ISSN 0970-3993

ejef^jtOIf ■

electronics

THE PRACTICAL MAGAZINE WITH THE PROFESSIONAL APPROACH

10.03

Front cover

A scientist at Bristol
Polytechnic's Transputer
Centre illustrates the
transputer's
capabilities in solving
the Mandelbrot Set,
sometimes described
as the most com-
plicated object in
mathematics.

Bristol Polytechnic's
Transputer Centre has
been set up to carry
out research and de-
velopment on uses of
the transputer in in-
dustry, help companies
to exploit the parallel
processing potential of
the transputer, and to
train students and per-
sonnel in its use.

OPEN SYSTEMS AT
CROSSROADS?

When early last year it was announced that a group of major European and
American computer manufacturers, the X/Open Group, had agreed to adopt
AT&T's UNIX operating system as an industry standard, it seemed that we were
at last moving towards an open standard of sorts. But, alas, these intentions
are in danger of going the same way as those of the makers of MSX
machines: to Never Never Land.

A number of the X/Open Group participants, among them IBM, DEC, Siemens,
and Honeywell Bull, have accused AT&T of attempting to influence
developments in computer hardware by using UNIX as a lever. They also
claim that certain computer companies in which AT&T has a stake, particu-
larly Sun Microsystems, are given advance notice and the opportunity of in-
fluencing future versions of UNIX.

AT&T, backed by Unisys, the world's second largest computer manufacturer,

ICL, and Xerox, says that all it is trying to do is to unify the many different
variants of UNIX into one consistent system that will allow all computers run-
ning it to be fully interoperational.

While AT&T maintains that it is fully committed to keeping UNIX open and giv-
ing all computer manufacturers unbiased access to future versions of UNIX,
the dissenting companies claim that they have been refused to lend a hand
in the development of UNIX, although Sun is doing so.

This whole rumpus is, of course, about money — lots of money. Dataquest, the
US market research organization, estimates that the world computer market
will amount to some $ 11 billion by 1992 and to perhaps more than $ 20
billion by the mid-1990s. Already, UNIX has more than five per cent of this
market, and this share is likely to grow to over ten per cent by 1992. That
would put AT&T in a very strong position to influence hardware development.

Since AT&T has apparently not agreed to grant the dissenting companies the
same facilities as Sun, these manufacturers, led by Hewlettt Packard and IBM,
have set up Open Software Foundation. They claim that this non-profit making
organization, in which they have invested hundreds of millions of dollars, is in-
tended to develop a new version of UNIX that will not be under the control of
AT&T, but will be truly open.

Although this would seem to indicate (encouragingly for users the world over)
that the major manufacturers are converging on an open system that their
customers have been clamouring for since the early 1980s, are we right in be-
ing optimistic? After all, the X/Open Group is not dead. In fact, IBM has only
just joined the 15 manufacturers, including AT&T and Unisys, that participate in
this group. So, there are now two powerful groups of major computer
manufacturers (seven of whom belong to both groups) whose aim it is to pro-
mote UNIX as a common standard. But which UNIX?

Users the world over can only hope that the two groups will be able to bury
the hatchet soon and together produce a universal operating system.

10.05

VIDEO THEATRES

Entertainment Revolution

An entertainment electronic revolu-
tion, virtually gone unnoticed so far
and holding the potential of sweeping
the country, has already been ushered
in. The introduction of video theaters
has not been heralded with any fan-
fere and if knowledge of its existence
and potential is still poor, it is mainly
because it is rooted in smaller towns
and mofussil areas. Paradoxically, the
video theater seeds have been sown in
the countryside and not in the met-
ropolitan cittes as is the wont of
those testing markets of electronic
products. Except for the video theat-
ers, almost every electronic innova-
tion has been introduced in large,
commercial cities.

“It may sound unusual that a technol-
ogy of such magnitude should have a
begining in smaller towns”, says Mr
S.C. Hira the pioneer in the Indian
Video Field who gave this country its
first video feel. Some ten years ago.
Hire’s Esquire ushered in video in
India through his factory at the San-
tacruz Electronic Export Processing
Zone (SEEPZ). The justification of
propagating video theaters in mofiusil
areas is that the medium is most
suited to small towns where cinemas
are in great demand but the halls sc-
reening them are far fewer.

Video theaters will transform audi-
ences for the cinema. “You will see
how dramatic this transformation is
when the triumph of this technology
is exquisite" points out Hira. He ad-
mits to considerable interest among
prospective cinema theater owners in
installing video projection systems.
While it is unlikely that the existing
owners of conventional theaters will
convert to the video technology, the
new proprietors of cinema might
favour videos right at the beginning.
The advantage of operating a video
system in a cinema theater is man-
ifold. Firstly the operator has only to
insert a video film into the projection
equipment system and not the messy
35 mm films in cans. The running
cost is much cheaper because of
lower power consumption, easier
maintenance, cheaper price for video
compared with that for the 35 mm
software and a smaller manpower
complement Above all, the system is
longer lasting and provides a relatively
trouble-free service.

In several parts of the world, video
theaters have already gained much
popularity. Entry of Esquire with the

backing of well-known National of
Japan, video theater technology is set
for a major breakthrough. A few other
parties, including the one in Madras
in South, have offered a similar
technology and path has been cleared
for more competition. However, Na-
tional claims to have incorporated
some special features in their package
said to be not available in other sys-

Most of the patronage for the video
theaters now comes from Gujarat,
Rajasthan and other states in North
India. Although the prospective
cinema theater owners have begun
acquiring the video theater technol-
ogy neither have they advertised their
projects nor have the suppliers in
Bombay commenced aggressive mar-
keting strategy. Perhaps everyone is
watching the other and adopting a
cautious attitude to business. How-
ever, the technology has been well
adopted in several parts of the world.
India is one of the last few nations 'to
go in for the video theaters.

The National technology being
adopted by Esquire following the lat-
ter’s receipt of a letter of intent in April
this year is capable of enlarging the
picture on a screen six times the nor-
mal Television size. 'Hie magnifica-
tion does not blur the picture and the
resolution is considered as good, and
even better, than the conventional
cinema in regular theatres. Tire
technology is ideal for community vie-
wing. A video projector connected to a
VCR can be used for either showing a
recorded programme or Doordarshan
telecast programme through the
VCR’s tuners.

The video projection systems (VPS)
for enlarged screenings of video films
is expected to gain in popularity in
coming years for also receiving gov-
ernment sponsored programmes for
rural developments, eradication of po-
verty, propagation of family welfare
and mass communication through
Doordarshan’ s regional and network
programme. Video projectors, in con-
trast with conventional movie projec-
tors, require less infrastructure in
terms of capital investment, land and
building, power and maintenance. Be-
sides the running cost is virtually neg-
ligible and the tape itself available
cheap, the video option for many
cinema owners is a strong one. The
audience capacity in these theatres
will be ideally 15 to 200. For the view-

ers in terms of eye strain, it will be the
same as watching TV. Moreover,
there will be a lifelike image projected
on the screen. The projection room
space requirement is also less in that
the VPS takes very little room and is
installed in the theater itself, between
the audience and the screen to be pre-
cise. The reproduction of colours on
the screen will be the same as on TV
and three primary colour emitting
lamps in the VPS takes care of all com-
binations. Unlike the TV, however, it
will be necessary to switch off the’light
during the screening. How low is the
power consumption is borne out by
the fact that a three-hour projection
will involve only one unit of current,
against some 1 2 units in conventional
cinema.

Several innovations are possible on
the tape itself at tire editing table.
Extra and special effects can be pro-
vided by manipulating the tape, cut-
ting scenes and rejoining them at pre-
determined sequences. Portions
erased could be restored. The
medium thus lends itself to endless
experimentation.

In due course, films for the VPS might
just as well be shot in video camera
making the entire project far cheaper.
Currently, the film processing
laboratories cost a lot for additional
prints. Much of this cost can be ob-
viated by shooting on video films and
re-shooting by erasing the earlier
sequence if the shot is not well taken.
The entire equipment is also mobile.
It can be packed and carried easily
from one venue to another. Anybody
with space for screening the VPS
could hire out the equipment. It is
also possible for corporate agencies to
acquire the equipment on lease for
projecting the product profile or for
training of manpower.

Video theatres might even push Out
cinema from the strongholds in cities.
This possibility is attributable to the
limited availability of theaters.

There are some 6,800 cinenia houses
spread over 3, 200 towns in India with
the seating capacity' ranging from 300
to 1,050. of these 50 per cent are in
the Southern region, 19 per cent of
the remaining in the western region
and 12 per cent in the eastern region.
In addition there are some 4,500
touring cinemas. These and the many
towns with populations of 5,000 and
20,000 currently not served by
cinema are potential patrons of the
VPS.

Br 1988 1 0.21

64 KBYTE STATIC RAM
EXTENSION FOR MSX COMPUTERS

Although the concept of MSX allows the addressing of up to
1 Mbyte of memory, the number of computers that use more than
128 Kbyte is surprisingly low, and ready-made RAM extension
modules thin on the ground. We decided to do something about
this, and developed a plug-in RAM extension that enables MSX
users to increase the total available memory of the computer in
steps of 32 or 64 Kbyte.

With a mere 64 Kbyte installed as a stan-
dard, and 128 Kbyte available on newer
models only, MSX computers do not
follow the trend towards the use of vast
amounts of system ' memory. The
diagram of Fig. 1 shows the theoretical
memory structure of the MSX concept,
which was originally designed for
1 Mbyte of addressable memory. In
practice, however, there is not a single
MSX computer that actually uses all of
the available system memory.

In principle, any MSX computer can
have up to four so-called primary slots,
which are, in turn, subdivided into four
blocks of 16 Kbyte. The BASIC and sys-
tem ROM are located in the address
range of the first slot (number 0). The
two ROMs use up half the memory in
this, occupying address range OOOOh to
7 FFFh, i.e., two blocks of 32 Kbyte.
Random access memory is usually
located in another slot, and in address
range 8000 h to FFFFh. After a reset,
the control system runs a test routine to
examine which slots hold RAM.

A slot can be expanded with the aid of
additional hardware. Slot expansion
makes it possible to use four equal banks
per slot. Like the slot itself, these banks
are in principle composed of four blocks
of 16 Kbyte. In practice, a slot expander
circuit enables extending the memory ca-
pacity of a primary slot from 64 to
256 Kbyte.

Table 1 lists the slot structure of a num-
ber of MSX computers, and also shows
which slots are expanded internally. The
function of the so-called memory map-
per in MSX-2 machines can be disregard-
ed as far as the present RAM extension
card is concerned. Most MSX computers
have one or two non-expanded slots, so
that 64 or 128 Kbyte of RAM can be
added without problems.

More memory, more
workspace?

When running in BASIC, MSX com-

10.22 elektor india October 1988

puters have relatively little free memory
— in practice, this hardly ever amounts
to more than 23 Kbyte. It may come as
a surprise that adding 128 Kbyte of
RAM does not resolve this limitation,
since BASIC can not address this ad-
ditional memory. Does this make any
RAM extension useless? Fortunately, the
answer is no. Evidently, the present cir-
cuit would not have been developed if
the computer could not benefit from it.
There are programs capable of using the
extra memory by bypassing the memory
handling routines in MSX BASIC. Still
other programs can only work when ad-
ditional memory is installed, and the
above limitations of BASIC are, of
course, unknown when machine code is
used.

In a number of cases, the RAM exten-
sion card described makes it possible to
run older programs on more recently in-
troduced computers. This is because the
first releases of some programs did not

assume that the 64 Kbyte of memory
was divided over several slots. This, how-
ever, is not strictly required according to
the MSX standard. In the case of the
present RAM extension, this rule is, of
course, observed.

In BASIC, the extension card offers an
interesting feature by allowing memory
to be made ‘read-only’ for testing
whether a machine code or BASIC
program can run from EPROM. Pro-
grams developed by the user and in-
tended for storing in EPROM can,
therefore, be tested in RAM, obviating
tire need to clear and load EPROMs for
every minor change in the program (an
EPROM programmer for MSX com-
puters was described in (l) ).

Because the internal memory is nearly
always in a ‘high’ slot number, the con-
trol system does not encounter it until all
other slots have been examined for the
presence of RAM. The control system
uses the first RAM bank found. Testing
is done in blocks of 16 Kbyte, i.e., in the
areas COOOh through FFFFh, and 8000n
through BFFFh. This means that the
32 Kbyte RAM may be divided over two
slots.

When a lower slot is selected, the control
system will find the extension card
before it finds the internal one, and use
it as workspace. When, for example, the
internal memory is located in slot 3, the
RAM extension can be used in slots 0, 1
and 2. The slot allocation of the internal
memory is given in Table 1 for a number
of commonly used MSX micros. For a
computer not listed, consult the techni-
cal reference manual supplied with it.
The internal RAM is always selected
when it is in slot #0 or #1.

Circuit description

The circuit diagram of the RAM exten-
sion card is given in Fig. 2. Composed of
only two 32 Kbyte static RAM chips,
one CMOS IC, two resistors, three
capacitors and one FET, the memory ex-
tension could hardly be simpler.
Connector Ki is formed by the (pretinn-

Table 1

Slot assignment of MSX computers

MSX1 RAM-

SLOT

AVT Daewoo DPC-200 1

Canon V20 3

Goldstrar FC 200 2

JVC HC-7-gb 2

Mitsubishi MFL-FX1 3-2

Mitsubishi MFL-48 0

Mitsubishi MFL-80 1

Panasonic CF2700 1

Philips VG8020 3

Philips VG8010 0

V68020/20 3-2

Sanyo MPC-100 3

Sony HB201p 3

Sony HB75p 2

Sony HB55p 0

Sony HBlOp 3

Sony HBSOIp 3

Spectravideo 738 1

Spectravideo 728 1

Toshiba HX-10 2

Yamaha CX5M 0

Yashlca YC-64 3

REMARKS

Slot 3 expanded, 64 Kb RAM
32 Kb RAM

32 Kb RAM, slot 2 not usable

1 6 Kb ROM firmware in slot 0
1 6 Kb ROM firmware in slot 0
16 Kb RAM, 16 Kb ROM
firmware in slot 0

Slot 3 expanded, RS232/Diskrom

32Kb RAM
Slot 1 not usable

MSX2

AVT Daewoo CPC-300

Sony HB-F500P
Sony HB-F700P

Sony HB-F900P
Sony HB-F9P

Philips VG8220

Philips VG8230
Philips VG8235/8245

Philips VG8250/8255
Philips VG8280

RAM- REMARKS
SLOT

0-2 Slot 0 expanded.

128 Kb Memory mapper
0-0 0-2 Slot 0 expanded.

3-3 Slot 3 expanded.

256 Kb Memory mapper
0-0 0 -2 Slot 0 expanded.

Video digitizer
3-2 Slot 3 expanded.

128 Kb Memory mapper
16 Kb ROM firmware
3-2 Slot 3 expanded.

16 Kb ROM firmware
3-2 Slot 3 expanded.

3-2 Slot 3 expanded.

