THE FUTURE BELONGS TO THE PHOTON
Volume-5. Number-2
February 1987
Electronics Technology
The future belongs to the photon 2.28
Digital signal processing 2.40
Software for the BBC computer 2.42
Flexicell to beat battery weight 2.66
Projects
Indoor unit for satellite TV reception 2.21
Universal control for stepper motors 2.31
Precision power supply 2.44
Computerscope 2 2.51
Information
:
i Editorial 2.05
News • News • News • 2.17
Meet • • • 2.65
New products 2.70
Licences & letter of intent 2.80
I Corrections 2.82
Guide-lines
Switchboard 2.77
Classified ads 2.82
Index of advertisers 2.82
Selex-20
Linear scale ohmmeter 2.58
The Cackling Generator 2.61
Power 2-63
Electronic Switch 2.64
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INDOOR UNIT FOR
SATELLITE TV RECEPTION — 3
by J & R v. Terborgh
This is the final, optional board in the IDU. As promised in the
preceding instdlments of the series, it comprises the AFC, scan
and remodulator facilities, as well as the LNB theft alarm.
The circuit board described in
this article is not, strictly speak-
ing. indispensable for a fully
operative indoor unit, But then,
the'optional add-on circuits are
relatively simple to build on a
single PCB, and may provide
you with a number of quite
RESET input, pin 4. Electronic
switch ESi is closed, while ES.
is opened, so that the DC-
coupled video signal. CVBS-1
(see Part 2) is routed to TV
modulator ICts. The operation
of this versatile RF chip will be
reverted to.
useful extensions.
Circuit description
The circuit diagram of the op-
tional extension board is shown
in Fig. 18. The various functions
it offers are best discussed by
starting from the three possible
positions of the front panel
mode switch, S4a b.
1 . TUNE: Bia b is set to position
1, as shown in the circuit
diagram. Oscillator IC9 is
disabled by the low level at its
The RF board tuning voltage, (
Vtune, is taken from the output
of summing opamp A;. which is
driven with the tuning control
voltage (terminal T. controls
P6-P7). and the output voltage of
AFC amplifier Ai.
If AFC switch Ss is opened
(AFC off), ESs and ES- are off
and on, respectively, which
means that the voltage at the +
input of A- is a fixed level, de-
termined With P9. Vtune will,
therefore, track the voitage at
point T, just as if there were no
amplifier of any type in func-
Switching on Si. however,
causes Bdc, rather than the
voltage at the wiper of P9, to be
fed to the + input of A-. This
creates a feedback loop in the
tuning voltage circuit. It will
be recalled that Bdc is the
smoothed direct voltage com-
ponent of the baseband video
signal. Tracing its origin will re-
veal that Bdc is the proportional
equivalent of the PLL-generated
tuning voltage across varactor
Dj. i.e. it can provide infor-
mation about the instantaneous
centre frequency of the PLL
subcarrier (see Pan 1).
Assuming the AFC function to
be switched on, and assuming
that the selected oscillator, LOl
or LOh, starts to deviate from its
set frequency— which may well
happen owing to thermal ef-
fects— the PLL will conse-
quently alter the voltage across
D?— and hence Bdc— to match
its VCO frequency with that of
the incoming carrier at about
610 MHz. The AFC circuit next
responds to the assumed fluc-
tuation of Bdc by correcting
Vtune such that the oscillator re-
mains at the set frequency, i.e.
Bdc also remains constant!
The practical limitations of the
proposed AFC circuit mainly
concern the response speed of
the loop, and the AFC hold
range. The AFC circuit should
be insensitive to the demodu-
lated video component, which,
of course, is also the PLL action
to an FM input signal. This func-
tion is taken care of by Cso (see
elekto. India februaiy 1987 2-21
Fig. 18. Circuit diagram of the or
Part 2), as well as Cei. Feedback
resistor Rea defines the AFC
hold range, i.e. the span of Vtune
that ensures a constant Bdc
voltage. The stated value of this
resistor fixes the amplification
of A. at about 3 KRw + RnVBu].
which will ensure sufficient
AFC action in most practical
2. SCAN: Su b is set to position
2. ESi is closed, and ICi
oscillates at about 10 Hz. The
triangular wave at pins 2 & 6 is
amplified to about 30 Vpp by
means of Ai, which conse-
quently causes the relevant os-
cillator, LOl or LOh, to produce
a swept output frequency over
its entire mixer injection band.
The purpose of the SCAN fa-
cility is to facilitate the initial
dish positioning procedure As
soon as the dish "sees" the
satellite, there will be a marked
change on the TV or monitor
screen from stable noise to a
rather unsteady flicker, caused
by the receiver sweeping
across the incoming transpon-
der signals. Also, the S-meter
will show some deflection and
hence can be used to find the
initial aerial position.
3. TEST REMODULATOR Si,, b
is set to position 3. ESi is
opened, causing ICi to oscillate
at 1S6.25 kHz. or 10 times the TV
' line frequency. Counter ICio
' supplies two sequential 7 ps
pulses; one for use as a line
blanking pulse (Qj), and one for
j a white vertical bar (00- These
| pulses are combined by means
! of ES? and ESi to form what can
hardly be referred to as a com-
posite video signal, yet is en-
tirely satisfactory for the pres-
j ent purpose. Resistors Rs, and
; Rss have been dimensioned for
a blanking/white ratio of about
1:3. ESi is closed, while ESi is so simple as to obviate the need
opened, so that the video test ' for a detailed description. With
signal is passed to TV modu- three jumpers installed as
lator IC i6 . shown by the dashed lines, LED
The remodulator test facility j Du and buzzer Bzt will warn of
enables ready tuning of the TV attempts to steal the costly LNB.
set to the modulator output fre- The jumper block and the po-
quency. thereby slightly allevi- tential-free relay contacts
ating the possible difficulty in 1 should enable a straightforward
setting up a satellite reception connection of the LNB theft
system for the first time. alarm to many types of existing
alarm system. Table 3 shows
some of the possible alarm con-
LNB theft alarm (ICis;Ti3). figurations plus associated
The relevant circuit section is ; jumper positions.
phone: 101 3481 9420 9425;
om EleclroValue; Telephone
41 33803 or 1061 432) 4945.
Remodulator (ICie).
The Type TDA5660 from
Siemens is an all-in-one TV
modulator chip which can be
configured for a wide variety of
TV standards. In this design, it
provides a double-sideband,
AM vision. FM sound. TV signal
at 48 MHz. which is roughly
channel 2 (48.2S MHz. Band I).
Operation on channel 3 or 4 is
also possible by simply using
an appropriate crystal in the Xi
position. The circuit may also
I be modified to output a UHF TV
signal (470-790 MHz), but this i
rather more complicated than
hanging the crystal, and i
refore, only recommended j
experienced RF construc-
. The matter will be reverted j
n the section on construc-
The audio input signal to the TV
modulator chip is passed
through a pre-emphasis net-
work. Rsj-Css (t =* 30 ps). The
modulator chip provides wide-
band FM modulation at the
audio sub-carrier frequency of
6.0 MHz, as set with W The
VHF output signal is available at
symmetrical outputs pins 13 and
15. A double pi-filter, Cn-Iai-
C96 and C»/-L 2 i-C 98 , precedes
300R-to-75R balun L20, form
which the TV signal is taken by
C.oo. Trimmer Css is used to set
the modulator output filter for
optimum balance. The dashed
lines around the remodulator
circuit denote metal screens
elektor mdia febfuary 1987 2-23
the preclude stray
which serve
radiation.
i Construction
If you have made it so far in
building the IDU, you are not
| likely to encounter serious diffi-
culties in getting the present
extension board up and run-
ning.
Fig. 19 shows how PC board
1 Type 86082-3 is to be com-
j pleted. Only three points re-
I quire special attention, namely
I making Lis and Iuo, and fitting
! the extension board on top of
j the vision-sound-PSU board de-
scribed in Part 2 of this series,
i In order to avoid unnecessarily
j repeating the suggestions for
i making one's own inductors, it
is recommended to re-read the
passage on preparing Lis; this
1 can be found in El'ektor India ,
j December 1986.
j With reterence to Fig. 20 and
I Table 4, oscillator coil Ins is
i made as follows (note that the
white ABS former as part of the
Type 7T1S inductor assembly is
divided into two equally long
| sections by means of a small
1. Starting from f , and observ-
[ ing the indicated winding di-
I rection, close-wind 11 turns in
1 upward direction onto the
I lower section of the former
body; doing so will neatly fill
this section. Connect to b (not
toe!).
2. Starting from e', and once
more observing the correct
winding direction, close-wind 4
turns upward onto the upper
section of the former; the first
turn should rest against the rim.
Connect to a.
3. Check for any short-circuits
between the windings, and
verify correct continuity at the
pins.
4. If you have a GDO, check
I whether the inductor can be
j tuned to about 50 MHz with a
I 18p capacitor temporarily fitted
: across f-b.
; 5. Mount the former plus
I screening can onto the PCB.
Adjust the yellow-tipped core
I until its top is level with the hole
in the screening can.
As to Iko, the construction of
this balun (balanced-to-unbal-
anced transformer) is evident
from the six-step instruction
shown in Fig. 21. Almost any
type of small, two-hole ferrite
bead rated for at least 100 MHz
can be used in this circuit. The
inductor is wound with bifilar
2-24 slsklor India Mnusry 1987
20
L18 Neosid 7T1S
«.d fioo. unde.
eath
•e
•"W*
.
•€»
•j — ^ •
b
start ol winding
21
T-V V
nip, ;>#c
it u u
^A A 2,SWG3 °
1 n
si.
in
UJ
IS 5
51
Fig. 21. Suggested constructior
J wire, which is simply made by
| twisting two lengths of enam-
! elled copper wire. After wind-
ing two times three turns
; through the bead holes, the
j string ends are split in order to
| identify the four individual
wires by means of a resistance
meter or a continuity tester
(step IV). At this stage, it is a
i good idea to check the wires
for internal short circuits
caused by the insulating
enamel coating being damaged
as the windings are tightened
around the ferrite bead.
After making the balun and fit-
ting it onto the board, it is time
to check whether this is cor-
rectly populated. There should
' be six wire links in all, and the
of balun Lzo.
jumpers in the LNB alarm cir-
cuit should be fitted as re-
quired. Positions Cn and Cm
are vacant as yet. Make sure 1
that all ceramic capacitors in j
the remodulator section are |
' mounted with the shortest poss-
j ible lead length. The crystal
case must not be grounded,
j The position of the 12 mm high
metal screen around the
remodulator circuit, and the
lengthwise fitted screen across
ICis, is governed by 9 soldering
pins. A single strip of 12 mm
( wide thin brass sheet or tin
plate is readily cut and bent to
] size. Remember to drill two
small (^ 3 mm) holes in the
| screen to enable feeding
1 through the shielded wire to
the audio input, and, if re- I
quired, the $ 3 mm coax cable j
from the RF output to Ki on the I
enclosure rear panel.
The completed extension
board is mounted on top of the
rear side of the vision-sound-
PSU board, i.e. as close as poss-
ible to the enclosure rear panel.
Remodulator output socket K»
can be fitted at a suitable lo-
cation in the rear panel, whilst
being connected direct to the
relevant pin on the PC board,
ie. without a length of coax
cable. Note, however, that this
mounting method requires
making a suitably sized hole in
the previously mentioned
screen, allowing for the passing
of the socket.
The lowest possible mounting
height of the present board
above the vision-sound-PSU
board is determined mainly by
the height of the fuseholder on
the latter PCB. Sufficient stab-
ility of the "sandwich" con-
struction is ensured by using
two conventional 15-20 mm long
PCB spacers in the two rear
positions.
It goes without saying that the
overall height of the two-board
unit should enable the IDU to
be closed properly. Also, the
vision-sound-PSU board should
be fully operative and correctly
aligned, since many of its ad-
justment controls are no longer
accessible with the extension
board fitted on top.
The wiring of the boards should
be fairly straightforward, requir-
ing no further remarks other
than that the audio, Bd’c and
V tune connections should be
made in conventional shielded
microphone cable, while the
CVBS-1 connection is made in
if 3 mm coax. In all cases,
ground the cable shield at the
lower board only.
Finally, the external loop con-
nection can be made with
whatever type of socket or ter-
minal strip is thought most con-
venient; a 3- or 5-way DIN
socket is satisfactory.
Setting up
Before detailing a suggested
setting up procedure for the
present board, it must be made
expressly clear that attempting
to use the completed extension
PCB along with as yet un-oper-
ative RF and vision-sound-PSU
boards needlessly complicates
getting the IDU to function cor-
rectly. Therefore always build
up the receiver as detailed in
Part 2, and familiarize yourself
with the various adjustment
points and their typical re-
sponse, before adding the pres-
1. Set S« to TUNE, and switch off
the AFC (Ss). Turn Pi (coarse
tuning) to check whether Vtune
varies from about 1-30 V. Tune
to a satellite programme and
check the presence of com-
posite video at pin 10 of ICia.
Do the same for the audio at
Measure Bdc, note the value,
and adjust Pa for an identical
voltage at its wiper. Switch on
the AFC and check its hold
range by turning P 7 ; reception
should remain unaltered over a
certain portion of the tuning
control travel, then suddenly be
lost.
