that a
universal charger must have an adjust-
able output current, because each
different type of cell requires a specific
charging rate.

NiCad cells are an economic alternative to batteries, but if you have to
buy a special charger for each type of cell, this cheap alternative turns
into an expensive one. The solution to this problem is a charger that is
able to charge the whole range of cells. As you may have suspected, this
article deals with such a device. To prevent any damage to the cells,
the charger is also protected in the event of an incorrect connection.

The circuit diagram

I Figure 1 shows the complete circuit
| diagram of the universal NiCad charger.
A current source is built around the
transistors T1, T2 and T3, which pro-
vide a constant charging current. The
current source only comes into oper-
ation when the NiCad cells are connec-
ted the right way round (positive to +
and negative to -). It is the task of IC1
to verify the connection by checking
the polarity of the voltage at the output
terminals. When the cells are connected
correctly, pin 2 of IC1 won't be as
positive as pin 3. Therefore the output
of IC1 becomes positive and supplies a
base current to T2, which switches on
the current source. The desired level
of the current source can be set with the
aid of SI. A current of 50 mA, 180 mA
j and 400 mA can be preset when the
values of R6, R7 and R8 are known.

| Putting SI in position 1 means that the
penlight cells will be charged, position 2
is for C cells and it is the D cells' turn in
position 3.

The current source functions very
simply. The circuit is a current feedback
system. Suppose that SI is in position 1
and IC1 output is positive. T2 and T3
! are now supplied with a base current
and start to conduct. The current
| through these transistors produces a
voltage across R6, thus causing T 1 to
conduct. An increasing current across
R6 means that T1 will conduct more
thereby reducing the base drive current
to transistors T2 and T3. The latter
transistors will now conduct less and the
original current increase is opposed.
A fairly constant current through T3
| and the connected NiCad cells is the
logical result.

Two LED's that are mounted on the
I current source show whether and how
| the NiCad charger is working. IC1
supplies a positive voltage when the cells
[ are connected correctly and D8 will
j light. With an incorrect connection,

I pin 2 of IC1 will be more positive than
,n dia december 1984 1 2.55

pin 3, so that the opamp, which is wired
as a comparator, has 0 V output. Now
•he current source isn't switched on and
LED D8 will not light. The same holds
good for the case when no cells are con-
nected, since pin 2 will have a higher
voltage than pin 3, caused by the volt-
age drop across 010. The charger will
only work when a cell containing at
least 1 V is connected.

LED D9 indicates that the current
source is functioning as a current source.
This may sound a little strange, but an
input current produced by I Cl isn't
sufficient, there also has to be a voltage
level high enough to stabilise the cur-
rent. This means that the supply must
always be higher than the voltage across
the NiCad cells. Only then will there be
a high anough level for the current
feedback T1 to function, causing D9to
light.

Practical points

Figure 2 illustrates the track pattern and

component overlay of the printed
circuit board. Except for the trans-
former, all components can be mounted
on the board. A heat sink for T3 is a
must, since the transistor will run warm,
especially when only a few cells are
being charged. It is therefore rec-
ommended that a transformer with a
centre tap is used, so that a lower supply
voltage can be selected by means of S2.
This centre tap not only prevents T3
from getting overheated, but it also
saves a lot of energy. Diode D9 lights to
indicate that there is sufficient supply
voltage.

As stated before the penlight cells are
charged with a current of 50 mA when
SI is in position 1. C and D type cells
can be charged with 180 mA and
400mA respectively (positions 2 and 3).
The value of R6. R7. or R8 must be
changed if other charging currents are
required. The desired value can be
found by dividing 0.7 V by the charging
current. For example, for a charging
current of 100mA a resistor value of

0.7 V : 0.1 A = 7 SI is required. Currents
up to 1 A are possible, however it must
be remembered that T3 will require a
larger heat sink. We will not complain
or object if you replace SI by a switch
with more than 3 positions.

Resistor Rx in figure 1 is shown in the
position for one further current rate if
desired.