128 Kb Memory mapper
3-2 Slot 3 expanded.

128 Kb Memory mapper
3-2 Slot 3 expanded.

1 28 Kb Memory mapper
Video-digitizer

ed) contact fingers of the double-sided,
through-plated, pri nted ci rcuit board.
Gate Ni combines SLTSL and MERQ
to enable addressing the memory chips.
Since thes e have a capacity of 32 Kbyte
each, and SLTSL is intended for a range
of 64 KByte, the selected address block
needs to be divided in two 32 Kbyte
blocks. This is accomplished by N3 and
Nj combi ning true and inverted signal
A15 and A15 with the output of Ni.
Write protect switch Si blocks the WR
signal for both memory chips via gate
Nz.

RAMs ICi and IC2 work independently,
and one of them may be omitted when
only 32 Kbyte of extra RAM is required.

A compact module

The construction of the RAM extension
module on PCB Type 87311 is straight-
forward because the board is through-
plated and available ready-made. Before
mounting the parts, use a jig-saw to cut
off the two corners beside the slot con-
nector along the lines printed on the
overlay. Do the same with the area
behind Si.

It is recommended to use good-quality
IC sockets for the RAM chips, ICi and
IC2. Although the solder resist mask on
the ready-made PCB affords protection
against excess solder short-circuiting
pins or closely running tracks, unex-
perienced constructors arc well advised
to work carefully here, and use a low-
power soldering iron with a small tip.
Switch Si is preferably a miniature slide
type that can be fitted securely in the
clearance at the rear of the PCB.

A problem may arise with MOSFET Ti.
The Type BS170 may be supplied in a
different enclosure under the type indi-
cation BS170P. The P version also has a
different pin-out — see the circuit
diagram. The component overlay of the
PCB for the RAM extension is correct
for the standard BS170.

Testing

The RAM extension should be tested
before it is fitted in an enclosure. Figure
4 shows the listing of a test program
typed in under MSX BASIC. The actual
test program is machine code loaded as
DATA with the aid of a POKE instruc-
tion in a FOR/NEXT loop.

Before switching the computer on, close
Si to turn the extension card into a
ROM block. After the computer has
Finished its initialisation, open Si, type
in or load the test program, and make
sure that it addresses the right primary
slot, which corresponds to the value
POKEd in line 130. It should be noted
that the program tests the entire
64 Kbyte space. When the RAM exten-
sion functions correctly, the program
shows the message MEMORY OK in the
top left-hand corner of the screen. When

Semiconductors:

Ti -BS170 or BS170P (see text)
ICi;IC2=43256 or 62256 32Kx8
(C-l Electonics, Happy Memories)
IC3=74HCT32 i

Miscellaneous:

Si » miniature slide
PCB Type 87311

Fig. 5. Component mounting plan of
double-sided, through-plated, printed circ
board for building the RAM extension.

a fault is encountered, it displays MEM-
ORY ERROR in the same location.

Finishing

The completed printed circuit board can
be made into a compact and sturdy plug-
in module by fitting it in a music cassette
box — see the introductory photograph
and the drawing of Fig. 3. After remov-
ing the lower panel of the box, the PCB
is fitted by means of four screws and
spacers. After spraying the box with
paint, the extension module is ready for

Sony HitBit MSX micro upgraded with 64 Kbyte of random-access memory.

SHIELDING COMPUTERS WITH
METALCOATED GLASS

by Bill Pressdee, BSc, CEng, MIEE

Shielding for electromagnetic and radio-frequency interference
(EMI/RFI) has two distinct functions. In the first instance, its purpose
is to keep unwanted electromagnetic fields and unwanted
transmission from interfering with sensitive electronic equipment. A
second reason that has come about in recent years is keeping
transmissions inside a given system.

For many years a shielded room, or
Faraday cage, has been a prerequisite for
exact setting up of electronic circuitry in
conditions free from extraneous inter-
ference. Early shielded rooms, totally
enclosed by an earthed metallic mesh
frame were, however, often expensive to
construct and claustrophobic.

The puncturing of the screen to in-
troduce ventilation often created ad-
ditional problems with the result that air
flow was minimal. As a result of these
indifferent working conditions much of
the purpose of the Faraday cage was fre-
quently confounded by personnel leav-
ing the shielded door open.

In recent years, with an emergence in the
diplomatic and military fields of ex-
tremely sophisticated eavesdropping

equipment, a second reason for
shielding has come into prominence. As
much emphasis is now placed on keeping
transmissions in as keeping them out.
This is equally true in the commercial
field where radiation from computer cir-
cuitry can easily be detected by relatively
simple equipment and valuable data
siphoned off by those involved in com-
mercial espionage.

In the United Kingdom and other
countries where a Data Protection Act
has been introduced it has, in any case,
become a legal obligation for data to be
adequately protected. A general tighten-
ing up of international standards
relating to RF transmissions from elec-
tronic equipment in general, with stricter
regulations governing this likely to be-

come mandatory in Britain and most of
Europe this year, has given a fillip to the
shielded enclosure market.

In some cases where, for example, major
computer installations are situated near
airports or sites where there is the pros-
pect of considerable interference from
radar equipment, beacons, broadcast
transmitters, mobile radios, and in-
dustrial or medical apparatus, there is a
double problem. The shielding must en-
sure that unwanted transmissions do not
corrupt data or hamper operation, and
at the same time must secure the integri-
ty of the computer data from eavesdrop-
ping.

10.26

Problems of shielding

Effective shielding of computer rooms
and sensitive electronic equipment can
be achieved by enclosing them in a
metallic cage of wire mesh. In the case of
equipment, windows in the cage need to
be provided to enable dials or displays to
be read. In the case of computer rooms,
particularly those manned on a round-
the-clock basis, total enclosure is
claustrophobic and oppressive -in the
absence of daylight.

Even where windows are provided, they
have to be covered with wire mesh to en-
sure the shielding is complete and this
may well add to the feeling of imprison-

An expensive alternative in the past
where windows have had to be shielded
against RFI was in the use of conduc-
tively coated glass. The preferred
materials for coating were gold and in-
dium tin oxide (ITO). The window size,
however, has been restricted by the di-
mensions of the vacuum chambers
available for deposition. In addition, in
the case of ITO the cost of large pieces
has been prohibitive because the depo-
sition rate is slow, which means tying up
expensive plant for long periods.

The problems of manufacturing large
pieces of metallic coated glass at
reasonable cost have been solved by Pilk-
ington Glass, one of the world’s
foremost glass manufacturers. The pro-
cess involves the use of very-large-scale
magnetron sputtering equipment,
capable of producing conductively
coated glass in sizes up to 3.6 m x
2.5 m.

The plant operates on a continuous basis
with vacuum locks on the main sputter-
ing chamber enabling a very high
manufacturing throughput. Many dif-
ferent materials can be deposited by this
process, but for RFI shielding windows,

materials possessing high electrical con-
ductivity combined with good optical
properties in thin film form are chosen.

The result of this is a material that can
be used in the manufacture of windows
with an attractive appearance and little
visual obscuration. While the shielding
properties may not satisfy the most
stringent specifications it is quite
satisfactory for a wide range of appli-
cations such as windows for data sensi-
tive areas like computer rooms, RFI
shielding cabinet doors and video dis-
play unit (VDU) faceplates. The
laminate protects the operator’s upper
body from bombardment by radiation, a
suspected cause of headaches and other
ills.

The coated glass is laminated to a sec-
ond piece of glass to protect coating and
a peripheral metallic mesh tape com-
pletes the shielding connection.

Types of coaled glass

Several types of coated glass are
available, depending on the thickness of
conducting layer which will determine
the degree of shielding from elec-
tromagnetic waves, and further types
with improved performance are under
development. The thickness of the layer
will determine the surface resistance
which is in the range 2 to 20 ohms/
square (where ohms/squarc is the unit
for measurement of surface resistance).
The densest coating (2 ohms/square)
naturally has the lowest optical trans-
mission, which is of the order of 50 %.
Apart from the standard laminated
sheet, an interesting case arises where a
laminate is formed from two metallized
glasses separated by a non-absorbing in-
terlayer less than 1 mm thick. Radiation
that penetrates the first surface becomes
trapped between the two surfaces, which
act as mirrors, and only escapes after
multiple reflections due to the high
reflectivity.

Since the reflections are as likely to
escape through either glass, roughly only
half of the energy managing to penetrate
the coating will be transmitted on into
the shielded area. In practice, im-
provements in attenuation of 8 to 10 dB
over the single coat laminate have been
recorded.

From an architectural viewpoint an im-
portant additional consideration in
selection of a glass type — provided the
screening requirements are met — is the
ratio of light to solar heat transmission.
In the case of the 2 ohms/square
coating, the relative percentages for re-
duction are 50/30, so by its employment
there is considerable offspin.

In summer, the solar heat gain of the
building through the windows is greatly
reduced (to about one-third), while in
the winter, the same applies to heat

losses from the building via the win-
dows, providing for lower fuel bills.

Window construction

The conductive coatings may also be'
protected from abrasion by incorpor-
ation in a double-glazed unit. For inter-
nal applications the preferred construc-
tion is laminated, since it is less bulky
and gives a much increased strength. The
knitted wire mesh around the perimeter
of the sheet is brought into good elec-
trical contact with the coating by the
pressure of the lamination process.
Double-glazed units are more suited to
external or architectural use since this
construction gives the additional benefit
of lower heat loss. The conductive con-
nection around the perimeter of the win-
dow unit may be of the wire mesh type
or by depositing a robust metal coating.
Where there is a need to provide RF
shielding for an existing room with nor-
mal windows, secondary glazing is a
possibility. The additional windows
would be of laminated construction,
mounted in a metal framework
grounded to the rest of the structure, and
constitute the least expensive option for
a retro-fit.

A number of different coatings are
under investigation. Where a coating
durable enough to be used without pro-
tection is essential, one employing ITO
can be offered, but cheaper coatings
with similar properties are being
developed. Also, a coating similar to the
ones at present in use, but able to with-
stand the high temperatures involved in
toughening and bending the glass for
special applications, is under develop-
ment.

Pilkington Glass Ltd, New Technology
Business Unit, Cowley Hill Works,
ST HELENS, WA10 3TT.

„r 1938 10.27

SELF-INDUCTANCE METER

Measuring self-inductance reliably is notoriously difficult and
inductance meters are, therefore, few and far between and also
quite expensive. The instrument described here offers reasonably
accurate (within 1%) measuring of low-frequency inductors from
10//H to 2H.

One of the reasons that the measurement
of inductance is so tricky is that the
value of inductance varies considerably
with the conditions of measurement.
The principal reason for the variation in
inductance is the variation of per-
meability, which changes with the level
of the test signal and the d.c. bias.

Principle of meter

When a non-constant current is passed
through an inductance, an e.m.f., u, is
induced whose magnitude depends on
the rate of change of current, d /, in a
unit of time, d/, i.e.

u =L(d/7dr).

If di/di is kept constant (=k) by increas-
ing or decreasing the current uniformly,

u=Ik

that is, the e.m.f. is directly proportional
to the inductance (see Fig. 1).

Fig. 1. A uniformly increasing or decreasing
current through an inductor causes a con-
stant voltage to be induced across the induc-

In practice, however, it is impossible to
create a uniformly increasing or de-
creasing current, but a good alternative
is a current whose waveform is
triangular (see Fig. 2). If such a current
is passed through an inductance, the in-
duced e.m.f. will have a rectangular
waveform as shown in Fig. 2. If that
e.m.f. is rectified, the resulting direct
voltage is a measure of the inductance.
Unfortunately, no inductance is pure: it

always has some internal resistance, R ,
in series with it. Thus,

U=UL+UR.

Fig. 2. A triangular current through an in-
ductor causes a rectangular voltage across
the inductor.

The three voltages are shown in Fig. 3.
Rectification produces a direct voltage
with a small sawtooth-shaped ripple,
which is caused by ur. The average
value of the direct voltage (shown
dashed in Fig. 3d), remains a true indi-
cation of the inductance, however.

This shows that in this method of
measurement the internal resistance of
the inductor (unless it becomes large)
does not affect the measurement.

Block schematic

The block diagram of the proposed
meter is given in Fig. 4.

The function generator, consisting of a
combination of an integrator and a
Schmitt trigger, generates a triangular
and a rectangular voltage.

The triangular voltage is converted into
a triangular current superimposed on a
direct current. The composite current,
which is thus always greater than 0, is
passed through the inductor on test, Lx.
Range switching is effected by reducing
the current by a factor 10 for each higher
measuring range.

The a.c. component of the voltage across
Lx is amplified and then applied to the
first of three electronic switches, ES> to
ESj.

The electronic switches are controlled by
the rectangular voltage from the func-
tion generator and provide half-wave
synchronous rectification of the alter-
nating voltage.

Since this type of rectification halves the
average value of the input voltage, the
preceding amplifier raises the magnitude
of the a.c. component across Lx by a
factor 2.

The rectified voltage is applied to a
digital voltmeter, DVM, which displays
the value of Lx in henrys.

Triangular current

If the DVM has a full-scale deflection,
f.s.d., of, say, 200 mV, the input to it

Fig. 3. In a practical inductor, its internal re-
sistance causes a deviation from the rec-
tangular shape of the induced e.m.f. The
average value of this e.m.f. does not change,
however.

when an inductance of 2 mH is being
measured on the lowest range, must be
200 mV.

In the following, it will be assumed that
the maximum current, /'■», through the
inductor is 20 mA (a reasonable value
for inductors of 2 mH or smaller).
Starting at one edge of the triangular
current,

di/dt=UL/L =200x10-V2x10- j = 100.
Since the current increases linearly,
/m/tc-d/'/d/ =100
so that,

/c=20xl0-7l00=2xl0- 4 = 200/rs,

where t* is the duration of the edge. The
frequency of the triangular current is
therefore

/•=l/2fc=l/2x2xl0- 4 =2500 Hz.

Circuit description

The function generator consists of in-
tegrator A1 and Schmitt trigger IC1. The
frequency of the generated signal is, as
calculated above, 2500 Hz.

The Schmitt trigger provides a square-
wave voltage that is used to control elec-
tronic switches ESI to ES3.

Resistor R3 provides a DC offset to en-
sure that the triangular voltage does not
drop below 0 V. This is necessary for
good control of the voltage-to-current
converter.

Circuit IC1 is a Type 3130 opamp, which
is one of the few devices whose output
voltage can really be driven positive and
negative. That output serves as reference
voltage for the following integrator.

The output of A1 is a triangular voltage,
which varies between 4.9 V and 2 V. The
voltage-to-current converter around A2
and T1 transforms this voltage into a
current that is passed through the induc-

The value of the current, and thus the
measuring range, is determined by re-
sistors R5 to R8. When the 2 H range is
selected, the current is 20/10* =
0.02 mA.