2. Set S. to SCAN, and switch off
the AFC. Use a scope to check
measuring points (Q) and ©.
Vtune should be an undistorted
triangular wave, ie. it should
have clearly defined points of
inflection, and no clipped tops
or appreciable offset. If
necessary, Rat and R$s may be
redimensioned to achieve the
correct wave-form and ampli-
tude respectively.
Set Pa to the centre of its travel
and observe the monitor screen
to see the effect of the SCAN
mode when a satellite is re-
ceived. You may want to experi-
ment a little with the value of
Cai to obtain the best
noticeable effect on the screen.
Try to remember what it looks
like!
3. Set Su to TEST REMOD.. and
connect a TV set to K<. Tune the
TV to channel 2. Adjust the core
in L 6 . Depending on the degree
of crystal activity, it may be
worth while to fit a damping re-
sistor (1K0-10K) across pins f and
bofL,..
Run a quick check on the oper-
ation of the LNB theft alarm by
disconnecting the downlead
cable at K> Please note that the
alarm circuit is fed from the
unswitched + 12 V supply.
Therefore the +Bzi terminal on
the PCB should be wired to the
buzzer as well as the ap-
propriate connection of Sa (see
Pan 2).
Finally, if the setting of Pa fails to
give a satisfactory compromise
between the operation of the
SCAN function and that of the
internal test pattern generator,
try fitting a number of small
capacitors in the Cn position.
Remodulator on UHF
The circuit diagram of Fig. 22
shows how to modify the on-
board. TDA5660-based, TV
modulator for operation in the
UHF TV band (470-790 MHz). As
this modification is not sup-
ported by the PCB layout, alter-
ing the circuit is recommended
for experienced RF construc-
tors only.
Preset P is used to set the
desired output frequency,
which must be well removed
from the PLL VCO frequency to
avoid carrier interference.
Therefore do not tune ICia to
the generally used modulator
channel 36.
The small ceramic NP0 capaci-
tors can be fitted in a three-di-
mensional construction, along
with oscillator inductor Luhf
which can be spaced or com-
pressed slightly to set the initial
output frequency. The lp5
capacitors are, of course, fitted
direct across the relevant IC
pins at the PCB track side.
The modulator output filter
must also be altered as shown
to allow for the higher fre-
quency. Use a suitably rated
bead for L 20 , and wind two turns
through each hole, rather than
three as in the VHF circuit. The
data for Luhf, Lx and Ly can be
found in Table 4.
Aerial positioning unit
The circuit diagram of Fig. 23a
and the photograph of Fig. 23b
show a simple, yet indispens-
ible accessory unit for the IDU.
It is a hand-held remote meter
circuit which is connected to
the IDU over a length of 6- or
7-way cable, enabling the user
to monitor the S-meter indi-
cation while lining up the aerial
for optimum reception.
It should be noted that the cir-
cuit diagram and practical re-
alization are but suggestions;
other configurations, as well as
more sophisticated controls are
perfectly feasible, and con-
structors should have little diffi-
culty in tailoring the aerial
Fig. 22. Modified circuit for the remodulator, if this is to operate ir
HH
Test set up to examine the performance of the BFG65 prestage in
the IDU. Display indications, left to right: frequency (MHz) as-
sociated gain idB); noise figure IdB). Courtesy of SSB Elec
Fig. 23. Circuit diagram (23a) and practical outlook (23b) of the
aerial positioning unit.
positioning unit to their specific 1
requirement.
With reference to Fig. 23a, the
meter should be a more sensi-
tive type than that incorporated
in the IDU Either a switch,
mounted onto the IDU rear
panel, or a socket contact, is
used to break the S meter driver
output to the front-panel
mounted meter, and route the
signal to the aerial positioning
unit. A buzzer is fitted to enable
the person remaining at the IDU
to notify the other person at the
aerial that the IDU is switched
from SCAN to TUNE following
the slightest sign of reception
on the TV or monitor screen.
In practice, the aerial position-
ing unit may be used as follows
(note that a detailed aerial posi-
tioning method will be dis-
cussed in next month’s final
instalment of this series):
1. Set the IDU to SCAN. LOl or
LOh depending on the satellite
to be received; connect the
positioning unit cable, and, if
possible, install a helper at the
IDU.
2. Take the positioning unit to
the aerial site (on the roof, in the
garden, or wherever reception
is thought feasible).
3. Set the unit to maximum
meter sensitivity and line up the
dish until some deflection is
seen. Hopefully, the person in-
side has noted the SCAN effect
on the screen, and, via the
buzzer, notified you that the
meter indication will be lost for
an instant as he tunes to some
transponder.
If no help is available, leave the
dish roughly positioned and go
inside to switch from SCAN to
TUNE yourself. Reception of
the satellite may still be weak at
this stage, but you have at least
managed to find a stable signal.
4. Co outside again and line up
j the aerial for highest meter
deflection, turning down the
sensitivity any time the meter
reaches its fsd indication.
Threshold extension
The following is a necessarily
brief examination of a number
of experiments with the PLL
demodulator. IC2, on the RF
board. As these experiments
are not supported by the PCB
layout, their being carried out is
only recommended for ex-
perienced RF constructors.
Also, since the objective of the
proposed modifications is to
further lower the PLL noise
threshold so as to improve upon I
reception with relatively low
C/n ratios (8-10 dB), there is no |
point in altering the PLL circuit
if your specific outdoor unit en-
sures a C/n output of more than ,
about 12 dB.
When the C/n ratio at the input
of the PLL demodulator ap-
proaches the noise threshold,
the received picture is more or
less impaired owing to noise j
spikes occurring primarily in j
the saturated colour areas. This
effect is mainly due to insuf-
ficient open loop gain of the
PLL at the chroma subcarrier,
4.433 MHz (PAL system).
Incorporating a chrominance
filter in the secondary PLL loop
may improve reception to some
extent, but it should be noted
that the effect depends on the
transponder deviation and
bandwidth. For instance, the
signal from Teleclub Switzer-
land could be slightly improved
by peaking the chroma filter
whilst observing the few re-
maining sparklies in the ochre
rectangle at the lower right of
the test chart. Correct tuning of
the series filter will enable the
sharp white-to-black transitions
in the chart to appear with a
clearly improved definition.
The practical circuit of the
chroma filter extension is
shown in Fig. 24a.
It will be recalled that C20 and
Czi define the secondary loop
response and hence the oper-
ation of the PLL at a specific
portant to realize that, at pres-
ent, there is no single standard
for the peak-to-peak deviation
of transponders, not even if
these are part of one and the
same satellite. Research carried
out by the EBU and the CCIR
has provided evidence for the
proposition that, given a
specific C/n ratio, S/N rises
with increasing deviation. It is,
therefore, arguable that future
satellites will hold transponders
with larger output bandwidth;
after all, a number of the pres-
ent generation of TV satellites
were originally designed to
j operate in data communication
networks.
It may be interesting to experi-
ment with the values of C20 and
j C2i while observing the signal
| from a relatively weak transpon-
der. The range of values that
can be fitted in the stated ca-
j pacitor positions is quite large
j —see the small inset table in
' Fig. 24a. Fig. 24b shows how
the secondary loop differential
amplifier is converted into a
single-sided type by decoup-
ling the IiFB? input and the V
output with lOOn ceramic ca-
pacitors. This modification is
called for when receiving satel-
lite signals with a peak-to-peak
deviation of the order of
25 MHz. It should be noted that
such a high deviation value
does not necessarily mean a
higher bandwidth; in next
month’s article we will examine
the exact relationship between
Finally, interested constructors
are advised that Plessey have
recently introduced the Type
SL1455 quadrature FM TV de-
modulator. which is stated to
achieve a noise threshold of
about 7.5 d& is. it is some 1 dB
bener than the SL1451 con-
figured for optimum operation
given a specific deviation.
RF board
measurements
The IF amplifier chain on the RF
board was studied with respect
to its frequency vs amplitude
characteristic. Use was made of
a 0-1800 MHz spectrum analyzer
plus associated sweep unit.
Fig. 25a shows the curve of a
wrongly adjusted IF chain; one
of the four bandfilter trimmers
has obviously been set at too
low a frequency, causing a
marked peak outside the re-
quisite pass band.
While adjusting the bandfilters
to obtain a satisfactory filter
response, it was found possible
to locate the pass band any-
where in the 450-650 MHz band,
while the bandwidth was never
less than about 35 MHz. There-
fore, constructors not in pos-
session of an RF sweep gener-
ator or other sophisticated
equipment to measure the IF
bandwidth need not worry too
much about the overall pass-
band of the RF board. As long
as all trimmers can be tuned for
stable noise output, the initial
alignment is satisfactory.
Fig. 25b shows the band pass
curve obtained after very care-
fully peaking the trimmers for
optimum reception of the test
chart broadcast by Teleclub
Switzerland on ECS-1. The
curve thus obtained may be
compared to the theoretically
required one shown in Fig. 25c.
The latter is used by the EBU to
specify the minimum require-
ment for Eutelsat-1 receiving
stations.
Next time
Next month's concluding article
in this series will tackle a wide
variety of questions raised in
connection with satellite TV
reception. So. should any
aspect of the present subject
matter still puzzle you, see
whether it is among the sub-
jects qualified for closer exam-
ination in Part 4.
RGK;Bu
25c
(dB) (dB) (dB) I (dB) (dB)
2.5 (I0.0)" (25)'
ment for out-of-channel filtering in the transmit equipmei
amended that out-of-channel filtering be provided in the
86082-3-25c
2-27
THE FUTURE BELONGS TO THE
PHOTON
Electronics has been the main engine ot innovation since the
invention ot the transistor 40 years ago. Most of tomorrow's
interesting technologies will work by manipulating light, not
electricity.
The electronics revolution is
young. The electron was ident-
ified less than a century ago
and the microchip, on which
today's information-technology
industry utterly depends, has
been around for fewer than 20
years. The successes crammed
into these two hectic decades
have created the impression
that electronics is a technology
capable of limitless improve-
It is not. Electronics will give
way to a superior technology
based not on electricity but on
light. Physicists did not realize
until early in this century that
light came in the separate
packets they now call photons.
But science has made startling
progress in manipulation
photons. A photonics revolution
is already in the making.
The first shot of the electronics
revolution was the transistor.
Photonics' first shot was the in-
vention, in 1960, of the laser. Un-
til then, those trying to do tricks
with light had to make do with a
jumble of disorderly wave-
lengths. Lasers create a source
of light with a uniform wave-
length and with each wave mov-
ing in step with its companions.
This is a tool of immense power.
Lasers can— or so President
Reagan hopes— destroy ballis-
tic missiles thousands of miles
away. They can cut metal in fac-
tories and repair blood vessels
in human eyes. Hospitals use
laser beams guided through
optical fibres to shatter people's
kidney stones. A French inven-
tor has replaced the strings of a
harp with laser beams. Like
transistors, lasers have shrunk:
they can now be generated by a
chip the size of a grain of sugar.
This is paving the way for a
wholesale switch from elec-
trons to photons.
Why is the switch worth mak-
ing? Because photons travel
faster than electrons; because
J they have no mass; because (un-
like electrons, which interfere
with each other) photons can be
made to pass through each
other unperturbed; because
light behaves both as a particle
and as an electromagnetic
j wave-which means that optical
devices could be based on
much the same operating prin-
ciples as those already used in
electronics.
Moreover, electronics is dis-
covering its limits. One is the
speed at which electrons travel
through semiconductor ma-
terials. So long as electrons re-
main the information carriers ol
computers, this sets an absolute
limit on the speed— and hence
power— of computing. Elec
ironies has not reached that
limit yet, but it is drawing close
enough to worry engineers.
The customary way to make
computers cheaper and fastei
is to squeeze electronic compo-
nents closer together. The
number that can be fitted on a
single chip has grown from
about a dozen 20 years ago to
2m today. But miniaturization,
too, is bumping against limits.
Engineers are running out ol
ways to etch into chips ever-
smaller paths along which elec-
trons can run. And when com-
ponents get too close, the chips
are plagued by "cross talk— the
leakage of charges from one
component to another.
If computers are to work faster
still, a new approach is needed.
The best bet is "parallel pro-
cessing'— the notion that com-
puters ought to be able to per-
form a lot of operations simul- I
taneously, instead of channel-
ling all their calculations
through one bottlenecked cen-
tral processing unit. Here, too,
the case for a photonic solution
is compelling. Sending several
electric currents through one
chip at the same time risks
cross-talk and disaster. Not so
with beams of light: a chip
could process several at once
without their interfering with
each other.
| Still sceptical? Consider how
j rapidly light has nudged elec-
I tronics out of two pillars of in-
formation technology: telecom-
munications and the storage of
information.
In communications, telephone
companies are tearing out their
j copper cables as quickly as
they can afford to and replacing
them with hair-thin optical
| fibres made of glass. Light is a
better messenger than elec-
tricity. It wastes less heat and is
| immune to electromagnetic in-
1 terference. Better still is light’s
enormous bandwidth. Because
it spans so many frequencies,
light can squeeze in far more in-
| formation than electricity can.
j The quality of the optical fibres
themselves has improved dra-
matically. In early (circa 1970s)
j fibres, light ran in a disorgan-
ized zig-zag through a relatively
large core within the fibre. The
resulting collisions with the
fibre's cladding absorbed much
of the light, requiring frequent
repeaters to refresh the signals.