Charging NiCad cells takes about
14 hours. It is wiser to use sintered cells,
because they won't be damaged if this
limit is passed. 14

2.57

SPEAKERS FOR COLOUR TV

LUXCO Electronics have introduced a
new range of speakers tor colour TV
These speakers are designed and
manufactured to meet the specific
needs of colour TV manufacturers. The
magnet of the speaker is shielded with
a compensating magnet of high
permeability to reduce stray magnetic
fields which cause distortion in the

The frequency response is FO-IO.OOO
CPS and the resonant frequency is 130
CPS. These speakers are available in
three different models —

10 * 15 LCT 6 D, 8 * 13 LCT 5 and 10
LCT5D.

and low power transistors. SCRs and
diodes, heat sinks for ICs have also
been developed Transistor insulating
sets and heat sink compound are also
available. Technical data of dimen-
sions. weight, thermal resistance etc.
are available in form of a catalog.

For further information, write to :
Jain Electronics.

F—37, Nand-Dham Industrial Estate.
Marol, Bombay 400 059.

monitoring winding temperature and
surface temperature.

For further information, write to :
Chowdhry Instrumentation Pvt. Ltd.

1 10, Model Basti. New Delhi 1 10 005.

DATA LOGGING PRINTER

The new Data Logging Printer from
ELECTRONUMERICS is suitable for
logging of fast changing digital data
and multichannel analogue data. 32
digit BCD data in single channel or
divided in 2 ro 4 channels of 16 or 8
digits each can be accessed and
printed at the set interval. The logging
interval can be as short as one
millisecond. The print initiation can be
through a push button on front panel or
through an external trigger pulse. Fast
paper feed and paper loss detection
facilities are provided in the printer.

For further information, write to:
LUX Ml 4 CO.

56, Johnstonganj,

Allahabad-211 003.

KNOBS

MARVEL Industries introduce a
complete range of knobs for electronic
instruments and entertainment pro-
ducts. The range includes alluminium
anodized diamond-cut knobs and
plastic moulded knobs with diamond
cut aluminium anodized fronts Knobs
are also manufactured as per cus-
tomer's specifications.

For further information, write to :

Marvel Industries

208, Allied Industrial Estate,

Mahim, Bombay 400 016.

HEAT SINKS

JAIN Electronics offer a range of 20
heat sink designs manufactured from
Alluminium alloy 6063 — T6. The
designs are suitable for high, medium

DC TO DC CONVERTER
S BAJ introduce their new DC to DC
converter with input - 50 Volt to — 24
Volt D.C. 6 Amp. It is designed with
P.W.M. technique with over current,
over voltage and short circuit
protection with LED indication. All
filters and transformers are designed
with ferrite cores. The efficiency is
claimed to be 90%. The DC to DC
converter finds application in Railway.
Telephone and Posts & Telegraph
departments.

For further information, write to:
Sbai Electronics.

19. Mother Gilt Bldg.

Opp. Novelty Cinema.

Grant Road. Bombay 400 007.

RESISTANCE THERMOMETER

The Resistance Thermometers from
CHOWDHRY Instrumentation Pvt. Ltd.
are electrical thermometers which over
cover a range between -220°C to *
750°C. The range can be extended upto
x 850°C in special cases. Resistance
elements are manufactured using
Platinum. Nickel or Copper and
formers can be of Mica. Ceramic or
Glass. The elements are provided in
protecting tubes and thermowells.
Special elements are available for

For further information, write to .

Electronumerics

Kammagondanahalli

Opp. HMT Industrial Estate.

Jalahalli West, Bangalore 5 60 015.

MAGNETIC COILS

Shepherd Transformers manufacture
Mahavir' brand magnetic coils, which
have the main application for creating
temporary magnets. The coils are
manufactured in various sizes, shapes
and capacities according to the
customer's specifications. Wound on
bakelite formers, the coils are also
insulated with interlayer insulation
Epoxy moulding can also be provided
for H.V, coils.

For further information, write to :
Shepherd Transformers
Shed No. 4, Vallabh Society.

90 feet Road, Ghatkopar (East),
Bombay 400 075.