Resistors R9 to Rll in parallel with I*
provide some damping. This is
necessary, because the inductor is also
shunted by various parasitic capaci-
tances (connecting wires, internal ca-
pacitance of the inductor, etc.) which
results in an LC circuit. The high-
impedance drive of this circuit (by a
practically ideal current source) would
certainly give rise to oscillations in the
absence of some damping. The value of
these resistors is chosen to ensure that
they have a negligible effect on the
measurement. Note that when the 2 H
range is selected, R14 serves as damping
element.

If an attempt is made to measure a small
inductance with a high range selected,
e.g., a coil of 1.5 mH in the 2 H range,
it may be that the value of the damping
resistor is too high, with the result that
oscillations may occur. It is, therefore,
advisable always to start in the lowest
range and then switch to a higher range
as required (shown by the absence of an
overflow indication on the DVM). This

5

Fig. 5. The various waveforms in Fig. 4: L is
Ihe voltage applied to the voltage-to-current
converter; a falling edge indicates an increas-
ing current.

method also ensures the highest possible
resolution.

The overflow indication is provided by
comparator A4, which connects the in-
put of the voltmeter to +5 V via ES4
when too low a range has been selected.
Opamp A3 raises the magnitude of the
measured alternating voltage by a factor
2. Note that C2 at its non-inverting input
blocks any direct voltages. The offset of
this opamp is compensated with the aid
of PI.

The half-wave rectifier is formed by ESI
and ES2, while ES3 serves to invert the
rectangular control pulses. During the
positive part of the measured alternating
voltage, ESI is closed and ES2 is open;
during the negative part, ESI is open
and ES2 is closed. The resulting steady
voltage is smoothed with the aid of R19
and C3 and converted into a readable
quantity by IC7.

The digital voltmeter consists of the
well-known Type 7106 IC and a 3 Vi-
digit LCD. The 7106 contains all that is
necessary for converting a steady voltage
into a digital quantity and displaying
this on the LCD. The decimal points of
the display are provided by XORs N1 to
N3. Which decimal point is visible
depends on the position of switch S2c.
LEDs D8 and D9 indicate whether the
display must be read in henrys or
millihenrys.

The voltmeter is powered by two 9-volt
PP3 batteries and two voltage regu-
lators. Note that the 7805 and 7905
regulators provide better interference
suppression than the smaller L types.

Construction

The meter, constructed on the PCB
shown in Fig. 7, fits in a small, hand-
held case.

All resistors and diodes are mounted
upright, except Rl, R3, and R24. Elec-
trolytic capacitors should be PCB types.
Sockets should be used for the ICs.

The display is best mounted on some
stacked terminal boards so that it is
located just under the window in the en-
closure.

10.29

8

Fig. 8. Prototype of the self-inductance
meter before it is fitted in the enclosure.

R22 is an array of four 100 k resistors;
this may be replaced by four discrete
100 k resistors that are mounted upright
with their upper terminals interconnec-
ted.

The LEDs are mounted at such a height
that they are seated just under the front
panel once the PCB has been installed in
the case.

The rotary switch is soldered direct to
the PCB to make the best possible use of
the available space and also to prevent
noise and interference from connecting

The centre pin of IC5 and IC6 should be
bent forward so that the pins form a
triangle (just as with a standard transis-

9

Fig. 9. Design of a possible front panel for
the self-inductance meter.

tor). The ICs are mounted on the PCB
with their case just clear of the board;
this means that the wider part of the
pins also goes into the relevant hole,
which allows for this. Where a very flat
enclosure is used (possible), it may be
necessary to carefully cut off the metal
tops of IC5 an IC6.

To secure the PCB, three holes must be
drilled in the case: the non-populated
board may be used as a template.
Switch SI is best mounted at one of the
sides of the case, while the two batteries
may be located at the underside of the
enclosure (after the small mouldings
have been removed).

Note that the mouldings on the lid

should also be removed before the dis-
play is mounted.

The spring action terminals for connect-
ing the inductor on test should be fitted
at one of the sides of the enclosure as
close as possible to the relevant pins on
•the PCB.

Calibration

An inductor of between 1 and 1.8 mH,
whose value is accurately known, is re-
quired for the calibration. A cross-over
filter coil with an accuracy of better than
3% may suffice, but some retailers can
provide an air-cored inductor of 1.5 mH
with an accuracy better than 1%.

It is also possible to determine the
inductance of an air coil of about
1—1.5 mH accurately as follows. Con-
nect it in parallel with a capacitor of
47 nF or 100 nF (accuracy 1% or 2%)
and connect this circuit via a series re-
sistor of about 3.3 k to a frequency gen-
erator. The resonance frequency of the
circuit is then determined with the aid of
an oscilloscope and a frequency meter.
The self-inductance of the air coil is then
calculated from

1 0 BC516^

* J 10|l|lOV

f

12V

1

'ft

] "

2L-

BC517

Fig. 10. This small circuit may be added to
ensure automatic switch-off of the batteries.

L=l/(2nf) 1 C [H],

Short-circuit the measuring terminals
and select the 20 mH range.

Adjust PI till the display read 0.000.
Gonnect the reference coil to the measur-
ing terminals and select the 2 mH range.
Adjust P2 till the display reads the exact
value of the reference coil.

Since the accuracy and precision of the
other ranges are determined by the toler-
ance of resistors R5 to R8, this com-
pletes the calibration.

Automatic switch-off

The meter draws about ±20 mA. A pair
of batteries will have a fairly long life, as
long as they are always switched off
when the meter is not in use.

Forgetful users may find the circuit in
Fig. 10 ideal: this automatically switches
the batteries off after about half a mi-
nute. The meter is switched on again by
pressing the reset button. This circuit
may be connected behind SI or simply
replace it altogether. The meter is then
always switched on .by pressing the reset
button.

A HIGH-SPEED DEPLETION-
MODE DMOS FET FOR SMALL
SIGNAL APPLICATIONS

by Alan Pritchard

This article describes a new ultrahigh-speed n-channel depletion-
mode lateral DMOS transistor geared for small-signal applications.
This device boasts high-performance characteristics, which in-
clude tunr-on speeds of less than 1 ns; low reverse-transfer
capacitance of less than 2.5 pF; high-frequency
transconductance greater than 1 0 ms; a wide dynamic range;
and low distortion.

Fig. la and lb show idealized cross-
sections of the ‘normally-on’ depletion
mode and ‘normally-off’ enhancement-
mode devices. Because these device
structures are similar, the device
characteristics are also similar. In fact,-
the depletion-mode device may be
thought of as an enhancement-mode
device with a negative threshold voltage.

Fig. la. Depletion-mode device cross-
section.

Fig. lb. Enhancement-mode device cross-
section.

Unlike enhancement-mode devices,
whose drain current falls to zero when
the gate-to-source voltage equals zero,
the new depletion-mode FET has ap-
preciable current at zero gate signal. In
fact, the drain-to-source resistance is.

typically 100 Q at zero voltage. As
shown on Fig. 2, the on-resistance
rDS(on) versus analogue signal range is
an almost flat response. This character-
istic, coupled with the low-capacitance
values of the new device, makes it par-
ticularly suitable as an analogue switch
for audio and video switching appli-
cations.

The depletion-mode ‘normally-on’
characteristic makes the FET useful for
single-device current regulators. This
type of circuit, usually associated with
junction FETs, is shown in Fig. 3. The
value for Rs can be calculated from:

Rs _ VGS(off) [1 — (Id/Ipss) i/2 1

10.32

where Id is the required value of
regulated current.

The major advantage of depletion-mode
MOSFETs in current-source circuits is

Fig. 3 Single-device current regulator.

their low drain capacitance, which
makes them suitable for biasing appli-
cations in low-input leakage, medium-
speed (>50 V/ps) circuits. Fig. 4 shows
a low-input-leakage current differential
front-end employing a dual low-leakage
junction FET.

Fig. 4. Low bias-current differential front-
end using an M440 OFET. Note Cs reduces
the maximum current swing available to
charge Cc, thus reducing the slew rate.

In general, each side of the JFET will be
biased at Id = 500 pA. Thus, the current
available for charging compensation and
stray capacitances is limited to 2 Id or, in
this case, 1.0 mA. The JFET's matching
characteristics are production-tested and
guaranteed on the data sheet.

Cs represents the output capacitance of
the input stage ‘tail’ current source. This
capacitance is important in non-
inverting amplifiers, because the input
stage undergoes considerable signal ex-
cursions in this connection, and the
charging currents in Cs may be large. If
standard current sources are used, this
tail capacitance may be responsible for
marked slew-rate degradation in non-
inverting applications (as opposed to in-
verting applications, where the charging
currents in Cs are very small).

The slew-rate reduction may be shown

1

1 + (Cs/Cc)

As long as Cs is small compared to Cc
(the compensation capacitor), little
change in slew rate occurs. Using the
DMOS FET, Cs is about 2 pF. This ap-
proach yields a significant slew-rate im-
provement.

Where IDSS currents greater than 1 to
5 mA are required, the device may be
biased into the enhancement mode to
produce up to 20 mA for a Vgs of
+2.5 V maximum, with low output
capacitance remaining a major feature.
Fig. 5 shows a suitable enhancement-
mode current source.

Fig. 5. Enhancement-mode current source.

A ‘normally-on’ analogue switch can be
constructed for applications where
default condition is required at supply

failure, such as for automatic ranging of
test equipment or for guaranteeing cor-
rect initialisation of logic circuits at
start-up.

The low negative threshold voltage of
the device gives simple drive re-
quirements and allows low voltage oper-
ation. Fig. 6 shows the typical bias con-
ditions for a depletion-mode DMOS
analogue switch.

To turn the device off, a negative voltage
is required on the gate. However, the on-
resistance can be reduced if the device is
further enhanced with a positive gate
potential, allowing it to be used in the
enhancement-mode region as well as in
the depletion-mode region. This effect is
shown in Fig. 7.

Fig. 6. •Normally-on' analogue switch.

Fig. 7. Current versus drain-to-source
voltage for the Siliconix SD2100 DMOS
FET.

The high-frequency gain of the device,
along with its low capacitance values,
produces a high ‘figure of merit’. This is
an important factor in VHF and UHF
amplification, and defines the gain-
bandwidth product (Gbw) of the device,
which may be expressed as:

For a common-source configured ampli-
fier, this becomes:

10.33

_ gfe (4) example, a ‘de-glitch’ circuit for the out-

2n(Ciss+Crss) ' 1 put of a high-speed digital-to-analogue

(D/A) convertor, such as those found in
where: video waveform generators, can take ad-

vantage of the device’s high speed, low
Ciss = short-circuit input capacitance, and low distortion.

(Miller)capacitance = Glitches at the D/A convertor output, as

Cgs + Cdg(l - Av); shown in Fig. 9, are generated during the

Cgs = gate-source capacitance; switching transition times, when time

Cdg = feedback capacitance; skew allows incoming and previous data

Crss = short circuit reverse transfer to overlap. The worst-case occurence is
capacitance = Cdg. at MSB (most significant bit) switching

(e.g. from 01111111 to 10000000).

samples the output some time after it
has settled. As D/A convertor perform-
ance improves, settling times ap-
proaching 10 ns have become possible;
therefore, fast-switching, low-
capacitance sample-and-hold circuits,
such as the one shown in Fig. 10, are re-
quired.

Alan Pritchard is with Siliconix.

It is evident that the gain-bandwidth
product is largely dependent on the
device gain and the feedback
capacitance. If typical values for the new
DMOS FET are substituted in Eq. 4, in-
cluding the low feedback capacitance of
2.5 pF, the gain-bandwidth product is
found to be greater than 400 MHz, a
useful value in VHF and UHF oper-

The high figure of merit is also reflected
in the nanosecond turn-on times which
are important in applications such as
sync-pulse generation for Ptgh-
definition video systems, signal routeing
for high-speed digital video recording
where data rates of greater than
100 Mbit/s are possible, and outside
broadcasting systems where signal
switching is required during blanking
periods. Fig. 8 shows a high-
performance video d.c. restorer. In these
applications, the low distortion
characteristics are important.

Fig. 8. High-performance video dx. restorer
using the SD2100.

The new device is also useful in appli-
cations that require both low charge in-
jection and high switching speeds. For

Fig. 9. Effect of time-skew glitches at D/A
convertor output.

A de-glitch circuit effectively forms a
sample-and hold function which

Fig. 10. ‘De-glitched‘ D/A convertor using
two SD2100 devices. Note: Charge injection
is reduced by complementary drive to Qi
and to Qr, which acts as a ‘dummy’, capaci-

10.34

MICROPHONE PREAMPLIFIER
WITH ACTIVE FILTER

by S.G. Dimitriou

When a microphone is used a good distance away from an
amplifier, its relatively small output signal is inevitably affected by
noise and attenuation caused by the cable.

The simple, yet versatile, preamplifier/line driver described here
can be used with a variety of microphones, has a user-defined
frequency response, and prevents signal degradation because its
ability to load long cables enables it to be installed near the
signal source.

Many types of modern audio equipment
have a built-in microphone, or allow an
external microphone to be attached
semi-permanently. This is the general
case with portable tape or cassette
recorders, video and movie cameras. In
spite of the apparent benefits of having
a built-in microphone, this will almost
always pick up mechanical noise from
the equipment it belongs to. Also, it is of
little or no use when sounds from remote
sources are to be recorded, as the level of
ambient noise is bound to exceed that of
the wanted sound.

Obviously, reasonable signal-to-noise
ratios and, therefore, good-quality recor-
dings, can only be achieved when the
microphone — or microphones — is in-
stalled relatively close to the source of
the sound, but this arrangement
necessitates the use of a long coaxial
cable between microphone and associ-
ated equipment. With cable capacitance
typically of the order of 200 pF/m, up to
100 m of coaxial cable with Z=600 Q
may be used without running into con-
siderable attenuation of the upper part
of the audio spectrum. In this case, the
cut-off frequency, ft, becomes:

/c=l/(2nRC) [Hz]

ft = l/(2n x 600 x 20 x 10' 9 ) [Hz]

/'c = 13. 263 kHz

In practice, however, the microphone im-
pedance often rises with frequency, so
that the cable must be kept shorter to
prevent bandwidth reduction. In any
case, it is not a very good idea to have
the small microphone signal travel
through any appreciable length of cable,
as this will cause degradation of the
signal-to-noise ratio.

A better method is to amplify the signal
locally, i.e., as close as possible to the

microphone, and drive the coaxial line
by means of an amplifier with low out-
put impedance. In this way, the wanted
signal on the line is too strong to be af-
fected by noise or capacitive loading.
The signal amplitude can be reduced
fairly easily at the receiver end with the
aid of a two-resistor voltage divider
(Fig. 1), in which

R:*0.9AR,

Fig. 1. A line driver with attenuator to over-
long coaxial cable.

where A is the amplification of the line
driver, and Ri = 100 Q (typical value).