In 1977. experimental fibres
transmitted up to 140 megabits
of data a second, and needed a
repeater every six miles or so.
Today, one experimental fibre
network installed in Britain
carries telephone traffic at
1200 megabits a second, with
30 miles between repeaters.
The first transAtlantic fibres will
be carrying data and telephone
conversations between Europe
and America in 1988. Yet the
technology is on the threshold
of another luminous leap.
This will not come from
changes in the fibre itself, but
from the devices used to send
and receive the optical signals.
The first step is to combine in a
single device all the parapher-
nalia that optical fibres re-
quire— lasers to send signals,
detectors for receiving them,
and a rag-bag of lenses, mirrors
and electronic controls.
I The second step is to transmit
light beams “coherently— ie. in
tightly-defined wavelengths—
[ into a receiver that can be
tuned to select the required
wavelengths and sort out the
separate streams of data. In
principle, coherent trans-
mission enables a single fibre
to carry 10m telephone conver-
| sations or 10 000 digital tele-
[ vision channels at once.
The optical assault on data
storage— that other pillar of in-
| formation technology— has
j been as impressive. Music
j lovers were in the van with their
compact discs. The music is
I turned into digital signals,
burned on the disc as a series
of minute pits and then de-
coded for playback by a low-
power laser.
Audio discs like these are only
the first big success of a
technology restlessly seeking
new applications and markets.
Optical discs are beginning to
replace magnetic ones as a way
to store computer archives.
Because they are tough, the
discs can be stored inside
specially-constructed juke-
boxes. One 4.7-inch disc can
store about 550m bytes of
data— the equivalent of 1500
floppy discs or about 250 000
printed pages. Which means a
jukebox can store the archives
of an entire government depart-
Optical discs suffer from one
drawback: erasing them or
writing new information on
j them is difficult. This has im-
| peded their marriage with com-
puters. but has also prompted
an imaginative hunt for appli-
cations in which data must be
stored permanently without
I alteration.
Discs sold under a standard
I format known as compact-
disc read-only memory (CD-
ROM) are enabling data-base
I companies to sell archival infor-
mation to subscribers cheaply
by post instead of expensively
by telephone. Grolier, an
| American publisher, has put its
j Academic American Encyclo-
I paedia (30000 articles, 10000
pages) on one-tenth of one disc,
which it sells for less than $200.
| A new generation of discs
J called WORMs (write-once-
read-many-times) is half-way
| there. These are sold blank, so
the end user can store whatever
j data he likes on them, although
the information, once stored, is
I there to stay. But the technology
| for a fully-erasable disc will
| probably be perfected by the
end of the decade. Two ideas
for making them are. already
showing particular promise.
One is based on a magneto-
optical process. The disc's re-
cording layer is an alloy of ter-
[ bium, iron and cobalt. To store
information, a laser heats up a j
tiny spot on this layer, creating a |
( vertical magnetic field. The in- J
[ formation is read by another
laser: whenever it encounters a
j magnetised spot, the light’s
plane of polarization is rotated.
The information can be erased 1
j by reheating the spot,
j The other approach is chemi-
cal. Here, a laser is used to
I switch the structure of a tel-
j lurium alloy back and forth be-
j tween amorphous and crystal- ,
■ line phases, which reflect light
differently.
Impressive as they are. the
progress made by optical discs
and fibres do not amount to a
revolution. Photonics will not j
come fully of age until it equals
and then surpasses, the central
triumph of the electronics revol-
ution: the computer.
At the heart of the computer |
sits the transistor. A transis-
tor, remember, is a switch, a \
device that can flip backwards
and forwards between two
states. Computers are chains of
switches. They treat sequences
of ons and offs to denote num-
bers (in which case ons and offs 1
are read as the ones and zeros
of binary counting) or to denote
"true or false" (in which case ■
chains of switches can be used
as the building blocks of
algebraic logic). The challenge
for photonics is to invent a
device that does for light what
the transistor does for elec-
Into the heart of the
! computer
It has virtually happened. At !
AT&T's Bell Laboratories and
Britain's Heriot-Watt University
in Edinburgh, small and primi- j
tive circuits of the kind that
could one day grow into com-
puters are already running on i
light. The switches they use-
known variously as bistable op-
tical devices (BODs) or trans-
phasors— are essentially optical
transistors. Light emerges from
them as a strong beam (on) or a
weak one (off). Put a bunch of
transphasors together, shine
laser beams through them, and
you have the basic ingredients
of an optical computer,
lb understand how a
transphasor works, think of it as
two partially-reflecting minors
facing each other. If a beam of
light is shone through them
some of it gets trapped, bounc-
ing backwards and forwards
between the mirrored surfaces
(see diagram on next page). As
these waves cross each other
they can either interfere with
and weaken the beam or align
with it and reinforce it. This
phenomenon is the basis of a
simple instrument— used to
measure wavelenghts— in-
vented by two French scientists,
Charles Fabry and Alfred Perot,
in 1896.
The Fabry-Perot interferometer
emits a strong beam or a weak
beam depending on whether
the waves are being reinforced
inside the cavity. On its own,
however, it is not a switch: a
useful switch needs to be ob-
viously on or obviously off.
Common sense says that a
gradual change in the intensity
of the beam shining in will pro-
duce a gradual change in the
beam getting out, not the ab-
rupt change that is needed. In
ordinary circumstances, com-
mon sense would be right. In
the case of the transphasor. it is
To make the Fabry-Perot in-
terferometer into a switch,
physicists hit on the idea of mar-
rying it with a phenomenon
known as optical bistability,
first observed at Bell Labora-
tories in 1976. The secret is in
the cavity between the mirrors.
I If this were filled with an or-
dinary medium— air, say, or I
most solids— the intensity of the
beam passing out of the minor
would, indeed, change in pro-
portion to changes in the inten-
sity of the beam shining in.
Transphasors, however, use a
family of materials (such as in-
dium antinomide and zinc sel-
enide) that are "non linear". If a
laser beam shines into these
materials, a slight change in its
intensity can trigger the wave-
reinforcement and make the
beam coming out of the trans-
phasor suddenly brighter— and
make it stay that way until the j
trigger is released.
Bell Laboratories and Heriot- |
Watt have made different sorts
of transphasors, but they both
work. Heriot-Watt's are entirely
optical: the laser beams are
shone into bistable plates made
of zinc selenide. Bell is trying a
hybrid approach. Its devices,
made of gallium arsenide, use
| electro-optical interference
| within the cavity to trigger the
I reinforcement effect. In an op-
j tical computer, these devices
[ would be the "chips", and the
"wires" would consist of laser
I To make a computer, it is not
I enough to be able to turn just
j one switch on or off. Computers
: are complex arrays of switches,
| each of which feeds signals into
the next. So optical switches
must be “cascadable— the
beams of light emerging from
one transphasor must be able to
flip the next, and so on. They
must also be able to receive
and send several signals at the
same time (properties known
respectively as "fan-in" and
“fan-out").
These obstacles are tumbling
fast. Last year, for example, the
team at Heriot-Watt University
showed that its zinc-selenide
transphasors could be kept
near their threshold by a
holding laser, then switched by
turning on a small extra beam.
Earlier this year, the team an-
nounced that it had placed
several transphasors in a cyc-
ling loop.
Optical switches should, in
theory, be able to operate 1000
times faster than electronic
ones. But do not throw your
electronic computer away just
yet. For the present, trans-
phasors are primitive. They still
have to be pumped by too
much light, and they are still
bulky, separate devices— they
have not yet been squeezed
together on chips in the way
electronics switches have. Even
so, optical switching works.
Hybrid vigour
Laboratories everywhere are
rushing to bring optical and
electronic switches together.
One motive is to make even bet-
ter use of optical fibres. Exist-
ing optical networks do not
work at the speed of light,
because the messages the
fibres carry are shuttled be-
tween machines such as tele-
phones and computers that
run— for now— on electricity,
not light. So at each end of even
the niftiest optical fibre sits a
cumbersome device whose job
is to transform optical pulses
into electronic ones and vice
To speed this procedure,
engineers are creating op-
toelectronic chips. To do so.
they have had to conquer a dis-
advantage of the photon— its in-
ability to carry an electrical |
charge. Picking signals off the j
end of an optical fibre demands
some way to sort out waves of
light and send them to different
destinations. Electrons can be
shunted by the application of an
electric field; chargeless
photons are impervious to such
methods.
The answer has been to chan- |
nel the light through "wave- j
guides" etched into chips
made of materials with unusual |
optical properties. These
materials change their ability to
conduct light when an electric
field is applied to them. Using
lithium niobate, engineers have |
been able to make a wide
range of optoelectronic modu-
lators, switches and other de- j
But there is another reason for 1
wanting to bring the photon and '
the electron together: parallel ,
processing. Britain's Plessey j
has developed a BOD in which j
the bistability comes from in- |
serting a photochromic ma-
ill
|
87009-2
terial— one whose chemical
form changes when exposed to
different wavelenghts of light—
into the cavity. Plessey believes
the device could be used for
parallel processing. The idea is
to squeeze an array of BODs on
a single two-dimensional plate.
Each then becomes an in-
dependent switching centre
that can be addressed simul-
taneously by an incoming laser
beam (see diagram below).
This approach comes into its
own in applications such as
image-processing, in which the
value of thousands of picture
elements (pixels) must be in-
dividually calculated to build
up a whole picture. Plessey
aims to get around this data-
processing bottleneck by using
light to process all the pixels
at once. The optical switches
are not yet as fast as elec-
tronic ones, but that hardly mat-
ters when they work simul-
taneously. Plessey reckons that
with its photochromic BOD, a
device the size of a finger-nail
could process 4m pixels in one
ten-thousandth of a second.
Photonics has come a long way
in the quarter century since the
arrival of the laser. But entirely
new ideas for manipulating and
exploiting light are still pop- j
ping up. These range from the
mundane (mechanical and bio- ]
logical sensors based on op- j
ticai fibres) to the frankly I
quixotic (travelling to the stars j
by giving spacecraft sails that
catch photons). Physicists have
begun to use laser beams to
trap individual atoms so they
can be observed in detail.
Engineers envisage massive
computer memories with data
encoded within the light-waves
of a hologram.
Why this sudden flowering? In
the 1970s, physics made a
wealth of discoveries about the
ways in which light interacted
with matter. These discoveries
are now finding applications.
The properties of non-linear
materials— which made the
transphasor possible— are one
example, but there are others.
In some circumstances, light
travelling through a material
sets up internal sound waves
that contour themselves like a
deformable minor, sending the
light backwards out of the
substance on the path along
which it entered. In 1972, Dr
Boris Zeldovich and colleagues
at the PN Lebedev Physical In-
stitute in Moscow used this pro-
perty to make something called
a phase-conjugate mirror.
This is no ordinary minor: it can
take an image that has been dis-
torted and then straighten out
the jumbled-up waves to recon-
stitute the original image. Like
so many technologies, the mir-
ror was treated as a laboratory
curiosity at first. It is now being
pressed into service by astron-
omers to take the twinkle out of
stars, and by star-wars generals
to shoot laser beams through
the turbulent atmosphere. The
minors can also be used to pro-
ject three-dimensional images
through optical fibres and to
etch tiny components on micro-
chips. One way or another, light
looks like the wave of the future.
Reproduced with permission
from The Economist
UNIVERSAL CONTROL
FOR STEPPER MOTORS
With good quality stepper motors widely available at reasonable
cost, this flexible, computer-driven, control board will make it
rather hard to hold on to the belief that stepper motors are the
exclusive realm of industrial electronics. If you are suspicious
about "universal", just glance at the speficications Table below; if
you are into industrial electronics, well. . .
Stepper motors come in an as-
tounding variety of types and
sizes, and they are frequently
spotted items in electronic
surplus stores and on hobby
venues. Sheer curiosity has
I prompted many a home con-
structor to purchase one at a
fraction of its original price.
However the number of wires
| coming from the device, and
I the fact that it is often found far
more difficult to get going than
often than not causes the per-
plexed owner to carefully put
his price possession in the
junkbox, together with other
"possibly useful" materials.
In Stepping Motors, Elektor
India , May 1985, the general
methods were examined for the
the driving of stepper motors.
Also that article provides a use-
) ful discussion of stepper motor
I terminology, used further on in
I this article.
' The main specifications of the
proposed control board are
summarized in the shaded
Table on this page. The board is
readily tailored to suit the user's
requirement, but it should be
made quite clear at the onset
that each of the following sec-
tions is to be read closely to be
able to decide on the most
favourable circuit configuration
for a specific application. A de-
tailed discussion of each of the
technical features is, therefore.
I indispensable to a good under-
standing of the operation of this
I fully user-configurable inter-
face board between computer
and, for instance, robot limbs, a
I pantograph, or a plotter.
eleklor mdia lebruary 1 987 2 -3 1
: rismm
I
L293E fitted: 1 A phase.