VHF/UHF TUNER

SIEL Electronics of Italy have
introduced a new VHF/UHF electronic
tuner type F-500 for use in colour TV
receivers. The tuner covers VHF and
UHF channels of C.C.I.R. system. The
circuit uses MOSFETs and chip
components Input filters are provided
with IF and FM suppresion circuits
Oscillator radiation, frequency drift
and microphonics meet international
standard specifications. The tuner is
fully tropicalised and can work
satisfactorily upto ambient temperature
of 50°C. The mounting can be directly
on the PCB of 2.5 mm grid.

PHOTOMETERS

United Detector Technology. U.S.A.
have added a new photometer S 351 F
to their existing S 351 series of
radiometers and photometers for laser,
fiber-optics. CRT luminance and UV
power measurements. The S 351 F has
a photometric detector with a 15°
field— of view lens and ambient light
shade. CRT luminance can be
measured directly by holding the
detector against the CRT. The shade is
made of soft rubber and does not
scratch the face of CRT.

For lurlher information, write t <
Surest} Electrics & Electronics
Post Box No. 9141
3 B, Camac Street.

Calcutta 700 016.

SOLDERING IRON STAND

The makers of Soldron' Soldering
Irons have introduced a Stand for their
soldering irons.

The stand consists of a heavy cast iron
base, painted in oven baked hammer
tone finish The thick plating of coil
prevents corrosion and the soldering
iron holder is made of turned
aluminium. A sponge is also provided
for cleaning tip the stand can be
mounted on a table or a vertical
surface.

The stand is useful on production line
for easy access and for prevention of
damage to expensive raw material due
to unexpected contact with the
soldering iron.

For further informant
Rao & Associates
79. 2nd Main
Defence Colony
Bangalore 560 038.

MODULAR JACK RETAINER

The FCC— 68 Modular Jack Retainer
and Receptacle Housing has been
introduced by Molex Inc. This is a new
addition to the Molex connector
product series. The jack retainer 90080
is designed to be custom harnessed.
The body is moulded in gray
polypropylene and the contacts are
selectively plated with electro tin in the
ID area and gold in jack contact area.
The retainer is to be used with Molex
receptacle housing 90079, which is
moulded in polyester.

meriiceg

PCB SUPPORT

SEE offers non-conductive printed
circuit board supports. These are
designed for secure and rapid
installation. The supports have a plug-
in fastening method and allow quick
access for maintenance or equipment
modifications. Its tension retaining
system allows positive fastening with
the advantage of being detachable
when required.

For further information, write to:
Toshni—Tek International
267, Kilpauk Garden Road,

Madras 600 010.

DIGITAL CLOCK

ION Electricals offer a mains operated
digital clock module with quartz crystal
oscillator for high accuracy. The
operation is independent of mains
frequency or mains voltage variation.
The clock is designed to display time in
12 hours mode with AM or PM
indication. 15 mm green fluorescent
display is used, which can be changed
to orange or blue using suitable filters.
Back up battery can also be provided.
The module is available in open
construction and can be fixed in any
suitable enclosure.

For further information, write to:
Jay Electric Wire Corporation Ltd.
I 202. Maker Tower 'E' 20th floor,
Cuffe Parade, Bombay 400 005.

For further information, write to:
Bombay Insulated Cables & Wires Co.
74. Podar Chambers,

S.A. Berlvi Road, Fort,

Bombay 400 001.

For further information, write to:
ION Electricals

307, Owner's Industrial Premises
505, Gabrial Road.

Mahim, Bombay 400 016.

12.59

s»-

COXCO

SP€RK€RS FOR:

COLOUR TV

Speakers for B/W TV: 10 * 15 LG 6 TV (4" * 6" Oval), . 10 LG 5 TV (4" Square).
7» 10 LG 2 TV (21/2" x 4" Oval). 8» 13 LG 5 TV (31/4" x 5" Oval),

8 LG 1 TV (3'4" square) For Portable TV.

The LUXCO Range also covers speakers for:

Transistors. Tape recorders. Stereo systems.
Car Stereos. Intercoms and P.A. systems.

□ Manufactured by:

LUXCO Electronics
Allahabad-211 003

O Sole Selling Agents:

LUXMI & CO.

56. Johnstonganj
Allahabad— 21 1 003.

Phone: 54041. Telex: 540-286

□ Distributors for Delhi & Haryana:
Railton Electronics

Radio Place. ChandniChowk
Delhi-110 006.