Bandwidth and filtering

The line driver is readily modified to op-
erate as an active filter that can help to
improve the signal-to-noise ratio of the
sound picked up by the microphone.
This is particularly useful in applications
involving the recording of speech
signals. From acoustic engineering it is
known that the frequency spectrum of
speech has relatively large redundant
parts. The dynamic and spectrum-
related characteristics of speech have
been widely studied, but a further dis-
cussion of this interesting field is, unfor-
tunately, beyond the scope of this article.
Here, it is sufficient to say that a simple
Wien-type bandpass filter can aid in
shaping the spectrum such that speech
becomes more intelligible due to the
elimination of redundant signals and a
good deal of ambient noise. In this way,
the signal-to-noise ratio of the sound
picked up by the microphone is
significantly improved.

The response of the standard, passive,
Wien filter is fairly smooth (Fig. 2),
making it suitable for use with music
signals without unduly affecting the
original sound.

Practical circuit

The preamplifier/active filter proprosed
here is mainly intended for use with low-
impedance dynamic or electret
microphones. Electret microphones of-
fer a wide frequency response, supply a
relatively large output signal, and are of
small size. They do have the disadvan-
tage of requiring a biasing voltage, but
this is no problem here as a supply is re-

quired anyway for the line driver.

With reference to the circuit diagram of
Fig. 3, the line driver is built around
low-noise operational amplifier ICi,
which is configured to work as an invert-
ing active Wien filter. With the values
shown for Ri, R2, Ci and C2, the centre
•frequency of the filter will be 3.3 kHz
nominally:

/o=159,155/(RiCi)

with Ri in kilo-ohms and Ci in nano-
farads. Related to the acoustic behaviour
of the human ear, and with reference to
the audible threshold curves set up by
Fletcher and Munson 3.3 kHz cor-
responds to the point of maximum sensi-
tivity. The filter suppresses both low fre-
quencies (whose reverberant nature tends
to impair intelligibility of speech) and
frequencies above 10 kHz, which can be
considered as noise in the context of the
spectral redundancy of speech.

The voltage amplification, A , of the line
driver is about 10 at the passband centre
frequency, and the value of the gain- and
frequency- determining components is
calculated from

Ri= 24R2 and C2=24 Ci

Components Cs and Rs ensure DC
blocking at the output and phase
stabilization respectively. The latter
function is required to isolate the
distributed capacitance of the coaxial
cable from the feedback network of
ICi, thus preventing parasitic high-
frequency oscillation. Evidently, the
value of Rs should be kept as low as
possible, because otherwise the benefit
of the low output impedance of ICi (as
seen from the line) is lost. The resistor
may be omitted if the LF356 is replaced
with a (less expensive) 741, but this, un-
fortunately, increases the output noise
level.

Potential divider Rj-Rj biases the non-
inverting input of ICi at half the supply
voltage. LED Di is the power indicator.
It is connected in series with the rest of
the circuit to minimize the total current
drain from the 9 V (PP3) battery. In this
way, the preamplifier draws only

4.5 mA, which is still sufficient to light
a LED with good efficiency. Owing to
the drop across Di, the preamplifier
works from a supply voltage of 7 to

7.5 V.

Microphone and supply
options

The basic circuit diagram of Fig. 3
shows a 3-terminal electret microphone.
This will typically have an output im-
pedance of 500 Q or less, which is low
with respect to the value of R2, so that
the microphone has very little effect on
the centre frequency of the active Filter.
Where a 2-terminal electret microphone

10.36 elektor india October 1988

Fig. 4. Methods of connecting three types of microphone to the preamplifier input. Fig. 4a: 2-terminal electrel microphone; Fig. 4b: low-
impedance dynamic microphone; Fig. 4c: high-impedance dynamic or crystal microphone.

Fig. 6. The electrel element and preamplifier
mounted on a small piece of verohoard that
can be inserted in a lube, together with the
battery.

is used, the value of R: must be reduced
to compensate the higher output im-
pedance — see Fig. 4a. Figures 4b and
4c show the input circuits required for a
low-impedance dynamic microphone
and a high-impedance (or crystal-)
microphone respectively.

As to the power supply for the preampli-
fier, this can be fed either by a battery as
already discussed, or by an existing 12 to
15 V supply available in the power
amplifier. In this case, only an ad-

Fig. 5. The preamplifier can be powered via a separate wire in the cable to the power
amplifier.

ditional 9 V regulator is required near
to, or as part of, the microphone pre-
amplifier. The +12 to 15 V input voltage
for this regulator is conveniently carried
via the centre core of one of the wires in
the stereo shielded cable to the power
amplifier.

Shaping the filter response

The centre frequency of the active filter
set up around ICi can be selected for
the application in question by redimen-
sioning Ci, C2, Ri and R2 on the basis
of the previously shown formulas. Some
commonly used frequencies and time
constants are 723 Hz (220 ns), 1 kHz
(150 ns), 1.5 kHz (110 ns), 2.2 kHz
(72 ns), 3.3 kHz (47 ns) and 4.8 kHz
(33 ns).

The filter can also be given a different
frequency response. For example, it
could be made wider to pass the spec-
trum from 200 Hz to 6100 Hz, which
has been recommended for optimum in-
telligibility of speech <:i . For narrow-
band music reproduction, a centre fre-
quency of 1.5 kHz should be a good
compromise between minimum required
bandwidth and frequency response of
loudspeakers typically used in public-

address systems. Table 1 gives some
useful design values and possible appli-
cations (voltage gain A =5).

Construction

Construction of the preamplifier should
be relatively easy on a small piece of
Veroboard. This can be fitted in a cylin-
drical enclosure, together with the power
switch, LED, battery and the micro-
phone element — Fig. 6 shows a
suggested arrangement. It is rec-
ommended to cover the battery and the
board in small plastic bags to prevent
any likelihood of a short-circuit. The ex-
cess material is wrapped around these
elements, which are then carefully push-
ed into the tube. In this way, all parts are
held securely in place without the need
for mounting hardware.

References:

111 Audio handbook, pp. 2-45. National
Semiconductor Publication.

(2) The Design of Speech Communi-
cation Systems. Proceedings of the IRE,
vol. 35, pp. 880-890, ed. 1947.

10.37

temperature-controlled
soldering iron

Since the days when soldering irons
were heated up on gas rings, the design
of this virtually indispensable piece of
equipment has come a long way. There
is now a wide variety of different types
of iron available, allowing power rating
as well as size, shape and composition of
the bit to be selected to suit a particular
application. Despite this plethora of
different irons, it is nonetheless possible
to discern two basic categories, namely
continuous heat and temperature-con-
trolled soldering irons. With the former
type, the heating element is connected
continuously to the supply, with the
result that the iron tends to run very
hot when not being used.

This means that the first joint made
after the iron has been left standing may
be too hot, thereby incurring the risk of
a bad joint or of damage to delicate
components. If one attempts to combat
this problem by using a lower power
iron, there is the danger that, under
heavy load conditions, it may be unable
to supply sufficient heat and will make
a dry joint. A further disadvantage of
continuous heat irons is that their
tendency to overheat shortens the
effective life of the bit and causes a
reduction in the heating capability of
the iron.

Temperature-controlled irons on the
other hand suffer from none of these
drawbacks. The only reason that they
have not completely replaced continuous
heat types is the fact that they cost
much more. However, with the current
trend towards ever smaller and more
sensitive components, the decision to
purchase a temperature-controlled iron
may well prove a worthwhile long-term
investment (particularly if one considers
saving the cost which results from
building the control unit oneself).
Thermostatically controlled soldering
irons must not only be able to maintain
a constant bit temperature (to within a
few degrees Centigrade), it must also be
possible to vary the soldering tempera-
ture to suit different requirements.
Designing a suitable control unit, which
both meets the above conditions and
yet is a reasonable financial proposition
for the amateur constructor, is no easy
matter. However, the circuit described
in this article adequately fulfils all the

Electronic temperature-controlled
soldering irons offer a number of
advantages over continuous heat
types: delicate components are
protected against thermal damage;
they permit the use of higher
wattages, thereby eliminating the
danger of dry joints when working
under heavy load conditions; and
finally they increase the life of
both heating element and bit.

The following circuit is for a
thermostatic control unit, which is
both easy to build and uses
standard components. Suitable
soldering irons containing a
built-in heat sensor are readily
available from a number of
different manufacturers.

desired design criteria at a price which is
roughly halfway between the cost of a
conventional continuous heat iron and
that of a commercially available tem-
perature-controlled model. The control
unit is designed for use with a readily
available soldering iron incorporating a
heat sensor in the shaft adjacent to the
tip of the bit.

Electronic control unit

The principle of the electronic thermo-
static control unit is illustrated in the
block diagram shown in figure 1 .

A sensor mounted in the element as
near as possible to the bit tip provides a
voltage which is proportional to the bit
temperature. This voltage is then
compared with a (variable) reference
voltage on the other input of a com-
parator, the output of which is used to
control a switch which regulates the
flow of current to the heating element
in she iron. Thus, when the sensor
voltage is lower than the reference value,
the switch is closed, current flows to the
heating element and the bit temperature
rises; once the desired temperature is
reached, the comparator output changes
state, opening the switch and thereby
cutting off the flow of current to the
heating element. The bit temperature
then falls until' the threshold voltage of
the comparator is again reached and the
control switch is opened. In this wdy
the temperature of the bit can be
maintained within a certain fixed range.
The amount of hysteresis between a
change in temperature and the corre-
sponding change in sensor voltage is
determined by the thermal inertia of the
sensor itself and the thermal conduc-
tivity of the bit (which in turn is deter-
mined by the size and composition of
the bit).

The deviation from the nominal bit
temperature as a result of the hysteresis
of the control system is illustrated in
figure 2. As can be seen, the bit tem-
perature oscillates about a preset
nominal value; the steepness of the
rising edge of the triangular waveform is
largely determined by the output power
of the heating element, and that of the
falling edge by the rate at which heat is
lost to the atmosphere, solder, p. c. b.

etc. In practice however, the bit tem-
perature only deviates very slightly from
the desired nominal value, so that it is in
fact possible to speak of an average
working temperature of the iron.

As far as the choice of heat sensor is
concerned, various possibilities come
into consideration. The firm Weller, for
example, manufacture a heat sensor
which utilises an unusual property of
magnetic materials. Above a certain
temperature, known as its Curie point, a
ferromagnetic material loses the prop-
erty of magnetism. The bit of a Weller
iron contains a slug of magnetic material,
which, when the iron is cold, attracts a
magnet. This in turn closes a switch and
applies power to the heating element.
When the temperature of the bit reaches
the Curie point, the slug ceases to
attract the magnet, causing the switch
to be opened. The only disadvantage of
this system is that a different bit con;
taining a ferromagnetic slug with the
appropriate Curie point is needed to
change the soldering temperatures.

Other manufacturers employ heat
sensors consisting of a thermocouple or
of an NTC- or PTC thermistor, usually
as part of a bridge circuit. One branch
of the bridge is formed by a variable
resistor with which the bridge is bal-
anced, In practice this means that the
temperature range of the bit is deter-
mined by the range of the resistor.

Of the above-mentioned types of sensor,
the thermocouple represents the best
choice. The reasons for this are clear
when one compares it with temperature-
dependent resistors. Firstly, the dimen-
sions of the thermocouple are smaller
than those of an NTC or PTC thermistor,
which means that is is easier to mount
close to the tip of the bit, and also that,
because of its reduced mass, it responds
more quickly to changes in temperature.
The response of a thermocouple (volt-
age as a function of temperature) is, as
figure 3 clearly shows, linear over a wide
range of temperatures. NTC- and PTC
thermistors, on the other hand, exhibit
a far less linear characteristic. Further-
more, a thermocouple has no quiescent
current flow to speak of, and hence will
not generate any heat itself. The final
point in its favour is the lower cost of
thermocouples, a not insignificant factor

Figure 1. Block diagram of an electronic
thermostatic control unit for soldering irons.
The voltage from the heat sensor is compared
with a variable reference voltage. The output
of the comparator controls a switch which it
used to turn the flow of current to the

Figure 2. The response of a typical thermo-

small not to significantly affect the perform-
ance of the iron, which remains at a more or
lass constant 'average' temperature.

when temperatures of the order of
400°C are involved.

The Elektor control unit

In view of the above-mentioned points
an iron which was both readily avail-
able and which incorporates a thermo-
couple as heat sensor was taken as the
starting point of the Elektor control unit.
Several manufacturers in fact distribute
suitable soldering irons without the ac-
companying control unit. For example,
the firm Antex produce a 30 W
soldering iron (the CTC) which includes
a thermocouple, as well as a 50 W model
(XTC) which should be available shortly.
Ersa are another company who have a
suitable 50 W iron (TE 50).

In order to ensure the complete re-
liability of the Elektor control unit, it
was in fact sent to Antex for assessment.
Their verdict was summarised as follows:
‘The performance of the sample tested
should be perfectly adequate for the
Home Constructor’. Furthermore, the
control unit can also be used with
soldering irons from most of the other
manufacturers, even if they contain
NTC- or PTC thermistor sensors,
although in that case certain changes
will have to be made to the circuit.
Without entering into the theoretical
details, it should be noted that different
combinations of materials can be used
to construct thermocouples, and that
each will deliver a different output volt-
age for a given temperature. For their
CTC and XTC models, Antex use a K-
type thermocouple, which is composed
of nickel-chrome and nickel-aluminium.
The response shown in figure 3 was
obtained using this type of thermo-
couple.

Circuit diagram

The complete circuit diagram of the
thermostatic control unit is shown in
figure 4. Despite the small number of
components used, the operation of the
circuit is somewhat involved, and for
this reason figure 5, which contains an
overview of the waveforms found at the
test points shown, is included to facili-
tate explanation.

The first problem which arises is the

tlektor india October 1988 1 0.39

choice of switching element to regulate
the flow of current to the iron. The use
of a relay involves several drawbacks
(contact burning, contact bounce etc.)
which can be avoided by employing an
electronic switch such as a triac. An
additional advantage of a triac is that
the switching point can be controlled
with a high degree of accuracy, i.e. in
order to reduce the switch-on surge
current and r.f. interference to a mini-
mum, the triac can be triggered at the
zero-crossing point of the AC waveform.
This is in fact the arrangement adopted
in the circuit described here.

R4, D3, Tl and the emitter resistors of
T1 form an adjustable constant current
source. D3 is a LED used to set the DC
base bias voltage of Tl, but since it
draws very little current it will hardly
light up at all. The advantage of this
somewhat unusual approach is that the
LED possesses the same temperature
coefficient as Tl , hence the stability of
the current source is unaffected by
variations in temperature. This is only
true, however, if the ambient tempera-
ture of the circuit does not rise too £ST
above normal room temperature, since
in that case the temperature coefficient

of the LED will cease to match that of
Tl. Thus, if when the circuit and trans-
former have been mounted in a case, the
temperature should rise by more than
30°C, D3 should be replaced by an 8k2
resistor. This step will obviously be
necessary if the soldering stand is to be
mounted on top of the control unit case.
The current through P2 and R6 can be
varied by means of P 1 . P2 determines
the amplitude of the reference voltage
at the inverting input of IC1. The
thermocouple is connected across the
non-inverting input of IC1 and the
junction of R3/R6. Thus the voltage

10.40 ele

difference at the inputs of the compara-
tor equals the difference between, on
the one hand, the voltage dropped
across R6 plus the resistance of P2,and
on the other hand, the voltage devel-
oped by the thermocouple. That is to
say , it virtually equals the thermocouple
voltage.