L298 fitted: 2 A phase
Software-controlled polarity
10. 35 V with L293E fitted.
10. 45 V with L298 fitted.
Stepper motors:
some problems
The following is a necessarily
brief discussion of the main dif-
ficulties to be overcome when
using stepper motors.
Limited speed range: the stator
windings constitute an induc-
tive load, which limits the com-
mutation speed of the coil
current. Also, the revolving,
permanent magnet rotor causes
an inductive voltage which
further worsens the commuta-
tion. These effects limit the
maximum attainable step rate
(also: pull-out rate), but can be
overcome by utilising current
drive control.
Resonance the undamped
character of a stepper motor
operating at a relatively low
step rate causes its movement
to be rather halting. The upper
oscilloscope trace in Fig. 1
shows the considerable over-
shoot after each step. Should
the step frequency equal that of
the underdamped oscillations,
resonance inevitably occurs,
causing a powerful, jerky move-
ment of the spindle. Mechan-
ical damping devices have
been developed to ensure a
smoother spindle movement,
but these permanent loads
typically cause the already low
efficiency of the stepper motor
to fall below the acceptable
The lower oscilloscope trace
shown in Fig. 1 provides
evidence for the proposition
that micro-step operation can
provide a marked improvement
in linear spindle movement,
thus enabling the direct transfer
of motor power via a set of
Low efficiency: an energized
stepper motor dissipates an
amount of energy in the re-
sistive load formed by its stator
windings. When the spindle is
held stationary, this resistance
is the sole current limiting fac-
tor; also the stall torque is often
needlessly high. Current drive
systems may enhance the dy-
namic characteristic of the step-
per motor to some degree, but
linearly controlled current
sources, unfortunately, exhibit a
very low efficiency.
The present design is based on
the use of high efficiency,
switch-mode current sources,
thereby going round the prob-
Overshoot
j£>|
a
i
\ “ '
a
Fig. 2. Block diagram of the stepper motor control board
lems associated with the
previously mentioned systems.
Also, the proposed current
driver has the advantage of be-
ing uncritical of its input supply
voltage; extensive regulation
and smoothing circuits are,
therefore, not required— an im-
portant fact in view of the poss-
ibly high currents involved in
operating the stepper motor. As
the current through the wind-
ings is fully programmable, the
user can arrange for the overall
dissipation of the stalled motor
to be significantly reduced.
Limited resolution: stepper
motors are classified according
to the number of steps per
spindle revolution. Using the
micro-step mode, this specifi-
cation becomes less important,
and a specific type of motor
can, therefore, be tailored far
better to the task it is to per-
form.
Block diagram
After these preliminary con-
siderations, it is time to have a
look at the block diagram of the
stepper motor control board-
see Fig. 2. This design is in
essence a quad bipolar power
driver. Each driver consists of a
bridge circuit and can supply
both negative and positive out-
put current from a single
supply. Starting at the input
side, it is seen that each driver
comprises a latch and a D/A
converter to enable program-
ming the level and the polarity
of the current fed to each in-
dividual stator in the stepper
The switch-mode current
sources are essentially voltage-
controlled pulsewidth modu-
lators (PWMs), driven with the
difference between the object
amount of stator current and the
actually measured current.
These two values are obtained
from the D/A converter and a
DC current sense amplifier, re-
spectively. The four driving
PWMs are synchronized via a
common 40 kHz oscillator sig-
nal. which ensures a favour-
able switching frequency— the
switch losses are still accept-
able, and the signal is inaud-
ible— as well as the absence of
beat signals.
At the top of the block diagram,
there are some more circuit
functions common to the four
drivers. An address decoder
uses the two MS (most signifi-
cant) bits to discriminate be-
tween the control data sent to
each of the four driver circuits.
Provision has been made to use
handshaking with the computer
for optimum reliability of the of
data transfer to the board. A ref-
erence voltage source makes it
possible to use D/A converters
without an interna! reference
circuit. Finally, a S V supply
powers all logic circuits on the
Depending on the application
you have in mind for the step-
per motor control board, this
need not incorporate all of the
previously introduced circuits.
For instance, the relatively ex-
pensive D/A converters may be
omitted if you do not envisage
using the micro-step facility, but
would still want to be able to
program semi-step operation.
The proposed board makes it
possible to drive a four-stator
system, even with two separate
two-stator motors. It is possible
to operate one motor in the
micro-step mode, while the
other one is controlled in the
standard way, i.e. by means of a
"stripped down" driver circuit.
The user is offered a choice of
two possible types of driver IC,
which can be fitted as required
by the expected output current.
As you can see, our use of the
word "universal" in the title of
the present article is fully justi-
fied.
Circuit description
It is not very difficult to spot the
I various functional blocks in the
circuit diagram, Fig. 3. As to the
j aforementioned common cir-
cuits on the board, ICj is the 5 V
regulator, IC* the 40 kHz oscil-
lator, ICe the one-of-four driver
| decoder, and zener diodes Du
and Du may be used to provide
DACs IC1.-IC.4 with a highly
stable 2.5 V reference.
i On rece ipt o f a computer-gen-
erated STB or STB (strobe)
pulse. ICe decodes De and D> in
the sent dataword and enables
the corresponding sextuple
latch, ICi. ..ICio, to clock the
6-bit value which determines
the output current level sup-
plied by the driver (Do. . .D4) as
| well as the polarity (Do),
j Therefore, only five bits of the
J six or eight-bit DACs are used to
translate the latch output into a
voltage between 0 and 2.5 V in
[ 32 increments (2 5 ). Each of the
DAC output voltages is used to
drive the inverting (+) input of
opamps A2, A3, As and A7. How
these in turn are capable of |
determining the stator output i
current is detailed in the next
section.
Returning to the handshake cir-
cuit composed of ICe. N • and
N2, it is seen that both positive
and negative-going strobe ,
pulses can be used by fitting
the appropriate wire jumper, a |
(STB) or b (STB). Note, however, ;
that in many Z80-based systems !
STB is an input signal, and RDY
(ready) is an output signal, i.e.
the signals are reversed as com-
pared with the Centronics stan-
dard. Jumper a is to be fitted
when driving the stepper motor
board with either a Z80 PIO, or a
6522 VIA, while jumper b ac-
comodates the use of a Cen- ,
ironies port. More information
on the handshaking circuit can
be found in Table 4, while Z80
PIO users may consult MSX ex-
tensions - 4, elsewhere in this
issue.
PWMs and current
drive
In order to make clear the oper-
ation of the switch-mode cur-
rent driver circuits in this
design, it is necessary to study
Fig. 4. From a functional point
of view, the Types L298 and
L293E from SGS Ates are largely
identical; these devices merely
differ in respect of the maxi-
mum available output current.
The L298 is twice as powerful as
the L2993E and is. therefore,
housed in a Multiwatt -15 SIL
enclosure, rather than a 20-pin
DIL package as is the L293E.
Each IC holds two indepen-
dently controllable bridge cir-
cuits plus associated logic
drivers. Since these ICs are to
be driven with logic voltages
only, there would seem to be no
way of controlling the bridge
currents with a linear regulating
system. However in each driver
the emitters of the lower bridge
transistors are brought out to
pins, enabling the connection
of an external current sense re-
sistor which provides a voltage
drop proportional to the stator
current. Fig. 5 further illustrates
this principle, which forms the
basis of the negative feedback
controlled switch-mode current
Any duty cycle of the current
drive system starts with IC*
generating a 1 fis negative reset
pulse for all four monostable
multvibra.tors MMVi. . . MMV* .
Taking MMV. and the upper
section of ICi as an example,
the reset pulse causes C.2 to be
discharged to the zener voltage
of Du. Simultaneously, MMVi is
triggered, and provides an out-
put period determined with
network Ru-Cu as well as the
DC level applied to the control
voltage input, pin 3. This level is
internally compared with the
voltage across C12 and hence
determines the length of the
output period. Since the com-
parator internal to the Type 556
MMV is incapable of linear op-
eration with input control volt-
ages below 1.5 V, Du leaves
sufficient residual charge in Cu
for the MMV to produce suffi-
ciently short output periods.
From this it is seen that the
MMVs in the circuit essentially
function as voltage-controlled
pulsewidth modulators, en-
abling the power output stages
contained in IC> and IC2 for the
duration of their output periods.
I Therefore, current sense re-
sistor Re carries the stator cur-
j rent and hence produces a
| proportional voltage drop,
| which is averaged in network
Ci6-Ru and raised in amplifier
Ai.
J Opamp A 2 compares the
measured current (— input)
with the object current (+ in-
put), and corrects its output
j voltage to MMV. until these two
values equal. Simple as this may
seem at a first glance, there is,
however, a snag in the measur-
ing of the stator current. As long
as the bridge is enabled, stator
current Is flows through Rscnse,
and its voltage drop is simply
IsRsenso volts— see Fig. 5, line a.
The disabling of the bridge im-
mediately breaks the current
through R se nse, but not that
through the stator winding,
whose inductance causes it
to supply a lagging current,
which is driven into the supply
via free-wheeling diodes-see
Fig. S, dashed line b. In es-
sence. the self-inductance
of the stator winding has a
smoothing effect upon the
stator current. Therefore, the
average value of Urs«i»o is not a
direct measure for the stator
current, since it does not com-
j prise the free-wheeling current.
I With most types of stepper
| motors, the period L/R of the
I stator winding is long as com-
pared to that supplied by the
PWM drivers (T--- 1/40 kHz =
25 ns). In practice, the variation
| in free-wheeling current in be-
[ tween driver pulses hardly
causes any ripple, and the error
incurred by only measuring the
current through the sense re-
sistor is, therefore, caused by
the duty factor variation. In
general, a relatively small duty
) factor variation suffices to give a
considerable stator current
span. As soon as the duty factor
rises above some 50%, and the
free-wheeling period starts to
overlap the bridge on-time, Is
rises relatively quickly. The re-
quired duty cycle giving maxi-
mum stator current is a function
of the ohmic resistance of the
stator winding and the supply
voltage level. The higher that
| voltage, or the lower that resist-
ance, the stronger the tendency
[ to large variations in Is around a
50% duty factor.
The foregoing considerations
J can not but lead to the conclu-
sion that the output signal of Ai
[ need not be exactly pro-
portional to the stator current.
1 (DOr
"*n
Fortunately, the overall linearity
is still acceptable, and occa-
sional deviations can be com-
pensated by suitable software.
Returning to the circuit dia-
gram, Fig. 3, the remainder of
the circuit functions are quite
conventional designs.
Timer IC. provides the nega-
tive-going 40 kHz synchron-
ization signai for the R and T
inputs of the MMVs. In the
absence of a common sync
signal, the input supply would
be corrupted by a good many
inductive voltage peaks, which
would readily lead to the MMVs
being triggered in error and the j
entire circuit operation being j
upset in consequence.
Network R,-D« prevents 5 V
regulator ICi from being dam-
aged by too high an input volt-
age. As the maximum input volt-
age for IC3 is 35 V, the use of the
Type L298 stepper motor driver
(Vs:wx>=45 Vpe a k) necessitates
fitting the voitage limiting net-
work. But even with the L293E
fitted in the circuit, it is still a
good idea to use Ri and D22, as
they also afford protection
against inductive voltage peaks
on the unregulated supply rail.
The use of the 2.5 V reference
diodes Du and Du' is not
obligatory, and their use will be
reverted to in the section on
construction.
The logic sections of the circuit
are composed of CMOS ICs
only. This means that the logic
drive to the board must be
capable of supplying CMOS-
compatible signals. Should you
want to drive the board with
TTL signals from a Centronics
port, the stated CMOS ICs must
be replaced by the suggested
HCMOS versions.
Construction
Before embarking on the con-
struction of the present board.
IC. 555 or 7555
ICi 4069B or 74HCT04
IC. 4556B or 74HCT139
ICr. . .IC 10 incl. -40174B or
74HCT174
IC.. IC.* incl. -ZN436 or
busconnector (It required!
Heatsink for ICt;ICr as requirec
PCB Type 87003 (see Readers
Services!
the type and the number of economic Type L293E. which motor current is fully program- D« and ICi. then fit a wire link
stepper motors must be con- can be fitted in the 1C and mable, but in order to attain op- in the holes provided for the
sidered in order to be able to ICt' positions on the PCB. In timum resolution in the micro- two outer pins of the regulator,
decide on the most favourable most cases, the copper surface step mode, the maximum value As to the D/A converters, there
as well as the most economical soldered to pins 5,6. 15 and 16 • ' of Is must be defined by means are a number of types to choose
realization of the circuit. of these chips provides suf- of selecting appropriate re- j from. In principle, the Type
To begin with, there are the ficient cooling, while ICj is best sistors in the Rt and Rs, as well ZN436E gives satisfactory per-
L293E and the L298 to choose fitted with an insulated, stan- as in the Rat . . . Rr; positions ! formance for most applications,
between. The latter should be dard U-shaped vane radiator, -consult Table 2. As Isimaxi is ; Note, however, that it comes
used with currents in in excess ; Should you decide to use a L298 also related to the self-induct- ; without an internal reference,
of 1 A per phase. Two L298s can for two stator windings, and a ance of the windings, it is ad- so that Di; (Dw 1 ) must be fitted,
be bolted onto a common heat- L293E for the other two, do not visable to actually measure the and R« must be a 1K2 type,
sink, together with regulator forget to limit the input voltage current consumption of the while Rs must be omitted
ICi. As all conductive surfaces in accordance with the maxi- motor. —consult Table 3. Jumpers c
of these ICs are at ground po- ! mum specification of the latter. The +5 V suppiy rail is made and d are not used, and jumper
tential, there is no need for in- Depending on the type of out- available at a separate pin of the e is fitted to pass the refer-
sulating washers and the like, put driver fitted, dimension R- I/O connector. When feeding ence voltage to the REF IN pins
Relatively low ouput currents as per Table 1 the stepper control board from of IC» and IC14. The Type
can be handled by the more As already stated, the stepper ! an external 5 V supply, omit R., i ZN426-X (the suffix indicates the
Fig. 6. Track layout and component mounting plan for the motor control board.