Phone: 239944, 233187.

□ Distributors for Maharashtra,

Gujarat and South India:

precious

Electronics Corporation

• Chotani Building. 52. Proctor Road.
Grant Road (East). Bombay-400 007.
Phones: 367459, 369478

• 9, Athipattan. Street, Mount Road.
Madras-600 002, Phone: 842718.

Wanted stockists oil over Indio

sound technology from a sound source

Classified ads advertisers index

APLAB 12 13

ARUN ELECTRONICS 1 2 69

BALAJI 12 12

BRISK 1 2 06

COMPONENTS TECHNIQUE 1 2 68

COSMIC 1 2 72

DANNIES ELECTRONICS 12 04

DEVICE ELECTRONICS 1 2 05

DOMINION RADIOS 12 06

ELCIAR 12 68

ESSEN DEINKI 12 10

IEAP 12 10

INDUSTRIAL RADIO HOUSE 12 12

ION ELECTRICALS 12 69

KELTRON 12 09

LAKSHON ELECTRONICS 12 12

LUXCO 12.67

MELTRON 12 07

MFR 12 65

PADMA ELECTRONICS (P) LTD 1 2 06
PHILIPS 12 11

PRECIOUS 12 02

R P ASSOCIATES 1 2 04

SAI ELECTRONICS 12 69

SUDHIR SURTI 12 04

VASAVI 12 10

VISHA 12 71

ZODIAC 12 68

CONDITIONS OF ACCEPTANCE OF CLASSIFIED ADVERTISEMENTS

1) Advertisements are accepted
subject to the conditions
appearing on our current rate
card and on the express
understanding that the
Advertiser warrants that the
advertisement does not
contravene any trade act
inforce in the country.

2) The Publishers reserve the
right to refuse or withdraw any
advertisement.

3) Although every care is taken,
ihe Publishers shall not be liable
for clerical or printer's errors or
their consequences.

4) The Advertiser's full name
and address must accompany
each advertisement submitted.
The prepaid rate for classified
advertisement is Rs. 2.00 per
word (minimum 24 words).

Semi Display panels of 3 cms
by 1 column. Rs. 150.00 per
panel All cheques, money
orders, etc. to be made
payable to Elektor Electronics
Pvt. Ltd. Advertisements,
together with remittance, should
be sent to The Classified
Advertisement Manager. For
outstation cheques
please add Rs. 2 50

Electronics Tools like Soldering Irons
Pliers,, Cutters, Screw Drivers
Tweezers at Competitive Prices. Con
tact Aradhna Electronics (P) Ltd., 1C
Srinath Complex, Sarojinidevi Road
Secunderabad 500 003.

COVOX 1500

Components: 2 full range
woofers. 16 cms. 1 tweeter
•40-18,000 Hz.

• "30-40 Watts.

COVOX 3500

Components: Enclosure-
Infinite baffle, sealed. 1
acoustic - suspension
woofer 20.32 cms.. 1
acoustic-suspension mid-
range. 1 tweeter, with
divided network.
•30-18.000 Hz.

"60-100 Watts.

COVOX 4500

Components: Enclosure-
infinite baffle, sealed. 1
acoustic - suspension
woofer 25 cms. 1 acoustic-
suspension mid-range 16
cms.. 1 tweeter, with divided
network.

•30-20,000 Hz.

"60-200 Watts.

COVOX 6000

Components: Enclosure-
' infinite baffle, sealed. 1
acoustic-suspension woofer

COVOX 2500

Components: Enclosure-
infinite baffle, sealed, 1
acoustic-suspension woofer
16 cms.. 1 acoustic-
suspension midrarige. 1
cone-type tweeter, with
divided network,

•30-18.000 Hz.

• "40-60 Watts.

COVOX 5000

Components: Enclosure-
infinite baffle, sealed. 1 full
range woofer and mid-range
combined 25.4 cms., 1
tweeter, with divided
network.

•30-20,000 Hz.

”100-200 Watts.

COVOX 7000

Components: 1 full range
woofer and 1 mid-range
combined 30 cms., 1 dome
tweeter.