If the soldering iron is cold, the thermo-
couple voltage is very small, so that the
output of IC1 is low. When the tem-
perature of the iron rises, the thermo-
couple voltage, and hence the voltage
difference at the comparator inputs,
also rises, until the output of the

comparator swings high.

IC1 is followed by a Schmitt trigger, the
output of which goes low when the
input exceeds approx. 3.2 V, and high
when it falls below roughly 2.1 V. This
arrangement could be used directly to
control the triac were it not that we
have to first ensure that the load is
switched at the zero-crossing points of
the transformer voltage. To achieve this,
one or two extra provisions are required.
The transformer voltage (Ut r in figure 5)
is connected to the input of N3 via a
potential divider, R9 and RIO, one end
of which is connected to the stabilised

5.6 V supply rail. This means that the
voltage at point 1 (the input of N3)
exactly tracks the transformer voltage,
whilst remaining 2.8 V ‘up’ on the latter
(see figure 5). The portion of waveform
above 6.2 V and below —0.6 V is shown
as a dotted line, since CMOS Schmitt
triggers contain clamping diodes which
protect the inputs from voltages which
exceed these limits.

The advantage of the 2.8 V positive
offset is apparent from figure 5, since it
means that when the transformer
voltage is zero, the voltage at point 1 is
-2.8 V; since the Schmitt trigger changes
state at the threshold values of 2.1 V
and 3.1 V, we can say that, in spite of
the hysteresis, it is only triggered
around the zero-crossing point of the
transformer waveform (the small devi-
ation from the ideal switching point of
exactly 0 V can be eliminated by
making R9 variable and using a scope to
adjust it to the correct value; in practice,
however, this small error is of little
significance and does not materially
affect the operation of the circuit).

When both inputs of N3 are high (i.e.
greater than 3.1 V), the output is low,
and since N4 is connected as an inverter,
its output will be high, with the result
that C2 will be discharged. If point ©
then goes low, since C2 is still uncharged,
point @ will also go low, causing C2 to
charge up via R1 1 . The time constant of
R11/C2 is 18ms; shortly before this
time is reached the voltage at pin 1 2 of
N3 will have reached the logic T’
threshold and since, at that moment,
pin 1 3 has once more been taken high,
the output of N4 is also returned high.
Since capacitor C2 is already charged,
the voltage across it would continue to
rise, but for the clamping diode in N3.
The capacitor is rapidly discharged
(figure 5 @), and a new cycle begins.

The signals at points ® and ® form the
clock signals for flip-flops FF1 and FF2.
The J-inputs of these flip-flops are
connected to points © and ®, where the
voltage is determined by the tempera-
ture of the iron, whilst the K-inputs are
connected to ground. Only when the
J-inputs arc high can the clock pulses
have any effect and change the state of
the flip-flops. Since the voltage at point
© is an inverted version of that at point
©, when the former goes low the first
positive-going edge at point @ will take
the (j output of FF1 (point 9) low,
causing T2 to turn off and the triac to
be triggered. The soldering iron then
begins to heat up, so that the voltage at
point © rises until it reaches the trigger
threshold of N 1 . When that happens N I
changes state, taking point © low and
point @ high; the next positive going
pulse at point 3 will take the Q output
of FF2 high and reset FF1, thereby
taking point ® high and resetting FF2.
Thus T2 is turned on and the triac
turned off, interrupting the flow of
current to the heating element in the
iron. The temperature of the iron will
fall until the lower threshold value of

10.41

6

10.42 ole

The photo shown on the first page of
this article is a prototype model of the
Elektor control unit. For exhibition
purposes the unit was housed in perspex.
The soldering stand shown in this photo
is not particularly suited for low power
irons, since the contact between the
iron and the metal rings leads to con-
siderable heat loss and hence to the iron
being switched on and off with excess-
ive frequency. Soldering stands which
avoid direct metal-to-metal contact with
the iron should be given preference.
These can be bought separately from
most electronics shops.

Adjustment procedure

The setting up procedure for the control
unit is as follows :-

Firstly, with the soldering iron discon-
nected, the inputs of IC1 are shorted
together. The offset voltage is then
reduced to a minimum by adjusting P3
until D4 either just lights up or is just

extinguished (depending upon which
state it assumes when power is applied).
Next, the short is removed and the
wiper of P2 is turned fully towards R6
(anticlockwise). The soldering iron is
then plugged in and the tip is held
against a length of solder. Although
solder melts at approx. 189°C (60/40
alloy), at around 185°C it exhibits a
‘plastic’ consistency. By very gradually
adjusting PI, it is possible to set the
temperature of the iron such that the
solder is in this plastic state, just on the
point of melting (185°C). PI should be
adjusted in small steps, always allowing
tlie temperature of the iron to stabilise
before testing it against the solder and
performing another adjustment.

By means of P2, it is then possible to
vary the temperature of the iron be-
tween 18S°C and approx. 400°C. P2
can be calibrated using the following
equation:

T * 185 + jjij x 185°C (P2 is in «)

In conclusion

As was already mentioned, the proto-
type model of the control unit was
designed for use with the CTC or XTC
soldering iron from Antex. However it
can also be used with other types of
iron, particularly if they are provided
with a thermocouple heat sensor. If this
is the case, and if the iron operates off
24 V, then it can be connected direct to
the Elektor control unit. In the case of
an iron with the same operating voltage
but which employs a different sort of
sensor, the situation is a little more
complicated. With a PTC thermistor, D3
and D4 should be omitted, T1 teplaced
by a wire link between the emitter and
collector connections, and the value of
R2 altered accordingly. The same
procedure holds good for irons incor-
porating an NTC thermistor, with the
exception that R2 and the NTC should
be transposed.

In the case of an iron employing a
thermocouple and operating from a
40 V supply, the modifications shown
in table 1 should be adopted.

The above-described control unit is thus
suitable for use with a wide variety of
different types of soldering iron, and
represents a considerable saving in cost
over commercially available models.

The final point worth noting is that the
circuit can not only be used to regulate
the temperature of soldering irons, but
can be adapted for a number of other
applications requiring a thermostatic
control unit, such as. eg. clothes irons,
ovens, central heating etc. H

10.43

FAST NICD CHARGER

Most popular personal radios suffer from high current
consumption, so that it is sensible to power them from
rechargeable batteries. Unfortunately, with most battery chargers
on the market it takes up to 15 hours to recharge batteries. The
charger proposed here does it in under an hour.

Most of the smaller NiCd batteries on
the market today have sintered electrodes
that can withstand fairly high currents.
This makes it possible for such batteries
with capacities up to about 500 mAh to
be recharged to 80% of their capacity
within an hour.

The problem with fast charging of NiCd
batteries is switching off the charge cur-
rent at the right time. With these bat-
teries, unlike, for instance, lead-acid bat-
teries, it is not possible to determine this
from the charging voltage.

The circuit

The circuit consists of four distinct sec-
tions as shown in Fig. 3. In Fig. 3a are
the supply section with rectifier, Bi, and
5-V voltage regulator, ICi, and a Type
4060 timer, IC4. The sections in Fig. 3b
and Fig. 3c are identical to enable the
charging of two LR6-size (U7) batteries.
It should be noted that batteries cannot
be connected in series in a voltage-
controlled charger, because the batteries
are never fully charged at the same time.
Each of the sections in Fig. 3b and
Fig. 3c consists of a charging voltage
monitor and switch, IC2 (IC3), and a
Type BD680 darlington, Ti (T2), which
functions as the source of the charging
current.

The supply section is provided with an
‘on’ indicator, D11. The input comes
from a mains transformer with an 8-V,
1.5 A secondary. The rectified voltage is
smoothed by C3. The regulated 5-V out-
put of ICi is used as reference voltage
and to power IC4.

The clock in the 4060 operates at a fre-
quency of 2.5 Hz, determined by Rio
and Ce. After 2' 3 ( = 8192) clock pulses
(= about 54 mins) from reset key Si be-
ing pressed* output Q14 (pin 3 ) becomes
logic high.

The reference voltage for IC2 and IC3 is
derived from the regulated 5-V output of
ICi by potential divider Ri— Pi— R2.
The reference voltage is the value of the
battery voltage at which the charging
process must be terminated: it is in-
variably 1.5 V.

10.44 eleklo, India October 1988

The reference voltage is applied to the in-
verting input of IC2 (IC3) via R3 (R4).
The battery voltage is applied to the
non-inverting input of the opamps. The
ICs also function as bistabics, which,
together with IC4, are provided by Si
with a set pulse at the onset of the charg-
ing process.

When Si is pressed, the inverting input
of IC2 (IC3) is briefly connected to
+ 8 V via Di. That is sufficient to switch
the output of the opamps to 0 V. If the
battery voltage is lower than the refer-
ence voltage, the comparator remains in
this state and current source Ti is
switched on. A current of about 0.5 A
then flows through the battery (bat-
teries) and D4 (Ds) lights. This LED
does not only serve as charging indi-
cator, but, in conjunction with D3 (Da),
also serves as voltage reference for Ti
(T2). The magnitude of the charging cur-
rent is determined by the value of Rs

(Ri»).

When the potential across the battery
becomes greater than the reference

voltage, the relevant opamp (IC2 or IC3)
toggles. Its output then becomes logic
high and the charging process stops,
because no base current can flow in the
current source, which therefore switches
off. The feedback via R4 (Ris) and D2
(D7) maintains the opamp in this state. It
can only be reactuated by Si.

If, because of a disparity between the
battery and reference voltages the cur-
rent source is not switched off, the timer
comes into ' action. After about 54
minutes (see above), output Qi4 (pin 3)
becomes logic high, which causes the
opamps to be reset and thus terminate
the charging cycle.

Extra charge

After about 54 minutes, the batteries are
charged to something like 80% of their
capacity, which is sufficient for their use
in the personal radio. Considering that
many of such radios draw a current of
around 100 mA, the batteries will give

Fig. 1. Recommended swilch-off voltage and Fig. 2. Typical charging voltage vs charging

over-charge voltage pertaining to sintered current (as a percentage of battery capacity)

NiCd cells as a function of ambient tempera- characteristic of sintered NiCd cells for

turc during charging. It is not advisable to a charging current of IC ( = 0.5 A for a

use fast charging at ambient temperatures 500 mAh battery). A charge of about 80% of

below 10° C full capacity is reached when the battery'

voltage has risen to just above 1.5 V.

about 4 hours playing time. If they had
been charged to their full capacity of
500 mAh, they would have afforded 5
hours playing time.

To enable batteries to be charged to their
full capacity, a resistor (dashed across
the current source in Fig. 3b and Fig. 3c)
may be fitted. This resistor allows a
small charging current to continue to
flow after the current source has
switched off. During the fast charge, the
resistor is virtually short-circuited by the
darlington and is, therefore, of no practi-
cal consequence.

For a ‘normal’ charging current of 45-
50 mA, the value of this resistor is 150R
and rated at not less than 0.5 W, but
1 W is better. Normally, there is no risk
in leaving the batteries on charge via this
resistor for days on end. If this happens
habitually, however, it is belter to give
the resistor a value of 220R or even
270R, rated at 0.5 W.

A reasonably charged battery can be
kept at that level by giving the shunt re-
sistor a value of 330R.

Finally

Although it was said earlier on that the
supply is obtained via a mains trans-
former, it is also possible to obtain it
from a mains adapter that gives an out-
put of 8 V a.c. or d.c. (in the latter case,
there is, of course, no need for the recti-
fier in Fig. 3a).

Darlingtons Ti and T2 require a heat .

Fig. 3. The circuit diagram of the battery charger: if only one battery is required to be charged at any one time, the section in Fig. 3c may
be omitted.

10.45

4

Fig. 4. The primed circuit of Ihe baltery charger.

Fig. 5. Top inside view of Ihe prototype of the battery charger.

If there is no need for charging two bat-
teries simultaneously (some personal
radios work from one 1.5 V battery),
one of the sections in Fig. 3b or Fig. 3c
can be omitted. In that case, the input
current needs to be only about 0.7 A in-
stead of 1.5 A.

The battery voltage varies from manu-
facturer to manufacturer, but lies nor-
mally around 1.5 V. During the first few
charging cycles it is, therefore, rec-
ommended to set Pi to 1.5 V. Make sure
that the batteries are fully discharged
before connecting them to the charger,

and ascertain (with the aid of D< and Ds)
when the current sources switch off. The
optimum charging period is about 55
minutes. If the current sources switch
off after a shorter period, increase the
reference voltage slightly. If the reference
voltage was originally set too high, the
timer will switch off the charger. The
ideal situation is that the comparators
switch off just before the timer can do

In all this, it is assumed that the ambient
temperature remains at roughly the same
level. At lower temperatures, the charger

switches off slightly sooner; at higher
temperatures, somewhat later.
Construction of the charger on the
printed-circuit board should present no
difficulties. .After the board has been
populated, it may be fitted, together
with the mains transformer (if used),
reset switch, lights, on/off switch, and
battery connector in a neat enclosure as
shown in Fig. 5.

It should be possible to construct the
charger for about £15.00 (at UK prices:
in other countries, this cost may be quite
different).

MICROPROCESSOR-CONTROLLED
RADIO SYNTHESIZER — 2

by Peter Topping

This final instalment of the article deals with the construction and
setting up of the multi-purpose RF synthesizer. This has been
divided in a number of building blocks to allow its use in a large
number of applications, from upgrading surplus SW receivers to
providing state-of-the-art tuning on modern tunerheads for the
VHF FM band.

It will have been evident from Part 1 of
this article 111 that the microprocessor-
controlled synthesizer is a relatively
complex project with many possible con-
figurations and applications. To enable
its use with many types of receiver (SW,
SW/MW, VHF FM), and to allow the
user the choice between three types of
display, the synthesizer system is divided
in a number of sub-units:

1. Microprocessor board;

2. Keypad;

3. One or more displays (these are not
necessarily of the same type);

4. Power supply;

5. Synthesizer board;

6. VHF prescaler board (/lo up to
150 MHz);

7. SW prescaler board (/lo up to
40 MHz).

Items 1, 2, 3 and 4 are fitted in a separate
enclosure, while 5 and 6, 5 and 7, or 5
and 6 and 7, are incorporated in the
existing receiver. Item 4, the power
supply for the microprocessor/display
unit, is not discussed here as it assumed
that the constructor is capable of
building a simple regulated 5 VDC
power supply without the need for
repeating an application of the 7805.
Similarly, the 5 V supply for items 5, 6
and 7 should be relatively simple to ob-
tain from the receiver.