2-36 eletoor tndia february 1987
Fig 7 Where micro-step operation is not required, each of the
DACs in the circuit may be replaced by this resistor combination.
number of bits: 6, 7, or 8) is
also usable but is expected to
be somewhat more expensive,
as it holds an internal reference
circuit, which can be used by
fitting jumper c or d, depend-
ing on the position of the DAC
on the board, and using a
390R resistor in the R« or R*
position, whichever is appro-
priate. Should you want to do
without the micro-step facility
altogether, mount two 10K re-
sistors as shown in Fig. 7. Com-
pleting the stepper motor con-
trol board is very straightfor-
ward indeed when using ready-
made, through-plated PCB
Type 87003 (see Fig. 6) available
from our Readers Services.
When using the L293E driver
chip, solder it straight onto
the board to effect sufficient
cooling by the large copper
surfaces at the track side of the
PCB.
Connections
In general, the connection of
bipolar stepper motors is fairly
simple. A two-phase motor re-
quires to be driven with one
half of the control board cir-
cuitry. The actual connection of
the stator windings is largely
uncritical. Reversing the po-
larity of one stator winding, or
interchanging both windings
simply causes the motor to run
in reverse. A bipolar four-phase
motor requires to be driven
with the whole of the control
board. When using such a
motor, observe the correct
phase relationship between the
stator windings, else the
spindle will merely oscillate be-
tween two positions, rather than
revolve.
Basically, unipolar motors can
be connected in three ways,
as shown in Fig. 8. The first
method, shown in Fig. 8a, re-
quires passing less than normal
current through the series con-
nected windings to preclude
overheating and/or saturation
effects in the stator. Also, the
increased stator inductance
causes a considerably lower
pull-in rate.
The second method involves
creating a centre-tapped wind-
ing-see Fig. 8b. In principle,
this arrangement always results
ir. one half of the winding being
short-circuited to the positive
supply rail. As compared with
the above method, there is the
advantage of the lower overall
inductance, but the short-cir-
cuited half-winding gives rise to
an increased motor dissipation,
owing to the inevitably high in-
duced current, which is only
advantageous m that it ensures
good damping characteristics
and hence a relatively smooth
spindle movement.
The last alternative is shown in
Fig. 8c This method of connect-
ing a unipolar motor is based
upon the use of the individual
windings as if these were of the
bipolar type. In case the two
windings of a stator are not con-
nected internal to the motor,
anti-parallel connection is pref-
erable. A normal, parallel con-
nection immediately results in
the magnetic fields counterac-
ting, causing the spindle to re-
main stalled.
Provision has been made on the
PCB to fit a 64-way, a & c row
busconnector. Kj. Its connec-
tions are left vacant to enable
users to configure the bus wir-
ing as required. At the other
side of the board is K<. a 20-way
angled plug which is used for
the Centronics signals. De-
pending on the set-up of the
computer system in which the
present board is to be incor-
porated, wires may have to be
run from Ki to K>, or Ki may be
used for mechanical support
only. Those users intending to
make a stand-alone peripheral
device of the stepper motor
control may want to cut off the
PCB section provided for Kj
altogether.
The power supply
As already stated, the present
board is rather uncritical of its
input supply voltage. Extensive
regulation and smoothing of the
12 ... 35 (45) V input rail is not
recommended in view of the
overall system efficiency. When
designing the power supply in
question, merely observe that
the ripple voltage does not ex-
ceed 10 to 15% of the output
voltage.
It must be reiterated that the
maximum permissible peak in-
put voltage for the baord de-
pends on the type of bridge
driver IC fitted; for the L298,
V,n = 45V P oak; for the L293E,
Vin=36 Vpeak. In practice, it is
recommended to keep the in-
put voltage a few volts below
these values to allow for the in-
duced peaks caused by the
free-wheeling current.
A second factor to be con-
sidered in the establishing of
the supply voltage is the ohmic
resistance of the stator wind-
ings in the stepper motor. As a
rule of thumb, the supply volt-
age for the board must be at
least two times the typical op-
erating voltage of the motor op-
erated ' with voltage drive. In
principle, therefore, most com-
monly available 5 V stepper
elcktoi india (ebruaiy 1987 2-37
| motors should work all right
I with a board supply of 10-12 V,
[ but a higher supply is prefer-
| able for improved current drive
I characteristics and hence a
| higher pull-in rate.
{ The total current consumption
| of the system goes mainly on
| account of the stepper motor(s).
! Due account should be taken of
j the fact that the total current
; drain may amount to 8 A when
| using the board to drive 4 off
2 A stator windings. Obviously,
1 the mains supply should be de-
j signed to reliably cater for
j possibly high current peaks,
and the same goes for the
supply wiring. Also observe the
2 times 4 contacts on K<, re-
j served for the connection of the
input supply; keep the total
current drain in mind and, if
necessary, use soldering pins
to avoid overloading the rela-
tively thin connecting posts in
Kj.
Driving stepper
motors
As the stepper motor control
board is essentially only a per-
ipheral device, the computer-
or more precisely the soft-
ware— determines the move-
ments of the stepper motor
spindle.
The key to the driving of the
motor(s) is the 8-bit control
word sent to the board via the
computer’s parallel output port.
Fig. 9 shows the bit assignment
for that control word. The two
MS bits-D« and D;— are used to
address one of four stator driver
circuits. Bit Ds provides the
polarization control, while Do-
Da determine the stator current
in 32 (2 5 ) increments. Note that
some Centronics output ports
are open-collector types, re-
quiring the data input lines and
the STB line to be pulled high to
+ 5 V with 470R-1K0 resistors.
Quite essential to the operation
of the stepper motor is the
stator current timing sequence.
Fig. 10a shows the timing for full
step operation, in which the
stator current is arranged to re-
verse with every step. Semi-step
operation is illustrated in
Fig. 10b; during the reversal of
the stator current, this is held at
nought. This basic method is
further exploited in the quarter-
step mode shown in Fig. 10c,
while extrapolation of this prin-
ciple leads to the stator current
being reversed linear with time.
as shown in Fig. lOd. In prac- I
tice, however, the linear com- |
mutation is slightly problematic,
since the sub-steps at the cur-
rent cross-over point are in-
evitably larger than those dur-
ing the start and the end of the
commutation cycle. Moreover,
the available torque will vary
considerably during the sub-
steps, as the total stator current
is not constant.
During the current reversal, a
permanent load fitted to the
spindle will cause the rotor to
deviate more from the object j
position than during moments
of maximum current, resulting
in irregularity of the sub-step
size. This effect is generally
found to be rather more mani-
fest with dual-stator motors than
with four-stator types. Up to and
including quarter-step oper-
ation, dual-stator motors have an
adequate performance, but
four-stator types are clearly to
be preferred for all applications
mentioned so far. The reason
for this is the more constant j
average stator current of the lat-
ter motors. In conclusion, dual- i
stator motors are best operated j
with a constant total stator cur-
rent, as shown in Fig. lOe.
The commutation characteristic
required for equal step size
is mainly determined by the
specific type of motor to hand,
and some trial-and-error pro-
gramming may be required to
attain optimum performance. j
Sending bits to the
board
The simplest method of driving
the stepper motor is probably
the writing of a array which
holds all data for a full com-
mutation cycle. Such a cycle
essentially involves once re-
versing the current, and revers-
ing it again to return to the
original polarity. In a four-stator
motor, this corresponds to 8 full
steps. A programmed pointer is
used to send the datawords to
the board, and can be read, in-
cremented or decremented to
control the direction of the
spindle rotation. To get the
motor to run as required, the
pointer is programmed to ad-
dress the individual array en-
tries in a closed loop.
Table 5a is a data dump of an ar-
ray to control a four-stator motor
j according to the timing dia-
I gram of Fig. lOd. Note es-
' pecially the toggling of the
8>885288 | SS>SS2S8
| data
M
1 data
IF
ID
80
3F
3D
IE
19
82
3E
39
17
15
84
37
35
S
13
33
31
T
OF
0D
88
2F
2D
A
OB
8A
2B
29
T
07
05
8C
27
25
O
03
8E
23
21
21
23
01
03
25
27
92
05
07
29
2B
94
09
0B
2D
2F
96
31
33
98
1 1
35
37
9A
15
17
39
3B
9C
19
IB
3D
3F
9E
ID
IF
5F
50
A0
7F
7D
5B
59
7B
79
57
55
A4
77
75
s
53
51
Ab
73
71
T
4D
A8
6F
6D
A
4B
49
6B
69
T
47
45
AC
65
o
43
41
AE
63
61
R
61
B0
43
65
B2
4b
47
2
bB
B4
49
4B
6D
6F
B6
4D
4F
/I
73
B8
51
53
75
77
BA
55
57
79
7B
BC
59
7D
7F
BE
5D
5F
9F
9D
CO
BF
BD
9B
C2
BB
B9
97
95
C4
B7
B5
S
93
91
C6
B3
B1
T
8F
8D
C8
AF
AD
A
8B
89
CA
AB
T
87
85
CC
A7
A5
0
83
81
CE
A3
A1
A3
DO
81
83
A5
A7
D2
85
87
A9
AB
D4
89
8B
AD
AF
8D
8F
B1
B3
D8
91
93
B5
B7
DA
95
97
B9
BB
DC
99
9B
BD
BF
DE
9D
9F
DF
DD
E0
FF
FD
DB
D9
E2
: B
F9
D7
D5
E4
=7
F5
s
D3
D1
E6
=3
CF
CD
E8
EF
ED
V
CB
L9
EB
E9
r
C7
C5
:C
E7
E5
>
C3
Cl
EE
E3
El
i
El
E3
: 0
:i
C3
E5
E7
■2
:5
C7
i
E9
EB
: 4
:9
CB
ED
EF
■6
:d
CF
FI
F3
; 8
)i
D3
F5
F7
)5
F9
FB
C
19
DB
FL>
ff LFE_!
DD
DF
ion cycle. Table 5a is for a four-stator motor operating
or type operating as per Fig. lOe.
stator address bits and the cur-
rent polarity bit. Table 5b is a
similar dump intended as a
guide in controlling a dual-
stator motor according to the
timing diagram of Fig. lOe. For
both applications it it advisable
to provide for an interrupt-
based synchronization facility,
as offered by, for instance, the
Type 6S22 VIA.
Unfortunately, the fairly large
number of sub-steps often
makes it impossible for the
motor to attain its maximum
speed. In this context, there is
no doubt about the advantage
of machine language subrou-
tines over BASIC programs.
Should the need arise to have
the motor run at a relatively
high speed, it is possible to
program for more than one step
at a time. At high switching fre-
quencies, the stator inductance
limits the current to such an ex-
tent, that accurate current drive,
and hence micro-stepping, is
unattainable anyhow, However
this is of little consequence,
since the motor will nonethe-
less run smoothly with the step
rate well in excess of the res-
onance frequency. Micro-step-
ping is, therefore, primarily of
use either for relatively low
motor speeds, or for accurate
spindle positioning.
When skipping array entries to
realize sufficient motor speed,
care should be taken to finish
with the last byte of the rel-
evant stator phase. Large steps
should, therefore, always com-
prise sub-steps which are
powers of two (2, 4, 8. 16 or
32 steps at a time). TW
2-39
DIGITAL SIGNAL PROCESSING
Compact disc players have been with us for some time. Digital
television receivers are becoming commonplace. These, and
other apparatus, have an important aspect in common: digital
signal processing. But what is really involved in this?
Digital circuits only respond to
discrete values of input voltage
and produce discrete values of
output voltage. Usually, these
circuits operate between two
discrete voltage levels, ie„ high
and low (logic) levels. It is
therefore clear that before such
a circuit can operate the
analogue signals have to be
converted into digital (= binary)
signals.
Some fundamentals
Fig. 1 shows the basic set-up of
a digital processing circuit. The
incoming analogue signals at X
are digitized, in an analogue-to-
digital (A-D) converter, pro-
cessed in a (digital) signal pro-
cessor, and then reconverted
into analogue signals in a D-A
circuit.
The A-D converter produces a
stream of binary values by
quantization. In this method, the
incoming waveform is divided
into a finite number of
subranges each of which is
represented by an assigned
binary value within the
subrange. In a compact disc
player, a 16-bit A-D converter is
perfectly adequate, while in
video circuits 8-bit converters
are satisfactory.
Since the signal processor
operates by computation, it can
handle only a finite number of
pulses in unit time. It is the task
of the A-D converter to ensure
that the input capacity of the
processor is not exceeded, and
I this in turn determines the sam-
pling rate.
Sampling is a technique in
which only some portions of the
(analogue) input are used to
produce the set of binary
values to represent the infor-
mation contained in the whole
signal. To ensure that the output
values represent the input
signal without significant loss of
information, Nyquist’s Sampling
Theorem states that the rate of
Signal processors
As already mentioned, virtually
all requirements are met by the
basic operations of multipli-
cation and addition. Also, it was
shown that the signal processor
does not have all that much time
left for each computation. Sig-
nal processors have, therefore,
microprocessors with typical
instruction codes: they are rela-
tively small but, none the less,
quite fast.
Sequences such as:
"fetch value 1; fetch value 2;
multiply values 1 and 2; add
value 1 to the result; load the ac-
cumulator at the position of
value 1 and increase the ad-
dress counter"
as a rule have only one oper-
ational code. Moreover, while
an instruction is being pro-
cessed, the next instruction and
the next two values are
retracted from the memory
(pipelining). This means that
such an instruction takes three
clock pulses from start to finish.
With a 10 MHz clock, a 16-bit
multiplication and addition lasts
only 300 ns.
Even faster are signal pro-
cessors that use the Harvard in-
stead of the von Neumann
architecture. In the latter, data
and instructions are stored in a
common memory, whereas in
the former separate memories
are used (see Fig. 2). In
Harvard-type processors, in-
structions and data (in some
even two sets of 16-bit data) are
fetched from the memory simul-
taneously. This means that two
to three times as many oper-
ations can be carried out per
second as compared with a von
Neumann device.
The software for the required
function is first computed and
loaded into a normal computer,
with which the run of the pro-
cessing cycle is simulated
before the PROM of the signal
processor is loaded.
To conclude, and specially for
Fig. 3. Basic recursive filter. Output signal yltl is stored in
intermediate memory and used as input signal ylt-1) for t
next computing cycle.
4. Basic non-recursive filter. Output signal yltl is buil
im a succession of inputs: xlt). . .y(t-n). Secondary m<
s required for each of the inputs.
Dim X1%(64l
Dim Y1%(64)
Dim Z1%<64)
For 1% = 1 To 64
X1%(l%) = l%*10-5
Next 1%
Polyline 64,X1*.0,Y1%() Offset
O.V%
Do
Mouse X%. Y%. T%
If T% = 1
If Tl% = 1
X% = lnt(X% / 10) + 1
Y% = lnt(Y%/10)*10 + 5
Y1%(X%) = Y%-V%
Polyline 64,X1%(|,Y1%()
Offset 0,V%
Else
CIS
Input "Select filter order:
For I ^ 1 To Ord
For 1% * 1 To 64
Z1%II%I = B*Z1%(I%U
II -BI*Z1*>(I%-1)
Next 1%
Polyline 64.X1%(I.Y1%<) Offset
Alert 1, "Change filter?'', 2,
"New filter; end". Z
BASIC for an RC low-pass
filter of the n ,h order and it:
graphical representation.
(Fig. 5).
those readers who want to
design a digital filter and are
not too familiar with Fourier or
filter theory, a sample design
for a personal computer.
Basically, there are two types of
filter: recursive and non-
recursive. Figure 3 shows an
example of the simplest type of
recursive filter, where the out-
put signal is available for further
use a computation cycle T later.
This type of filter can be used |
for high- or low-pass purposes.
Non-recursive filters are formed
by inserting the input signal(s)
into two or more successive
filter sections as shown in
Fig. 4. Each section must, of
course, have a secondary
memory. This type of filter is
suitable for use as a transverse
or comb filter.
A low-pass filter is easily com- ]
puted from the following |
mathematical relation
yft)=ax(t)+by(t-\)
where yfl) is the output signal I
resulting from an input signal x '
y(t- 1) is the output signal one
computing cycle before /(/);
A=l— a;
0 until the direct
voltage at the output of opamp A2
(pin 6) is 0.00 V.
Next, inject a 1 kHz square-wave
signal into the drive unit, and set the
input sensitivity (lines Vo-Vi) to 0000
Adjust trimmers On and Cjo to obtain
a true square-wave signal at the out-
put (pin 6) of Aj.
Set the input sensitivity (lines Vo-Vi)
to 0001 and adjust Cn to regain a
proper square wave at pin 6 of As.
Repeat this procedure with sen-
sitivities of 0010 0100 and 1000 when
C24, C?s and Cj«, and Cu and C2« re-
spectively are adjusted.
At all times, adapt the level of the
square-wave input but take care to
avoid overloading the circuit (the
level at pin 6 of A? should not exceed
5 Vpp).
Redo all the adjustments mentioned
to make sure that all settings are
correct.
With the aid of a voltage divider
(made from 1% resistors: e.g. 22 0
and 2k7, or 82 8 and 10k) derive a
voltage of 40.0 mV from the power
supply, and apply this to the junction
R23-R24.
Set the signal on lines OFo-OFe to
Software
First, the PIA (if used) is initialized.
Make the RESET line low, which
results in all the PIA registers to be
set to nought. The adapter occupies
four addresses: I/O to 1/0+3 incl.
(see Fig. 14). Two of the locations
have consecutive registers, and
these are selected by making bit b2
:n the associated control register 1
(data register) or 0 (data direction
register).
Select DDRA as shown above and
write FF in this register: all A ports
are then set as outputs. Then write 06
in CRA which results in input CAi
reacting to a leading edge, as well as
data register DRA being selected. It
is then possible to write into this
register, for instance, 10 which pulls
the PA« line high.
The B pons are arranged as outputs
by making control register B logic
low, and writing FF in DDRB. They
are set as inputs by making bit b; in
CRB 0, and writing a 0 into 1/0+2
and a 4 into 1/0 + 3.
Arrange the A and B pons as out-
puts; disable the interrupt; and set
the interrupt flag (bit 7 of data
register A) to a leading edge at CAi.
A timing diagram of all important
control signals is given in Fig. 15: this
gives a good idea how communi-
cation between drive unit and com-
puter takes place.
All PA lines are made 0, after which
the data for setting the interface can
be written into the latches via the PB
50 ?ddrb = 6FF
60 ?drb = oFF + 64+ 128'ING:?dra = &14
70 ?drb = NIV a 64 - 128"TH:?dra = &12
80 ?drb -TB + 16’AM:?dra = &11
90 ?ddrb = 0:?dra = 0:?dra = &40:?dra = &10
100 HOLD = TIME + (T8 + iriO:REPEATUNTILTIME>HOLD
110 IFTRIG = 0THEN?dra = &30
120 IFTRIG = 1THEN?dra = &38
130 IFTRIG*2THEN140ELSEIFINKEY-99THEN?dra = &90ELSE130
140 REPEATUNTIL?ifr< >0
160 FORI -0TO255:PLOT69,2*I,4’ ■
170 'dra -- &20
180 FORI 256T0511:PL0T69.2'I .4
H 190 ?dra = &10
200 END
&40:?dra = O:NEXT
a &60:?dra - &20:N£XT
10 MODEO
[ 20 dra = &FCB0:ddra dratcra &FCBl:drb &FC62:ddrb drb:crb &FCB3
I 30 ?cra = 0:?ddra - &FF:?cra - 6:?dra &10
40 oFF : 0: ING 0NIV - 0:TH 0 TB 8: AM 10:TRIG - 0
50 ?crb-0:?ddrb &FF:?crb 4
60 ?drb = oFF * 64 + 128*ING:?dra &14
70 ?drb = NIV * 64 * 128'TH;’dra = 612
80 ?dlto=TB + 16‘AM;?dra = &11
90 ?crb = 0:?ddrb = 0:’crb - 4.?dra = 0:’dra = 640:?dia = 610
100 HOLD - TIME + (TB + If 10:REPEATUNTILTIME>HOLD
j 110 IFTRIG - 0THEN?dra - 630
120 IFTRIG - ITHEN?dia - 638
130 IFTRIG«THEN140ELSEIFINKEY 99THEN'dta &90ELSE130
140 REPEATUNTIL?cra>127
150 P = ?dra:?dra = 0:’dra = &20:?dra 0
160 FORI = 0TO255:PLOT69.2'l,4*?drb:?dra = 640:?dra - 0:NEXT
170 ?dra = 620
180 FORI =256T0511:PL0T69.2*l.4'?dib:?dra - 660:?dia = 620:NEXT
190 ?dra = &10
200 END
Fig. 12. The
printed-circuit
board for the
PIA
Ct - 100 n
1C. = 6821
PCB 86100
For Electron:
le 74LS04
ie 74LS133
Table 1. Test
program for the
BBC Micro.
Table 2. Test
program for the
Acorn Electron.
ports. These data relate to the time
base; the off-set; the trigger level;
leading- or trailing-edge triggering;
selection of input sensitivity; and
selection of AC or DC inputs. Note
that PAo to PAj incl. are used here as
clock signals. See also under Control
signals in Part 1. Tables 3-8 show the
correlation between data and selec-
ted settings.
The PB ports axe then set as inputs;
PA* is made logic 0; and PA* is
briefly made logic 1. This results in
the off-set data in the D-A converter
being read.
Next, make the PA* line high, which
creates a waiting period of at least
256 times the selected time base.
This ensures that the first memory
page no longer contains old data.
Make the PAs line (INH) logic high,
which results in the digitized input
signal being compared with the set
trigger level. As soon as these levels
are equal, the highest data bit in the
RAMs is made 1 (which makes it
possible later to determine exactly
where triggering took place); the
RAM counter is reset; writing is
discontinued; and the circuit pulls
the READY line (CAt) high to indi-
cate to the computer that the two
RAM pages are full. The computer
then makes lines PA* and PAs logic
low, which results in the READY line
being pulled low. The computer can
then read the RAMs.
First, however, the PAs line is briefly
made 1 to reset the RAM counter to
nought, so that the first memory lo-
cation can be read immediately.
After this, CPUL pulses on PA«
enable the data of successive ad-
dresses to be read at each leading
CPUL edge.
After the first memory page (255
bytes) has been read, make PAs
(INH) high: this serves as the eighth
address bit for the memory.
Subsequently, the second page of
256 bits is read in a similar manner.
All data can be stored or processed
immediately, depending upon the
available memory.
Finally, new data may be written
(with the PB lines arranged as in-
puts). A pulse on the PA6 will cause
the off-set data in the D-A converter
to be clocked. Making the PA* line
high will cause the PIA to start again
with writing into the first memory
page. After an interval of not less
than 256 time-base clock pulses, the
trigger may be enabled again.
As stated in Part 1, complete pro-
grams for the Acorn Electron, the
BBC Micro, the Commodore C64,
and MSX machines are supplied
with printed-circuit board 86083.
To enable owners of other makes of
computer to compile their own pro-
gram, a flow diagram of the program
T6 TS T4 T3 T2 T1
OF6 OF5 OF4 OF3 OF2 OF1 OFO
Table 3. Corre-
lation between
time bases and
data on TBo-TBa
lines.
Table 4. Corre-
lation between
sensitivity set-
tings. maximum
input voltage
and data on
Vo-Va lines.
Table 5. Corre-
lation between
trigger levels
and data on
To-Ts lines.
Table 6. Corre-
lation between
off-set voltages
and data on OFo-
OFt lines.
Table 7. Corre-
lation between
type of input and
data on AC /DC
line.
Table 8. Corre-
lation between
type of trig-
gering and data
on +/— line.
is given in Fig. 16. The quality of the
screen image will depend largely on
the resolution of the computer.
First of all, the location on the first
memory page where the trigger bit
(D7b) went high must be determined.
The next location is the first for a
complete picture, from which the
whole page may be read. The sec-
ond page can be started at the first
location, since all data there are in
correct sequence.
■2-57
selex-20
Linear Scale
Ohmmeter
Why should anyone
construct an ohmmeter,
when every multimeter has
several resistance ranges?
True, the multimeter has
higher end of the
resistance scale reads the
values very poorly. Due to
very closely spaced while
are widely spread.
ohmmeter, the divisions are
equispaced over the entire I
scale. This advantage is due
to the small I.C. OpAmp
The Circuit
A simplified schematic
diagram of the linear scale
ohmmeter is shown in
figure 2 The actual circuit
is shown in figure 3, which
looks much more complex
than it really is. First, let us
concentrate on the circuit
shown in figure 2. The main
component of the circuit is
the Op Amp IC1. IC1
contains a multistage
differential amplifier circuit.
A differential amplifier
amplifies the voltage
difference between its two
inputs. The voltage on the
non-inverting input (*)
increases the output
voltage, whereas a voltage
on the inverting input (-)
reduces the output voltage.
The gain of such amplifiers
is a few hundred thousands.
Gain of one hundred
thousand means that a
difference of 10 microvolts
Volt at the output.
A voltage divider made of
Rx and R is connected
across the output and the
voltage at the
interconnection of Rx and R
is fed back to the amplifier
at its inverting input. This is
feedback. This has an effect
on the circuit which makes
the voltage on the inverting
input practically equal to
that on the non-inverting
input. To understand exactly
what happens, let us
2-58
selex
from 5.6 fo 6.6 V. i.e. 1 V.