As already noted in Part 1, the supply
voltage for the opamp in the synthesizer
module (Fig. 3) is governed by the maxi-
mum reverse voltage required on the
varicap that tunes the local oscillator.
Remember that this auxiliary voltage is
also applied to varicaps Di and D2 in
the RIT circuit, so that it must remain
below +10 V. Where +30 V is to be
used, make sure that this is only applied
to Cn and IC2.

Prescalers

The circuit diagram of the prescaler for
SW receivers is shown in Fig. 8. Transi-
stor T11 ensures that the local oscillator

Prototypes of all boards, completed and ready for wiring. From the left to the right: Synthesi-
zer, microprocessor, keyboard, SW prescaler. VHF prescaler, LED display, LCD display.

in the receiver is not excessively loaded,
and at the same time functions as an
amplifier/digital driver for divider IC12.
The prescaler has a divide-by-five and a
divide-by-ten output (refer to Table 1 in
Part 1). It can handle LO input signals
of up to about 40 MHz, and has a sensi-
tivity of 150 mV™, at 20 MHz. The
maximum usable frequency can be in-
creased to over 60 MHz by using a Type
74F90 in position IC12.

The VHF prescaler is a rather more
elaborate circuit — see Fig. 9. Ahead of
the divide-by-ten ECL counter, ICu, is
a two-stage direct-coupled wideband
amplifier, Ts-To. Although the SP8660 is
stated to have a TTL- and CMOS com-
patible open collector output, some in-
terfacing and filtering of the signal is re-
quired before it can be applied to the LO
input of the MC145157 (ICi). Sensitivity
of the VHF prescaler decreases from
30 mV™ at about 100 MHz to
500 mVrnu at 190 MHz (note that the
latter frequency exceeds the maximum
specification of the SP8660). In an ex-
perimental set-up, the VHF prescaler
was found to have an abolute maximum

Corrigenda lo Part 1:

• Pull-up resistor R« (Fig. 4) should

be labelled R-o.

• The IF offset table in the EPROM

starts at l9DBu, not 1E05 h.

• R« (Fig. 3) is a 3K9 resistor.

• Pin II of ICu should be connected

to ground.

10.47

input frequency of 250 MHz. The am-
plification of T9 is defined mainly by
the value of Rss.

Five modules on one PCB

Printed circuit board 880120 (Fig. 10) is
quite large because it holds the following
sub-units (the associated circuit dia-
grams are given in parentheses):

■ Synthesizer (Fig. 3);

■ Microprocessor circuit (Fig. 4);

■ Keyboard (in upper left-hand corner
of Fig. 4);

■ SW prescaler (Fig. 8);

■ VHF prescaler (Fig. 9).

With the exception of the section for the
keyboard, the PCB is double-sided, but
not through-plated. The prescaler and
synthesizer boards have large pretinned
copper earth planes at the component
side to prevent stray radiation.
Commence the construction with care-
fully cutting the large PCB in six to ob-
tain the previously mentioned boards.

Microprocessor board:

The construction of the microprocessor
board is not difficult, but should be
done strictly in the order given below to
avoid difficulties caused by the absence
of through-plating (this was not used
here to keep the cost of the PCB within
limits). Through-contacting of tracks at
the component and solder side of the
board is effected by soldering the rel-
evant component terminals or pins of IC
sockets at both sides of the board-
Start by fitting the 40-way socket for the
microprocessor, ICj. A normal IC
socket will not be very useful here since
it does not allow soldering to tracks at
the component side. Two 20- way ter-
minal strips, or a 40-way wire-wrap
socket, are suitable alternatives. A
similar way of mounting applies to the
other three ICs: first mount the socket
for the 74HC373 (IC4), then that for the
74HC00 (ICs) and, lastly, that for the
EPROM 27C64 (ICs). Constructors
with lots of confidence may, of course,
solder all ICs direct on to the board, but
this will make their removal at later stage
very difficult.

Now fit the passive components, starting
with the eight 100 k£2 pull-down
resistors R19 to Ris, which are mounted
vertically and commoned at the top side
by a horizontal running ground wire
(alternatively, use a 9-pin SIL resistor ar-
ray).

In a number of cases, one or both ter-
minals of a component will have to be
soldered at both sides of the board to
connect tracks. At the component side,
some terminals run quite close to, or
over, tracks they should not be connec-
ted to. To avoid short-circuits, bend the
wires accurately, and mount components
slightly above the board surface. The
mounting of the 4 diodes and the quartz

crystal (two possible enclosure sizes are
allowed) should not present problems.
The connection of the microprocessor-
board to the keyboard is made in a 10-
way flat ribbon cable, terminated in 10-
way press-on (IDC) sockets at either end
for pushing on associated headers on the
boards. The pinning of the connection is
given in Table 3. All other connections to
the microprocessor board are made via
solder terminals.

Before mounting the ICs in their sockets,
carefully inspect the microprocessor
board for solder faults and/or short-
circuits.

Synthesizer board:

No through-contacting is required here,
with the exception of the three solder ter-
minals for the ground connections
(supply, lo input, lo tone output). The

board is relatively densely populated,
but its completion should not present
problems. A few hints, though: do not
use a socket for ICi; the type indication
printed on varicap diodes D1-D2 should
face the quartz crystal.

SW prescaler board:

Solder IC12 direct on to the board. The
terminals for the input and output coax
cables are soldered at both sides of the
PCB.

VHF prescaler board:

First, wind L2 and L3 as 6 turns of
0.2 mm dia (SWG36) enamelled copper
wire through 3 mm long ferrite beads.
When mounting these chokes, make sure
that the copper wire can not touch the
earth plane at the component side of the
PCB. Next, mount the soldering ter-

between addresses 19DB and 1A0A.

Parts list

KEYBOARD

Si . . .Sie incl. = Digitast momentary action
key (ITT Schadow or ITW).

K2 = 10-way straight header; 2x5 pins in 0.1

MICROPROCESSOR BOARD

Resistors (±5%);

R 12 ■= 10M

Ri3. . ,R|7 incl. = 1 OK
Ria. . .R30 incl.-IOOK
R70 - 27K

Capacitors;

Ci4=10p; 16 V; radial
Cia. . . Cia lncl.”100n
Ci8;C20 - 39p
C42 = 1 0n

Semiconductors:

D4. . ,D7 incl. = 1 N41 48
IC3=MC146805E2 (Motorola;

IC4 = 74HC373

ICa= programmed EPROM Type 27C64;

ICe = 74HC00
Miscellaneous:

Ki= 10-way straight hsader; 2x5 pins in 0.1

X2= quartz crystal 1 MHz.

Si7= externally fitted push-to-make button.

toggle switch.

SYNTHESIZER BOARD

Resistors (±5%):

Ri = 10K
R2- 100K
R3 = 68K
R4;R5 = 1M0
Ra=270R
R7-24K"

R8 = 39K"

R9 = 3K9‘

Rto=2K7"

Rn =6K8"

Pi = externally fitted 100K linear
potentiometer.

' See text.

Capacitors:

Ci-47p; 35 V; tantalum bead
C2;C3- not fitted
C4 = 2p2; 16 V; radial

C5;C9=100n
Cs;Cio;Cli = 10n
C7;C8= InO ceramic
Cl2;Cl3=1pO; MKT

Semiconductors:

Di;D2=KV1235Z (Toko product; available from
Cirkit pic or Bonex Ltd. These diodes are
sometimes supplied in sets of three matched
devices held together as a Chocobreak* unit).
Da= externally fitted red LED
ICi =MC145157 (Motorolal
IC2 = 741
Ti=BC547B

Inductor:

Li = 22pH axial choke.

Miscellaneous:

Xi * 10 MHz quartz crystal.

VHF PRESCALER

Resistors l±5%):

Rs4= 1M0
R58 = 220R
R57 - 270R
RS8 = 68K'

R59 = 3K3
Reo=27K
R81 = 10K
R62= 100R
R63=470R
Ra4 = 6K8

Capacitors:

C27;C28;C30= 1 0Op
C29;C3i;C32=1nO
C33 = 6p8

C34;C35;C36 = 1 0On
C43=10n

Inductors:

L2;L3= 6 turns 0.2 mm dia enamelled copper
wire through a 3 mm long ferrite bead.

Semiconductors:

ICi3 = SP8660 (Plessey;

Ta = BF256C

T9=BFX89 ICricklewoodl
Tl0=BF199

Resistors l±5%):
R68 = 470R
R68-27K
R67 = 1 0K
Raa=100R
R68 = 6K8

Capacitors:

C37 = 22p; 16 V; radial
C38,C40;C4i = lOOn
C39=10n

Semiconductors:

IC12-74LS90
Til =BF199

ALL OF THE ABOVE CIRCUITS ARE FITTED ON
THE RELEVANT SUB-PCBs CUT OFF FROM
TYPE 880120-1 |

STATIC LC DISPLAY BOARD

Resistors l±5%):

Rsi;Rs2;RB3= 100K

Capacitors:

C24 = 100n
C25 = 10n

Semiconductors:

IC9;ICio;ICii =MC1441 15P (Motorolal
LCD= general-purpose 6-digit static liquid
crystal display, e.g. Type LTD229-R12 (Philips
Components), RS587-327 1. ..

Miscellaneous:

PCB Type 880120-2

LED DISPLAY BOARD

R3i;R32 = 27K
R49;R6o:R56 = 100K
R33. . . R48 incl. = 270R*

' See text.

Capacitors:

C2i = 100n
C22,C23 - 22n

Semiconductors:

IC7;IC8=MC14499P (Motorolal
LD 1 . . . LDs incl. = HD 1 1 07R (Siemens;
ElectroValue 0784 33603 or 061 432 4945)
T2. . .T7 incl. = BC1 82

Miscellaneous:

S22 - externally fitted miniature SPST switch
(see text).

PCB Type 8801 20-3'

minals (three ground posts are soldered
at both sides of the PCB). Proceed with
fitting the resistors and capacitors, fol-
lowed by the transistors. Lastly, solder
the prescaler chip direct on to the board.

Keyboard;

The construction of this unit is so simple
as obviate the need for further dis-
cussion.

Multiplexed display board:

As already stated in Part 1, this unit is

1 0.50 elektor India October 1988

not supported by a ready-made PCB
because the multiplexed LC display is a
relatively hard to obtain item. Construc-
tors in possession of the Type 4200-365-
920 from Hamlin may mount it on a
piece of Veroboard, together with the
three passive components and the dis-
play controller Type MC145000. As there
are relatively few connections to be made
(compare the circuit diagram. Fig. 7, to
that of the static LC display unit.
Fig. 6), the actual construction should
not prove too difficult. When using the

multiplexed display, be sure to fit it in a
metal enclosure to reduce stray radi-

LED and static LC display

The single-sided printed circuit board
for the static LC display is shown in
Fig. 11. This is a very compact unit,
whose construction is commenced with
the mounting of the ten wire links, fol-
lowed by the three 24-way IC sockets,
five passive components and five

soldering pins.

THE DASHED LINES ON THE COM-
PONENT MOUNTING PLAN INDI-
CATE THAT THE LIQUID CRYSTAL
DISPLAY IS MOUNTED AT THE
TRACK SIDE OF THE BOARD. BE
EXTREMELY CAREFUL HAND-
LING THE FRAGILE GLASS
DEVICE, AND MAKE SURE THAT
IT IS FITTED THE RIGHT WAY
AROUND.

Pin 1 of the LCD is practically opposite
pin 13 of IC«, as shown on the compo-
nent overlay. Hold the display slightly
oblique in clear light to make the in-
dividual segments visible. Turn the dis-
play so that the row of decimal points is
horizontal and towards you. Pin 1 of the
unit is the leftmost terminal, below the
lowest horizontal segment of the first
(most-significant) digit. The above
description is given because not all 6-
digit LC displays have a marker for pin
1 .

The socket for the 50-pin LC display can
be made from the terminal strips of a 40-
way and a 14- or 16-way IC socket.
Mount these strips slightly above the
board surface to enable soldering to the
tracks. Then fit the display carefully, ob-
serving the previously mentioned orien-
tation. Be sure that the display is sup-
plied with side pins, so that it can be
mounted as an integrated circuit.

Constructors opting for a 7-segment
LED display should have little difficulty
completing the printed circuit board
shown in Fig. 12. First fit the 6 wire
links, then the sockets for the displays
(cut off 14-way IC sockets to make your
own 10-way types).

Initial test

Make all the necessary connections be-
tween the completed microprocessor
board, synthesizer board, keyboard and
the static LCD or LED display. The
prescalers are not required as yet. Do not
forget to temporarily connect the reset
switch and band/I. F. switches to the mi-
croprocessor board, and be sure to ob-
serve the terminal designations printed
on the PCBs. Note that the LED display
board is driven with the s4t signal pro-
vided by the synthesizer board via an op-
tional switch, S22 (see Part 1).

It is recommended to power all the units
from a single 5 V supply. Where a
separate 10 V supply is not available,
connect the +10 V terminal on the syn-
thesizer board to +5 V also.

Apply power. The display should be
cleared after operating the MODE but-
ton. If this does not happen , press the
reset key. Verify that RESET of the mi-
croprocessor is logic high. Now type a
few numbers on the keyboard, and check
that these are displayed correctly. Read
up the section on the use of the com-
mand keys (Part 1), and check that the

Side view of the completed static LC display
board, showing that the controller chips and
the glass LC display are mounted at opposite
sides of the board.

special mode indication symbols appear
on the display (decimal points, dash on
the LED type and small square on the
LC type). The out of lock LED should
light because the local oscillator (and
prescaler) is yet missing from the phase-
locked loop.

It can safely be assumed that the micro-
processor board, keypad and display
function correctly if the above test
checks out.

In the receiver

Since the microprocessor-controlled
radio synthesizer is a general-purpose
design, users must rely on their own
knowledge and experience when it comes
to incorporating the prescaler and syn-
thesizer modules in an existing receiver.
A few general observations can be made,
though:

1. Be sure to understand how the receiver
is actually tuned. If it has mechanical

tuning (inductor/variable capacitor),
this must be replaced with a varicap sys-
tem as shown in Fig. 1 in Part 1. For SW
receivers, it is recommended to use a
modern varicap with a relatively high
Cam/Cmin ratio, e.g. Toko’s KV1235 or
KV1236, to enable using a low control
voltage (max. 10 V or 25 V respectively).
If the local oscillator inductor provides a
DC path to ground for the varicap, and
C is not required for defining the tuning
rate, then C and R can be omitted.

It is strongly recommended to first con-
vert the receiver as shown, then use an
external potentiometer to find out what
range of the tuning voltage is required to
ensure the receiver’s original frequency
coverage, and only then attempt to bias
the varicap by the synthesizer’s output
loop filter.

If the receiver already has electronic tun-
ing, i.e., if it is tuned by a single tuning
voltage obtained from a potentiometer
or a channel preset unit, simply measure
the tuning voltage range, and connect
the tuning voltage input to the output of
the loop filter via a short length of
screened cable. Dimension the supply to
opamp IC2 as explained above.