The output voltage will try
to increase by 100000 V.
The voltage on the inverting
simultaneously try to rise
depending on the ratio of Rx
and R. This in turn will try
to bring down the output
voltage. The result of this
will be that the voltage on
the inverting input will also
rise to almost the same
voltage which is on the
non-inverting input.
In case of the linear scale
ohmmeter circuit, the input
voltage on the non-inverting
input remains constant at
5.6 V. The voltage across R
is thus fixed at 5.6 V as we
above experiment. This
means that the output
voltage at the output of the
amplifier depends entirely
on the value of Rx. The
relation between these
values can be calculated as
follows:
Voltage on R = 5.6V
Voltage on Rx and R = Uout
^»5.6V
Which clearly shows that
Uout is directly proportional
to Rx if the constant value
of 5.6V is taken care of
during calibration with Rx =
Oil To take care of this, the
meter is placed on the
non-inverting input in the
actual circuit, so that the
voltage of 5.6V does not
play any part in the
measurement. The zener
diode produces the stable
input voltage current for D1
is supplied by R5.
The output voltage is
measured through the
combination R6 - PI - Ml.
Diode 02 protects the meter
Ml from very high voltages,
which can occur when the
ohmmeter is connected
without a test resistance.
Construction
As usual, the mechanical
work involved in
construction is much more
compared to the job of
soldering the electronic
components together on the
PCB. The mechanical work
can be simplified by using a
plastic enclosure, which is
easier to handle than metal
enclosures. Suitable holes
must be drilled in the lid for
sockets, switches SI, S2
and the meter Ml. A large
cutout of 50 mm diameter
must be carefully made for
the meter body.
'2-59
A standard Selex PCB can
accomodate all the circuit
component. Layout and
wiring is quite simple and is
shown in figure 4. Pin
| D1. D2 must be properly
observed.
Three different Op Amps
| have been specified in the
| component list for IC1 .
These are all pin
compatible. The commonly
used Op Amp 741 will not
work in this circuit. The
resistances used must be of
very close tolerances,
typically 2.5% or less, for R1
to R4. This ensures that the
scale is unformly divided.
The construction details are
shown in figure 5. The
Selex PCB is fixed on the
bottom of the enclosure and
the batteries are clamped
using an aluminium clamp.
After wiring and assembly,
the potentiometer PI is
adjusted such that the meter
shows full scale deflection
for a 1 K!! resistance in
Range 4 (0 to 1 K). The test
resistance of IK used here
Table 1 shows the setting of
switch S2 and the range
covered by the setting. Also
indicated is the current
through the test resistance
for each range. The scale of
our prototype is suitable for
range 2 (0-1 00K) For other
ranges, the reading must be
multiplied by 10 (Range 1 ),
0.1 (Range 3)0.01 (Range
4).
selex
The
Cackling
Generator
Functional
description
The audio frequency noise I
signal generated by the
third oscillator can be
adjusted by the
potentiometer PI to set the
desired sound level.
cackling quality, four short
and then a long cackling
noise with rising sound
level must be generated.
This is achieved by the
connection of first two
oscillator via a RC network |
consisting of R4. R8. RIO, j
| C3, C4 and D2. D3, D4.
I The sound of cackle can be
j modified by changing C7 by i
i two cackling cycles.
the second oscillator (using
N3 & N4| provides the
envelopes for the four
different cackling sounds
full cackling cycle The
selex
charged. When RIO has
more positve voltage than
that on R4. 03 blocks and
C4 is charged 04 prevents
the voltage on R8 from
becoming negative
The audio signal finally
amplified and fed lo the
loudspeaker. Observe the
shape of the transistor T1 ir
the circuit diagram.
Construction
This circuit has many
components to be
accommodated and requires
a double size SELEX PCB
The layout is showsn in
capacitors require the
maximum PCB space. While
soldering their polarity must
be correctly observed The
current consumption is
between 5 to 1 5 mA, and a
small 9V battery pack is
adequate to supply this
If one wants to combine this
circuit with a kitchen timer,
the relay contacts of the
timer c.l?eulf can be suitably
connected into this circuit
starts when the set time
If you want to pack this
circuit nicely in shape of a
hen as shown in the
photograph (5). it should be
assembled on two small
SELEX PCBs and
interconnections should be
made with wires. The
loudspeakers and battery can
be filed as shown in the
photograph. Potentiometer
PI can be fitted in front as
shown, so that sound level
can be conveniently
adjusted.
2-62 elettc.
selex
Electrical Power
characterises the use or
supply ol electricity. In the
abbreviated form it is
represented by the letter P
and the units for measuring
electrical power are Watts
(W|. Th higher the power of
a drilling machine, the
generated by it. The higher
the Wattage of a bulb, the
brighter is its glow. A water
heater with 3000 W rating
gives more heat than a
1000 W heater. The higher
the power of a stereo
amplifier, the louder is the
However, all the previous
examples are not identical
In case of the drilling
specified power from mains [
supply when it is drilling a
hole in a hard material. The
power consumption is much
running state This is not
true in case of the bulb or
tne water heater, because
'hey draw the specified
as they are switched on.
Also the example of the
amplifier is still different,
e power drawn by the
POWER
amplifier can be controlled
externally by the setting of
the volume control knob,
between a minimum and
maximum amount The
specified power of the
amplifier generally refers to
the maximum power. The
useful power is much less
than the power drawn from
mams When the amplifier
draws 30 W from the
mains, it does not supply 30
W to the loudspeakers. Even
the bulb with 100 W rating
does not convert all the 1 00
W of power into light, most
a part of it is given as light
There can be two meanings
to the power specification of
any electrical appliance It
can be the actual power
drawn by the appliance
from mains or it can be the
maximum power the
appliance is capable of
drawing from the mains
supply Another distinction
W from
vei. only a few
en out as light
lining power is
o heat So if
lying the power
the bulb to be
jt the stereo
kers It generally
twice as much as
ul Thus an
which is capable
>g 30 W from
II not deliver more
ut 16 W to the
loudspeakers.
converted from one form to
power loss, what we really
the power is not put to any
useless form of energy, as
in case of the bulb where
converted into
heat. However, if for any
reason, we were using a
100 W bulb to heat
something, then we would
say that most of the 100 W
of power is converted to
useful heat and some part
is lost as light! It all
depends on which form of
energy the appliance is
expected to deliver
The stereo amplifier draws
electrical power at 50 Hz
from the mains It gives out
electrical power at the audio
frequencies to the
loudspeaker The
loudspeaker in turn takes up
the electrical power from
amplifier and converts a
part of it into sound energy,
of the loudspeaker. Though
2 DJ
selex
Electronic
Switch
4
Described here is the
construction of a simple
electronic switch which is
electrically isolated from t
electrically conductive
voltage has no effect on the
switching mechanism
is directly placed in
mplest way to connect and
disconnect an appliance
Irom the mains supply.
However, the disadvantage
of such type of switching is
that the full supply voltage
is always present on one
terminal of the switch. This
may not always be
acceptable, especially in
case of switching to be
activated by sensitive
circuits like computers. In
such cases one can also
relay, but the relay contacts
can create problems when
they get worn out. Even
during normal operation,
the closing and opening of
relay contacts can produce
electrical disturbances
which may in turn affect the
actuating circuits of the
computer.
The better way is to use an
electronic switch similar to
the one described here.
Even though the practical
circuit of an electronic
switch used for controlling
computer is not as simple
as this, the principle
remains same. What is
described here is a simple
battery operated version.
The circuit still ensures full
electrical isolation from
I The Circuit
| The circuit of the eletronic
The Control Unit.
a) 16-Bit microprocessor (INTEL 8086)
b) Present on card memory of 1 6K (EPROM) and 8K (RAM
c) Provides communication between MARS and user witf
appropriate displays on monitor
d) Easily expandable to control large number of MARS
systems simultaneously.
el Speed control by simple command from user
f) Uses a * 1 2. -1 2 and *5 volt for motor and control care
g) User has three modes of operation to choose
1 1 TEST mode
2i MANUAL mode
3) TEACH mode
The Mechanical Unit
An omnidirectional ground transporting robot on four
wheels are powered by a pair of stepper motors. Each of
Ibese motors are capable of independent motion thus
very easily MARS can turn about any point
The ARM unit is capable of handling loads of upto
500gm held at the gripper High degree of accuracy and
low mechanical power input is achieved by the use of
gears. ARM unit has 90 degree freedom of movement up
and down making it capable of lifting objects from the
floor level.
The BASE unit has 360 degree freedom of movement.
The very cost effective design of gripper achieves a high
degree of compliance to suit any application. Driven by
high speed DC motors this gripper makes hold' and
release' action almost instantaneous.
selex
Construction
As always, all the rules for
construction of a circuit
which connects to the
mains, must be observed
strictly. The circuit can be
inside a suitable plastic
enclosure Standard
plug/ socket combinations
connected to points 1 . 2 and
3 shown in the circuit
diagram of figure 1
This gives us three
alternatives for switching on
the load.
1 Directly by switching SI
2. Through an external
switch or relay contact
qonneted across sockets
1 and 2
3. By applying an external
voltage of 4.5 to 6V across
sockets 2 and 3.
Triac TIC 206M, or
equivalent, can handle loads
upto 200W Triac TIC 226M.
loads upto 300 W
An important point to
remember here is that the
lamp takes a little time to
extinguish when
disconnected from battery,
and this will introduce a
short delay between turning
off switch SI and switching
off the load from the mains.
FLEXICELLS TO BEAT
BATTERY WEIGHT
by Dr Alan Hooper, Materials Developments Division, Harwell
Engineers designing electrical and electronics
equipment, from electric traction vehicles to port-
able radios for domestic or military use, have
always been frustrated by the weight and size of
batteries that have to be carried. Now under
development at Britain's largest laboratories, in
collaboration with other scientists in the UK and in
Denmark, all-solid-state rechargeable lithium
batteries bring pollution-free driving a great deal
nearer and may trigger many new and exciting
ideas for battery-powered equipment.
| Battery-powered electric
vehicles (EVs) are already
| in use in many countries.
I One example, in the UK, is
| the humble milkdelivery
| wagon, or 'milk float' It is
i successful because to do
its job it needs to work
| over only a short range
and a low speed is
acceptable in built-up
I areas, where it has the
added advantage over
! the internal combustion
engine of not causing
pollution. It is efficient and
j convenient tor continual
| stop-start operation and a
| commercial fleet of such
vehicles is easy to
maintain.
On the other hand, its
restricted performance
j causes considerable
frustration to motorists who
meet it on the open road,
for it cannot travel at the
speed of the rest of the
traffic Across the Atlantic,
the golf-cart would hardly
be welcomed on the
freeway. So the view of the
general public is that
electric vehicles have a
poor performance but are
I acceptable tor specialist
! duties.
.' 66 r . ! 987
It is the source of power,
the battery, which lies at
the heart ot the problem.
To put it simply, traction
batteries are too heavy
and too large for the
amount of energy they
store or the power they
can provide: a large frac-
tion ot the energy stored
in a typical traction bat-
tery is needed just to pro-
pel the battery itself
Aqueous
electrolytes
For practical purposes, the
present choice of batteries
for EV traction is between
two systems, each employ-
ing an aqueous elec-
trolyte. which is either
leadacid or nickel'iron.
This situation has
remained essentially
unchanged since the
beginning of the 20th cen-
tury despite many
attempts, especially over
the last 25 years, to
develop new systems. Over
that period, stimulation by
the appearance of poten-
tial rivals has led to signifi-
cant improvements in the
performance of existing
> systems and of vehicles
with good short-range,
traffic-compatible
i capabilities. Most of the
vehicles now available
are urban delivery vans
but one ot the latest is a
version of the popular
Peugeot 205 car. powered
by a nickel'iron battery.
There are certain practical
drawbacks specific to
individual systems, but the
main, general problem is
still that of limited range
I EVs are still, in general,
economically uncompeti-
tive with their internal
combustion engined
counterparts.
The performance offered
by the enormous energy
density of petroleum, with
more than 10 000 Wh kg
(watt hours per kilogram)
compared with
20-40 Wh kg for leadacid
! traction batteries and a
i highrate recharge
j capability (two minutes at
the pump in contrast to a
battery charge ot several
. hours), will never be
j matched by that ot any
I battery system, in spite ot
I an on-board energy con-
version efficiency that Is |
five times better. However, i
if a battery were available 1
! with high energy density
I (100 to 200 Wh'kg) it would
j significantly affect the
j practical value of EVs in a \
wide variety of appli-
| cations from wheelchairs
and bicycles to commuter I
I cars, taxis and delivery
I vehicles.
1 Not only would longer
i ranges and greater load-
| carrying capabilities be
1 realised, but the improve-
ments in gravimetric en-
ergy density would open j
j up considerable scope for
; innovative engineering in |
vehicle design, using
lighter and cheaper
materials. It is this, rather I
than cheaper batteries,
which would lead to a
j cost-competitive electric |
vehicle.