2. The local oscillator in the receiver
must be ‘tapped’ to provide the input

signal for the relevant prescaler. It is im-
portant to ensure that this signal is of

sufficient amplitude, but every care
should be taken to prevent the oscillator
being significantly loaded. Most tran-
sistorized receivers have a buffer stage
between the local oscillator and the
mixer. The input of the prescaler is then
conveniently connected to a low-im-
pedance point at the buffer output by
means of a short length of thin coaxial
cable. Coupling out of the LO signal via
a tank inductor in the oscillator is not
recommended as it will degrade the qual-
ity (Q) factor — this may limit the tun-
ing range, and reduce the oscillator out-
put power to the mixer.

For some SW and MW applications, it is
possible to omit the 40 MHz prescaler
and drive the MC145157 direct with the
oscillator signal. This can, however, only
be done when the LO signal has a fre-
quency lower than 15 MHz, and an am-
plitude of at least 500 mV. The author
developed and debugged his prototype
of the synthesizer using a simple
LW/MW/SW radio based on the Type
TDA1083 one-chip receiver IC. This op-
erated satisfactorily with pin 5 AC-
coupled direct to the MC145157 with no
buffering.

For VHF FM applications, the prescaler
used (Fig. 9) is sufficiently sensitive to
enable driving it by relatively small
amplitudes of the LO signal. For in-
stance, in the case of the LP1186
tunerhead, the LO signal can be taken
from the emitter of oscillator transistor
BF195 (near the centre of the PCB).

It is strongly recommended to check that
the prescaler used (whether the SW or
VHF type, or both) functions correctly.
Temporarily fit it in the receiver, near the
local oscillator, and measure the output
signal frequency (SW: +5 or +10; VHF:
+10) to verify that it receives enough LO
signal, and does not in any way affect
the receiver’s normal behaviour. Use a
15 MHz oscilloscope to check that the
output signal is of digital amplitude
(5 Vpp), and free from spurious pulses
and noise, which could point to parasitic
oscillation.

Before closing (he loop. . .

Be sure that the following- questions are
answered in the affirmative before actu-
ally connecting the completed sythesizer
to the receiver:

a. Does the receiver work as before with
varicap tuning installed, and can it be

tuned by a temporarily fitted poten-
tiometer?

b. Is the supply voltage for the active
loop filter (IC2) in accordance with

the maximum required tuning voltage,
and are the prescaler and synthesizer
boards correctly powered?

c. Does the prescaler supply a correct
output signal at all settings of the re-
ceiver tuning?

d. Is the band/I. F. setting for the micro-

10.51

processor board in accordance with
the actual intermediate frequency of the
receiver? (consult Table 1 and the techni-
cal specification of the receiver).

As a final check, leave the output of the
loop filter disconnected from the LO
tuning input, and program a frequency
within the receiver’s tuning range. Con-
nect a voltmeter or a DC-coupled os-
cilloscope to the loop filter output. As
the external potentiometer is operated to
tune the radio through this frequency,
the filter output should switch from one
extreme to another. Until all of the above
tests pass, it is not useful to close the
loop, as it is then very hard to
distinguish the cause of a problem from
its effects.

The microprocessor board, keypad, dis-
play, and 5 V power supply are housed
in a desk-style ABS enclosure. The
band/I. F. selection switches are fitted as
a 4-way DIP switch block on the sloping
front panel.

The connection to the synthesizer board
in the receiver is made in a 6-wire cable
terminated in a 9-pin D-connector
plugged into a mating socket at the rear
of the enclosure. The signals carried in
this cable are:

■ LE, DATA and CLK (from micropro-
cessor to ICi);

■ S/R (from ICi to the LED display
board);

■ RESET (from Ti to the microproces-
sor);

■ ground.

It should be noted that S/R is not re-
quired when the static display board is
used.

The quartz crystal on the microproces-
sor board may be replaced with a
1.8432 MHz or 2 MHz type, which is
generally less expensive than a 1 MHz
type.

The only problems that could be ex-
perienced with the synthesizer are in-
stability of the LO frequency and aud-
ible reference frequency on the output of
the radio. Either of these problems
should be resolved by empirically ad-
justing R- through Rio. R« and Rio
should normally be in the range from
1 kQ to 10 kQ, and Rt and R* in the
range from 10 kQ to 50 kQ. Accurate
values can not be predicted as these de-
pend on factors which vary between
oscillators. The most significant of these
is the tuning rate expressed in MHz per
volt. The values shown in Fig. 3 were
used with a dual-conversion shortwave
receiver with a tuning rate of about
1 MHz/volt.

If, after adjusting the above resistors,

the reference frequency can still be
heard, the tuning rate may need to be re-
duced by using a smaller valued C
(Fig. 1), and adding a fixed capacitor
across the oscillator inductor. This will
increase the Q of the oscillator and re-
duce phase noise. If the tuning range
becomes too small, it can be restored by
switching oscillator inductors.

Finally, the prescaler and synthesizer
boards installed in the receiver should be
fitted in screened metal enclosures.

Reference:

"’Microprocessor-controlled radio syn-
thesizer — 1. Elektor India,

September 1988.

motorphone

It is almost always impossible
for a motor cyclist and his
passenger to maintain aural contact
without dangerous acrobatics. The cir-
cuit shown in the figure affords effort-
less voice contact. The microphone
amplifier of post I is formed by an
amplifier stage T I followed by a super
emitter follower. The DC coupling
determines the current through R1 and

the base-emitter voltage of Tl . So when
one party speaks he hears himself, so
that he knows the system is working.
Since the two posts are connected in
series, the signal generated in post 1
travels through both telephones, so
that this ‘intercom’ needs only one wire
between the two posts.

The main function of the supply with
T7, T8 and T9 is noise suppression.

whilst at the same time the system is
made short-circuit proof. Via R13 and
D2 an audible indication of the
trafficators is obtained. The interphone
posts can be made so small they can be
mounted in the crash helmets. The
supply is mounted on the machine.

The microphones are crystal types.

COMPUTER-GENERATED COLOUR
TEST CHART

For ATVers and other video enthusiasts in possession of a BBC,
Electron or Archimedes computer we have developed a program
that generates a test chart with resolution test blocks and
selectable foreground and background colours.

Although a computer can not replace a
pattern generator for adjustment of con-
vergence or RGB circuits in a modern
colour TV, it is eminently suitable as a
low-cost means of generating a steady
high-resolution picture, which is often
required for testing video circuits. One
particularly interesting use of the com-
puter as a video generator is linearity
and bandwidth alignment of an ATV
(amateur television) transmitter. The

colour output signal of the BBC, Elec-
tron and Archimedes computers is of ex-
cellent quality, and has more than suf-
ficient bandwidth, so why not do
something useful with those complex
and expensive video chips? Software
alone can do it.

The REM (remark) lines in the listing ex-
plain the operation of the program. The
colour can be changed by pressing the Z
or W key on the keyboard.

1 10.53

sine-square-triangle generator

Unlike the more usual type of
function generator, in which the
sinusoidal output is derived by shaping
a triangular waveform, the basis of this
circuit is a Wien-bridge oscillator, which
provides a sinusoidal output. The square
and triangular waveforms are then
derived from this.

The Wien-bridge oscillator is built
around CMOS NAND-gates N 1 to N4,
and amplitude stabilization is provided
by T1 , D1 and D2. These diodes should,
if possible, be a matched pair, for
minimum distortion. The frequency
adjustment potentiometer P 1 should
also be a good quality stereo poten-
tiometer with the tracks matched to
within 5%. The preset R3 provides
adjustment for minimum distortion and
if matched components are used for D1 ,

also varies with frequency. In practice
the amplitude variation is relatively
unimportant, since the generator will
usually be used with a millivoltmeter or
oscilloscope and the output can be
monitored. Adjustment of the triangle
amplitude is provided by P3.

As the CMOS gates cannot drive very
low load impedances an output buffer
amplifier is provided, which greatly
increases the usefulness of the
generator. The amplifier is capable of
driving loads of 4 fl or greater, which
makes it particularly useful for loud-
speaker testing. If the generator is
likely to be used to generate square-
waves at low frequencies (less than
100 Hz) it may be worth increasing the
value of CIS to make the top of the
square wave flatter. Quiescent current

adjustment is provided by P4 and this
should be set to about SO mA. PS
controls the output amplitude.

As the impedances around the CMOS
circuits are fairly high the generator
should be mounted in a screened
metal box to avoid interference pickup.
If a mains power supply is used this
should be screened from the rest of the
circuit to avoid hum pickup. For
optimum results at high frequencies
C a (8p2) and Cb (33 p) can be added;
note that these components are not
shown on the layout for the p.c. board.

selex-38

OHM-ADAPTER

Resistance measurement is
a common feature of every
multimeter. Just a simple
zero adjustment is all that it
requires to start the
resistance measurement.
However, in almost all types
of multimeters it is difficult
to obtain a clear reading in
the lower and upper ranges.
This happens mainly due to
the logarithmic nature of
dial calibration in the
resistance range.

The Ohm-Adapter circuit
presented here is meant for
removing this defect. The
multimeter to be used with
this adapter must have a 0
to IV range, or at least a 0 to
2 V or 0 to 3 V range of
voltage measurement. With
the adapter, it will be
possible to read resistance
values from 0 to 1 0 Ohms, 0
to 100 Ohms, 0 to 1 Mega
Ohms and 0 to 10 Mega
Ohms without any problem.

The Measuring
Principle

The measuring principle is
very simple, as shown in
figure 1. A constant current
source sends its current
through Rx, the resistance
under test. Due to the
constant current, the voltage
drop across the resistor is
always proportional to its
Ohms value. The voltage is
further amplified by the op
amp A and fed to the
multimeter for
measurement.

Circuit Principle

Figure 2 shows the circuit
principle in more detailed
form. IC1 is the constant

current generator and IC2 is
the amplifier, both are op
amps. The constant current
source functions as follows:
at one input of the op amp
lies a constant reference
voltage u. In the feedback
branch, we have the
resistance under test. The
current I also flows over the
fixed resistance R1. The
voltage developed across
R1 also appears at the
inverting input of the op

The op amp now makes an
effort to make this voltage
across R1, equal to the
reference voltage /*. This is valid
irrespective of the value of
Rx because R1 is constant.
The result is that the voltage
across R1 always remains
equal to n and in doing this,
the voltage across Rx
becomes always
proportional to its ohms
value. This must happen
because the current flowing
through Rx is constant and
independent of its ohms
value. Ohm's Law tells us
why this happens.

In this manner, we have
been able to obtain a
voltage value which is
proportional to the ohms
value of Rx. Unfortunately
this voltage cannot be
connected directly to the
multimeter, because of the
presence of jtalso at the
output of the first oP amp.
The output of the first of op
amp is made up of the
reference voltage n plus the
voltage across Rx. We must
now have a way of
removing the effect of

voltage u, before feeding
the voltage to multimeter.
This task is given to the
second op amp stage made
of IC2, which is connected
as a differential amplifier.
This differential amplifier
amplifies the difference
between the two input
voltages - one of which is
the output of IC1 connected
through the voltage divider
R2' and R3',. The second
input is u, the reference
voltage connected through
R2/R3 voltage divider. As
long as the ratio R2/R3 is
same as R27R3', the
difference voltage is
proportional to the voltage
across Rx. The ratio R3/R2
decides the amplification
factor of IC2 and thus we get
an amplified voltage at the
output of IC2 which is still

proportional to the voltage
across Rx. R2 should
preferably, be equal to R2'
and R3 equal to R3'.

The Practical Circuit :

The detailed diagram of the
circuit of the "Ohms
Adapter" is shown in figure
3. Though the circuit of
figure 3 looks more
complicated compared to
that in figure 2, the basic
principle of operation is
same.

The power supply here is a
9V battery pack. SI is used
to connect/disconnect the
power supply. IC1 in this
circuit is not the same as of ICI
of figure 2, this 1C is an
adjustable voltage
regulator, which sets the
reference voltage u to a
value of 4.75 Volts. The

selex

second stage is IC2 which
does the function of IC1 in
figure 2. This is the constant
current generator.
Resistances R4, R5, R6, R7
are selected through switch
S2a. These are used to
decide the constant current
that will flow through the
test resistance. This must
change with the range
selected. R4 gives a constant
current of 10 mA, while the
resistance R7 gives 0.1
microamp. Through
selection of these four
resistances, we can select
four different measuring
ranges, as described earlier.
Transistor T1 is used to
avoid overloading of IC2 by
the current flowing through
the resistance under test.
After the constant current
source, comes the op amp
IC3 which serves to amplify
the output voltage
proportional to Rx. The
resistances R8 and R9 can
be compared to the
resistances R2 and R2' of
figure 2, Resistances R12 •
and R13, and the pair RIO
and R1 1 correspond to R3
and R3' in figure 2. The
correlation is direct.
Selection of either R12 or
R13 and RIO or R1 1 through
the range selector switch S2
changes the factor of
amplification. This factor is
10 for range A and B, where
as it has to be reduced to 1
for the higher ranges C and
D.

At the output of IC3 we have
a resistance R14 and
multimeter M with parallel
diode array of D1, D2 and
D3. The resistance R14 and
the diodes serve as
protection for the meter
when there is no resistance
connected at Rx. The diodes
do not allow the voltage
across the meter to rise
above 2v. In the case where
Rx is absent, the output
voltage of IC3 can rise to 9V.

ar198B 10.57

selex

Construction and
Alignment.

Figure 4 shows the
component layout to be
followed for construction of
the circuit. The entire circuit
requires a SELEX PCB of
size 2. The soldering must
be done in our usual
sequence - jumpers first,
then resistors, capacitors,
diodes, transistors and then
the ICs. IC2 and IC3 should
be preferably mounted on
Sockets. The resistance R7
(47 Mega ohms) is difficult
to obtain as a single
resistance and must be split
into four 10 M Cl resistors
and one 6.8 M It resistor to
get approximately 46.8 M ft
This is shown in the
component layout as R7a to
R7c.

Connections 1 to 8, Ml to
M3 and 0, + and the

multimeter connection are
going out from the PCB.
Wiring of the switches must
be done very carefully. The
complete circuit must be
checked carefully before
assembling the same into
an enclosure. The
component side of the PCB
as well as the soldering side
must be checked carefully.
When every thing is found
to be in order, the PCB can
be fitted into the enclosure
and wiring with switches
and sockets can be
completed. Alignment can
now be taken up.

- Short circuit Ax output
and Bx output. (Shown in
figure 4).

- Connect a multimeter to
the output of 1C 1 and
adjust PI till you get
4.75V at this point.

- With Ax and Bx short
circuited, measure the
voltage between the
output of IC1 and the
short circuited Ax & Bx.
Adjust P2 till it reads
exactly zero volts.

- Connect a known
resistance to Ax and Bx
now, and connect the
multimeter to the output
sockets provided for the
multimeter. If you have
connected a 68 O resistor,
(1%) put S2 in B position
and adjust P3 till meter
reads 680 mV.

The alignment is now
complete, check also for the
remaining three ranges
with known accurate
resistances.

For proper operation of the
circuit it is important to use
1% tolerance resistances

10.58 ele

selex

where ever specified. If it
becomes impossible to
obtain 1 % resistors, you
compromise for 5%
resistors.

After your adapter comes •
into fully working condition,
you can make some more
trials with known resistances,
using different ranges for the
same resistor and see the
results, wrong range
selection does not damage

the instrument but the
readings are not reliable.

If your multimeter has no IV
range but has only 2V or 3V
range, there are two
possibilities:

1. Use only half or one third
of the full scale for your
measurements.

2. Change R8 and R9 to 5k
for 2V range and 3.3k for
3V range.

This allows you to use the
full scale for measurement,
but you must always divide
the indicated value by either
2 or 3 depending on the
range you have.

The

Digilex-PCB

Is available!

Price:

Ra. 85.00 + Maharashtra Sales Tax.
Delivery charges extra: Rs. 6.00

Send full amount by DD/MO/PO.

Available from:

precious °

ELECTRONICS CORPORATION

Journal Division

Bombay-400 007
Ph. 367459. 369478
Tele* (011)76661 ELEK IN

10.59

NEW PRODUCTS • NEW PRODUCTS • N

Contactors

Larsen & Toubro Limited has intro-
duced bar-type contactors which arc spe-
cially designed for heavy duty under ad-'
verse conditions. These contactors, de-
signed series N, are suitable for both AC
and DC applications up to 1000 V and
can withstand up to 600 operations per
hour (Continuous) and up to 1200 opera-
tions per hour (occasional). Their DC2/3
ratings are equal to AC3 ratings and DC
4/5 ratings are equal to AC 4 ratings.
They are available in AC3/DC3 ratings
of 1 10A, 180A, 320 A, 600A. 900A in
various NO and NC pole combinations
up to 4 poles.

L&T's bar-type contactors are available
with accessories like mechanical inter-
lock and a mechanical latch.

Larsen & Toubro Limited • Switchgear
(S) Division • P O Box 278 • Bombay 400
038.

The Motwane Manufacturing Co Pvt.
Ltd. • Cyan Ghar • Plot 434A • 14th
Road • Khar • Bombay 400 052.

Temperature Transmitter

JNM’s two-wire temperature transmitter
is a design for 4-20mA current output
suitable for various ranges of ther-
mocouples, mV and RTD and has field
adjustable zero and span with a turn
down ratio of 3:1 . The two wire concept
helps to minimise the cable costs which is
a great advantage. The output is current
limited and upscale sensor fault indica-
tion is standard.

The RTD version features a built-in
lineariser and the specified overall accu-
racy includes linearisation errors and
having the output being linear with the
input temperature signal.

maintenance and inventory costs. This
unit can be conveniently mounted on a
pipe in a horizontal or vertical direction.

J.N. Marshall Pvt. Limited • P.B. No. 1
• Bombay - Pune Road • Kasarwadi •
Pune-411034.

Transformers

MAHAVIR INSTRUMENT TRANS-
FORMERS are manufactured and de-
signed by SHEPHERD TRANSFOR-
MERS. Applications are in ELEC-
TRONIC ELECTRO-MECHANICAL
AND ELECTRICAL INSTRUMENT
EQUIPMENTS.

•MAHAVIR' Transformers are tested as
per 1 ,S. Specification for resistance. Vol-
tage. No Load. On Load Open and Short
Circuit. H.V. Humidity Heat Run etc.
These transformers can also be supplied
with Electro Static shield.

M/s. Shephered Transformers •
Nityanand Nagar • OIT Link Bridge •
Ghatkopar (West) • Bombay- 400 086.

Digital Multimeters

Motwane have recently introduced,
three handheld Digital Multimeters;
model DM 452, a TRMS 4Zi digit and
DM 352 & 350, both 3'/: digit. These
multimeters are heavy duty, top of the
line, 7 function, 28 range handheld.
LCD readout, with shrouded leads and
terminals.

All the three Multimeters have been de-
veloped indigenously.

Additional features are AC/DC currents
upto 10A, Resistances upto 20M.ohm..
continuity, conductance and diode test-
ing, basic accuracy of 0.05% for the DM
452 with frequency response upto 10
KHz and 0.25% for the DM 352 and 350,
these instruments, with help of probes
can measure temperatures upto 1200°C,
AC currents upto 600A, high voltages
upto 40 KV AC/DC, frequencies upto 20
KHz and RF upto 200 MHz.

The transmitter has a removable PCB as-
sembly housed in a weatherproof
NEMA-4 enclosure. This arrangement
reduces the installation, commissioning.

Fasteners

FTC offer a very wide range of fasteners
to suit every requirement. Enquiries
may be forwarded to.

mtsiiiii

liiiffffff

Forged & Turned Components • Marine
House • Shop No F • 11-A, Navroji Hill
Road • 93 Dr Maheshwari Road • P.B.
No 5153 • Chinch Bunder • Bombay-400
009.

10.60

NEW PRODUCTS • NEW PRODUCTS • N

Z8 Microcomputer Has On-
Board Eprom

ARGATE BZ-APRIL 1987-SGS has
extended the popular Z8 microcomputer
family with the introduction of the
Z86E11, a complete single-chip mic-
rocomputer containing a powerful CPU,
RAM, serial and parallel I/o ports, two
counter/timers and a 4K UV EPROM.
The Z86E11 can be configured as a
stand-alone microcomputer, as a tradi-
tional microprocessor that manages up
to 1 20 k bytes of external memory or as a
processor element in parallel processing
systems.

The Z86E11 contains a 144-byte RAM.
organised as four I/O port registers, 16
control and status registers directly or in-
directly via an 8-bit address field. In ad-
dition, the register file can be considered
as nine 16-register workspaces and indi-
vidual registers within the selected work-
space can be addressed via a short 4-bit
addressing mode. The workspace or-
ganisation leads to compact programs
and also simplifies context switching dur-
ing interrupt and subroutine calls.

The 4 K x 8 on-board EPROM can be
programmed in three different ways,
first using a conventional EPROM prog-
ramming procedure, second using the
self-programming mode that allows
single bytes to be altered during normal
program execution, and in addition an
autoloading operation using a simple

An important feature of the Z86E11 is
the programmable readout protection
facility which allows users to inhibit ex-
teral access to proprietarty program
code. Readout protection is activated by
programming two non-volatile transis-
tors. Once set these security locks can be
reset only by erasing the entire EPROM
array.

M/S. SGS SEMICONDUCTORS (PTE)
LTD., • 29, Ang Ko kio Industrial Park
2, • Singapore 2056.

Clock Module

ION clock module F-Cl k/M6 is primar-
ily meant for automobiles. F-Clk/M6
also finds application in UPS systems,
emergency lights, battery panels etc.
The display used is 6mm vacuum fluores-
cent type with green glow. For variety, a
transparent acrylic filter of amber, yel-
low, green, blue or violet can be used to
change the display colour to individual
taste. Time is shown in hours and mi-
nutes in the 12 hours format. An addi-
tional feature is the display of month &
date in the calender mode.

The module measures 45(H) x 81
(W) x 14 (D) mm. Four push button
switches and a casing are required to
complete the clock. Wiring and operat-
ing instructions are provided with every
piece. To save the car battery from cur-
rent drain, the display is connected
through the ignition switch so that it
glows only when the car engine is on.
PROMOTION, • Blk # 4, Fir # 1, • 10,
Subash Cross Lane, • Bombay 400 057.

Push Buttons & Indicators

Efficient Engineering have developed
the Series 34 Lighted Push Buttons and
indicators with rectangular bazel to DIN
Standard 48 x 24 mm.

Two independent lamp circuits and one
or two pole SPDT Self cleaning and snap
acting microswitches, momentary or
maintained action or any combination,
offer 100% redundancy and a com-
prehensive functional range as shown.
Such comprehensive range, aesthetic
look and ample provision for engraving
on front face make these Lighted Push
Buttons and Indicators ideally suitable
for process Control Instrumentation,
Supervisory remote Control Systems,
Data Acquisition and Control Systems.
Sequencing Logic Controls, Hierarchy
Controls and Turn key Instrumentation.
M/s. SAI ELECTRONICS, A Div. of
Starch & Allied Industries)

• Thakor Estate, Kurla Kirol road, •
Vidyavihar(West), • Bombay 400086. •
Phone : 5136601/5131219.

Voltmeters Etc.

MECO manufactures a range of Porta-
ble Instruments for measuring various
Electrical Parameters like Amps, Volts,
Watts, Power Factor, Frequency, etc.,
Instruments are made in Moving Iron
type. Moving Coil type, Moving Coil
type. Moving Coil Transducer type &
Electrodynamimeter type. The com-
plete body is moulded from bakelite and
the movement is placed in a separate
compartment making it completely dust
proof. Instrument conforms to BSS &
ISS specification and is available in both
Industrial Grade Accuracy & Precision
Grade Accuracy.

MECO INSTRUMENTS PVT. LTD.

• Bharat Industrial Estate, • T.J. Road,
Sewree. • Bombay 400 015. • Phone:
4137423, 4132435. 4137253, •

Telex: 117101 MECO IN.

10.62 ele

NEW PRODUCTS • NEW PRODUCTS • N

Desoldering Station

Zevac Auslotsysteme GMBH, (WEST
GERMANY) manufacturers desolder-
ing tools, desoldering station and
machines, Zevac Tools are sturdy and
designed for high efficiency, ALS desol-
dering stations consist of vacuum unit
desoldering iron and stand, controlled
by a foot-switch with 220/24 Volts-sup-
ply, PVSG-60 is a desoldering system
with an integrated vacuum transducer
mounted on the soldering iron handle
with a finger operated control to enable
complete single handed operation with a
supply of 220 or 24 Volts. Zevac Desol-
dering machines are used for desoldering
standard integrated components, mul-
tilead connectors as well as chips,
capacitors, flatpacks etc.

M/s. Arun Electronics Pvt. Ltd. • 2 E,
Court Chambers • 35 New Marine Lines
« Bombay 40 020. Tel: 252160/259207.

Digital Multimeter

Ledtron Electronics has introduced au-
toranging Digital Multimeter Model DT
860A using latest American LSI
Technology. Its unique features are au-
toranging/manual mode, memory for re-
lative measurements, transistor HFE/
diode check, datahold, razor sharp LCD
with automatic range/function display,

10-ohm range for in circuit resistance
measurements etc. It can measure DC/
AC voltage up to 1000 V/750 V, DC/AC
current up to 15 A, resistance upto 20
megohm and audible continuity check by
buzzer. It is housed in compact, portable
high impact plastic case with till stand for
table top as well as field applications.

mi

GES ■

I

Ledtron Electronic • 170 Lohar Chawl.
• Bombay- 400 002.

Preset Counters

Micronix offers a range of Digital preset
counters for counting and controlling ap-
plications. The counters find applica-
tions in Machine tools. Pharmaceuticals
and food processing industries. Press op-
erations, process control panels, plastic
and rubber moulding industries, au-
tomobile industries etc.

These counters use CMOS technology.
Modular construction makes servicing
very easy. The presetting is done
through a set of Thumbwheel switches.
The actual count is displayed on a 0.5
inch seven segment LED display. It ac-
cepts a variety of input sensors such as
proximity switch, microswitch or optical
sensors.

It gives a set of chageover contacts for
control applications. These counters are

housed in DIN standard panel-mounta-
ble enclosures. Working voltages are
user selectable.

M/s. Micronix • D-74, Angol Industrial
Estate, • Udyam bag • Belgaum • Kar-
nataka- 590 008.

Temperature Controller

The ARTECH series 101 Temperature
Controller is a simple, rugged and accu-
rate instrument which can find applica-
tions in all area of Temperature Control.
The unit is fully solid staate, selectivity
using integrated Circuits. The circuitry
can withstand normal machine vibration
without adverse effects. Automatic cold
junction protection, lead resistance com-
pensation and open sensor protection
are standard features. The dimensions
are suited for flush panel mounting in a
92 x 92 mm cutout as per DIN 43700.

Artech Labs • A-3 Udyog Sadan No. 3
• Central Road • M I D C • Andheri
(East) • Bombay 400 093.

CORRECTIONS

advertisers’ index

Multi-function frequency meter

January 1988 p 1.41

The PRIME circuit, which is used for measuring large
time intervals, has been connected the' wrong way

• Pin 6 of IC2 should be connected to pin 9 of ICi,
not pin 5.

• Pin 2 of IC2 should be connected to pin 5 of ICi,
not pin 9.

The relevant connections between ICi and R12 on the
PCB are readily swapped - the two tracks immedi-
ately next to ICi are cut and then connected
crossways.

ABC ELECTRONICS .-,0 08

ADVANCED VIDEO I AB 10 68

BALAJI ENGINEERING 10 68

BHARAT ELECTRONICS 10 15

BINATONE 10.19

COMTECH 10 12

CROTECK 10 10

DYNALOG MICRO .10.11

DYNATRON ELECTRONICS 10 66

ECONOMY ELECTRONICS 10 04

ELTEK BOOKS-N-KITS 10 06

ESKAY AGENCIES 10 06

ESQUIRE 10.20

EXCEL 10.10

G S ELECTRONICS 10 08

IEAP 10.68

INDIAN TELEPHONE 10.13

LEADER ELECTRONICS 10 12

I FDTRON ELECTRONICS 10.04

MECO INSTRUMENTS 10 6/

NEW AGE ELECTRONICS 10 67

N.J. INTERNATIONAL . 10.36

OSWAL ELECTRONICS 10 1?

PACIFIC ELECTRONICS 10 65

PLASTART 'C 08

PRECIOUS 10 71

RAJPRI El ECTRONICS 10 08

ROLAND ELECTRONICS 10 67

SIEMENS 1009

SILICON AIDS 10 16

SMJ ELECTRONICS 10.02

1ANIIA ELECTRONICS 10 66

TELETONF RADIO ND 10 04

UNI IMITED ELECTRONICS 10 16

VASAVI ELECTRONICS 10 10

VISHA ELECTRONICS 1 0.07

ZENITH 10.61

>ad. Khar. Bombay 400 052.

I, Lower Parcl, Bombay 400 013.

M

DO IT YOURSELF

MH BV WEST ? 28
IIC No 91

LEARN-BUILD-
PROGRAM

The Junior Computer book
is for anyone wishing to
become familiar with
microcomputers, this book gives
the opportunity to build and
program a personal computer at
a very reasonable cost.

The Indian reprint comes to you from

elektof

Send full payment by
M.07I.P.0./D.D. No Cheque Please.
Packing & Postage free

to: ElEkTOR ElECTRONiCS pVT lid.
52-C, Proctor Road. Grant Road |E),
Bombay-400 007.