Portable
electronics
Similar problems are to be
found in other technologi-
j cally important areas. The
vast demand for portable '
I electronics equipment in !
the computing and com-
munications fields bring
with it a need for small,
lightweight, rechargeable
power sources. Both the
business executive and
the infantryman in the
field would benefit from a
lighter load to carry.
It is not only important to
achieve lower absolute
weights and smaller
volumes, to avoid the
hand-held cellular radio-
telephone or 'wrist-watch'
device having a suitcase-
size battery, but to be able
to provide batteries that
are suitably shaped, too.
For example, a flat-screen
television ideally requires
a flat battery pack.
There are also growing
markets in the telecom-
munications and other
industries tor standby
power sources. Here, too,
there is a trend towards
smaller electronics
packages and corre-
spondingly small power
sources.
NiCd batteries have been
used traditionally in these
markets and. more re-
cently, NifHi batteries too
for space applications
such as power sources for
satellites where cycle life
and reliability are also ot
prime importance; but the
low energy densities so far
achieved have restricted
the electrical load
capabilities ot missions.
Space stations and deep
space probes will require
power sources with higher
energy densities.
Much better energy den-
sities are theoretically
available from alkali-metal
couples, but materials
problems have restricted
their use mainly to pri-
mary battery systems and
to secondary batteries
operating at high tem-
perature. Of the latter, the
sodiumsulphur battery
is the best developed. It
uses an Na‘-ion conduc-
ting solid, sodium-beta-
alumina, as a solid elec-
trolyte and has to be
operated at 350°C
Predicted energy densities
are more than 100 Wh/kg;
more prototype traction
batteries have been made
and vehicle demon-
strations carried out in
several countries.
However, sodiumsulphur
batteries are still not com-
mercially available, even
after some 17 years'
research and develop-
ment by large teams of
scientists around the
world. Remaining prob-
lems include the repro-
ducibility of manufacture
and reliability in use of
beta-alumina ceramic
tubes, and the thermal
control and safety ot large
batteries High tempera-
ture systems of this kind
will, even it successful, be
useful only where large
batteries are needed
I A small, room-temperoture.
. rechargeable lithium bat-
l tery with a liquid organic
electrolyte has recently
, become commercially
j available in Canada. Its
! cathode material (M 0 S 2 )
I leads to a low open-
circuit voltage and
I moderate energy density,
i A useful life of more than
100 charge discharge
cycles is quoted but little
information is yet
available from field trials j
Applications under con- j
sideration include
photographic flashguns
and electric wheelchairs j
Radical
departure
Rechargeable all-solid- 1
state lithium batteries now |
being developed at
Harwell constitute what is j
perhaps the most radical '
new departure in battery
technology tor decades.
They also promise very
exciting commercial pro-
spects Based on thick-film
polymer technology, with
no liquid components,
they offer very high energy
density, mechanical flexi-
bility and variable
geometry as well as being j
robust and safe.
This work has evolved from j
a programme begun here
in 1978 to investigate
materials for advanced
alkali-metal rechargeable
batteries It was shared
between Harwell, univer-
sities in the UK and
research and develop-
ment establishments in
Denmark. The Anglo-
Danish Battery Programme
as it became known was
jointly sponsored at
Harwell by the UK Depart- I
ment of Trade and Industry
(DTI) and the European
Community.
The aim of the programme
was to examine the pro- |
perties and behaviour of 1
several promising solid
electrolytes and electrode
materials described in the
literature, to obtain a
sound idea of their pro-
perties. to define the prob-
lems to do with their use
in batteries and to assess
their compatibility with
other materials in cells.
Such work would enable
us to find out reliably
which materials might be j
technologically useful for
electric vehicle batteries
in the future. It was hoped j
to obtain a fairly hard-
headed assessment of |
whether alkali metal bat-
teries could be developed |
thal would achieve their
potential energy density |
advantages and to ident-
ify which materials could
best be chosen for future
cell development studies.
A working temperature
range of 100°C to 200°C
was considered accep-
table for a first generation
EV battery.
All-solid-state
Because of persisting diffi-
culties with organic liquid
electrolyte batteries, all- |
solid-state cells were seen
as the only practical way
forward tor operation at
ambient and moderate
temperatures. The cells
developed in the pro-
gramme have lithium
anodes and a so-called
intercalation or insertion
compound as reversible
cathodes. Examples are
V 6 0.3 and TiSz.
Although the early stages
of the programme studied
,‘l«nd,a tetwusiv 1987 2-67
in depth the very
interesting crystalline
inorganic lithium-ion-
conducting electrolytes
LiiN and Lil(AlzOa), the
choice of this type of cell
was made more realistic
by the discovery ot
polymer-based solid elec-
trolytes by Michel Armand
and fellow workers in
France. Certain polar
organic materials such as
polyethylene oxide) will
dissolve alkali metal salts
and manifest rapid olkali-
ion conductivity.
The absolute conduc-
tivities of such polymer-
based materials are not in
general as high as those
of crystalline solid elec-
trolytes, but they may be
made into thin, pinhole-
free plastic sheets with
good enough conduc-
tance for use in cells and
batteries. Equally import-
ant is that the plasticity of
the polymers overcomes
the other big problem of
solid-state battery systems,
namely how to maintain
good contact between
faces
Harwell staff have concen-
trated over the last four
years on developing the
technology for making the
polymer-electrolyte plastic
battery* and have built
and tested cells. Tech-
niques for continuous pro-
duction of the electrolyte
and cathode components
in the form of thin films
have been developed
and their dimensions can
be scaled-up when
required. The thickness of
iplete ceil is only
including printed and
packaging materials and
photographic film.
It has been shown that in
laboratory-scale cells,
operating at around
120°G there is a high
utilization of the active
cell materials at dis-
charge rates of a few
hours and with lives of
over 100 deep discharge
cycles. Larger cells, of up
to 500 cm 2 area, and
series-connecled multi-cell
stacks have also been
successfully mode and
tested. From these results
we predict usable-energy
densities tor solid-stale
traction batteries that
would make them one-fifth
of the weight and one-
third of the size of
lead acid batteries now
in service
Temperature
range
At present the cells, which
are polyfethylene oxide)-
based, operate most effec-
tively at 100°C or just
above, so they are quite
suitable in that respect for
vehicle traction service
and for use in satellites.
Earliest specialist appli-
cations may also be
found where the environ-
ment is hostile with tem-
peratures of up to 150°C, a
region where most con-
ventional batteries fail.
They may include down-
hole instrumentation in the
oil industry and certain
standby power sources
Furthermore, lower-
temperature performance
can be achieved with
existing materials and
cells when the power
requirements are low, as
tor many micro-electronics
jobs.
One attractive possibility
in this field is the inte-
gration of the battery with
the circuit it powers: the
thin-film planar tech-
nology is compatible with
conventional printed cir-
cuit board and hybrid
electronic circuitry. For
example, the technology
lends itself to the develop-
ment of a self-powered
intelligent credit card
incorporating a micropro-
But for many other pro-
spective uses, operation af
room temperature and
below is required, at high
power levels. This will
mean developing new
cell materials, especially
new polymer electrolytes.
Work is now going on in
many countries and a
research and develop-
ment programme here is j
being sponsored by an
industrial group or 'club' of j
battery users, manufac-
turers and materials
specialists. Supported by
the DTI, our Solid-State Bat-
tery Working Parly aims to
provide the basic tech-
nology to make all-solid-
state lithium batteries,
based on polymeric elec-
trolytes, for as many ap-
plications as possible.
Studies will concentrate at
first on developing better
electrolytes but expand as
membership of the group
grows
Success in this area will
open up many new uses
in the military, industrial
and domestic sectors. It
might well lead to cord-
less' vacuum cleaners,
lawnmowers and power
tools, and to new
flashlights, toys and elec-
tronics and communi-
cations equipment.
The idea of batteries
based on an all-solid-state '
polymer electrolyte,
perhaps using various i
materials and construction I
technologies tor different
applications, holds out
one of the most versatile
and exciting prospects for
battery development this
century.
Always a move ahead
Resistance Measurement
ill
For further details write to: r—
THE MOTWANE
I MANUFACTURING COMPANY
MOTWANE
R
61084 Telex 752-247 MMPL IN Grams: MOT-
WANE or Gyan Ghar. Plot. 434 A. 14th Road. t
Khar, Bombay-400 052. Grams: MOTESTEM L_
:w PRODUCTS • NEW PRODUCTS • NE\
IEC Strip Connectors are
available in wide range, from 5
Amps to 30 Amps in 12 ways,
moulded in Bakelite & P VC. The
metal parts are made of brass
and screws of M S. duly plated
to prevent corrosion. The strip
connectors are tested to
withstand High Voltage for 2
Lock Switch provides add
safety to electrical and
electronic equipments an
prevents unauthorised us
Panel Projection 6 mm
Action 3 Pcs SPOT
Contact Resistance 20
Milliohm
Rating 125 VAC 5A, 251
For further information, please
contact: -
ASIA ELECTRIC COMPANY
Kataru Mansion
132 A, Dr A B Road. Worli Naha.
Bombay 400 018
PLA introduces Series 101
Miniature relay in a slim style
design with overall dimensions
26(1} x 1 2 5(W! x 24.5(H) mm
Available with t changeover
contact rated for 6 amps, at
240V Ac 28V DC
It is ideal for high density PCB
applications in the field of
communication and Industrial
Control Systems as well as
house-hold electrical
appliances
Excel have come out with
'Henry Flat Cables'. Satisfying
UL and CSA standards, these
cables are generally available in
6 to 1 2 ways either in soft
Copper alloy, with silver or gold
plating and rated at 300V, 5A
Capable of operation in a
temperature range of -45°C to
100°C. the cables can be used
as jumpers for interconnections
in electronics instruments,
communication equipments,
business machines and
computers
BETA TESTER
This Transistor Beta Tester
measures static gain (Beta)
upto 300, at collector currents
upto 10 Amps and base
currents upto 1 Amp, at VCE of
4 Volts, as per international
specifications The currents are
pulsed at 2% duty cycle at 50
For further inform ..
ELCOM
103. J ay y opal Indt.
M S SAI ELECTRONICS
Thakor Estate Kurla Kirol Road
Bombay 400 086
Phone. 5131219 5136601
For details contact
M/S. EXCEL ELECTRICALS.
C 4 Raj Mahal Apartment.
Coves Road. Jogeshwari IE).
Bombay -400 060
TEMPERATURE DATA
LOGGER
SCR's Temp Data Logger is a
sw PRODUCTS » NEW PRODUCTS « NEW
1 — J
DIGITAL MULTIMETER
MECO has just introduced
the nodt I Mil . ( 1 Digits
Multimeter wlncti features a
single knob operation for all
functions.
It measures AC & DC
currents Tom 200 uA to 10 A
with a min resolution of 0 I
mA. AC voltage upto 750 V
and DC voltage upto 1000V;
Resistance from 200 ohms to
20 megohms; diode checks
and continuity tests
It has an accuracy of 0.5”'. •
1 dgt for DC Amps Volt &
Resistance measurements
and 1% • 3 dgt for AC
Volt /Amp measurments. It
temperature
f Wli
1
& & - -- 1 .**'
It is over load protected on
all the ranges except the 10A
range It has facilities for low
battery indication and
overload indications. It
operates on a 9V cell with
battery life in excess of 800
for further information.
MECO INSTRUMENTS
PVT LTD
Bharat Industrial Estate.
T.J. Road. Sewree.
Bombay 400 OIS
Phones: 413-7423.
413-2435. 413-0747.
construction m
coolants, v.brat
AC and DC ver«
for further inform, itn
INDIAN ENGINEERING
COMPANY
Katara Mansion.
PB NO 16551 rt •'!
Bombay 400 018
POWER PUSH BUTTON
SWITCH
Rajkumar engineers offer a
new power push button
switch incorporating the
latest advances in switching
The switch is rated at 5
amperes continuous load and
has double pole single throw
switching configuration
bimetal sandwitclied contacts
A tor fail safe operations it
preventing electrical cross
| for further information
I Rajkumar Engineers
j 106. Ba/sons Ind Estate
Chakafa Road.
i Andheri lEastl
| Bombay 400 099 Indio
I DIGITAL FREQUENCY
| COUNTER VDC18
VDC18 is the smallest sire ever
made in India. Features include
I BATTERY OPERATION cum
I mains operation through
[ adaptor. 7digit 0.5 inch LEO
I display. 30 MHz frequency
| range, light weight, resolution
selection etc.etc. VDC18
incorporate latest L.S I
circuitry Model VDC19 has
frequency range upto 500 MHz
and PERIOD. FREQUENCY
for details contact:
VASA VI ELEC TRONICS
(Marketing Division J
630.Alkarim Trade Centre
Rantganj
Secunderabad 500 003
Phone 70995
PLASTIC INSTRUMENT BOX
FOR BACK MOUNTING
Comtech T-77' is an elegantly
designed plastic moulded
instrument box suitable for
back mounted instruments such
as Timers. & various other
control instruments, having
overall dimensions of 110 mm.
I x 77mm W x 100 mm B It
consists of a moulded box. a
cover. & a M S plate for back
mounting The box has an
mm for various components. A
six way terminal strip fixed at
the top & bottom, infront of the
box provides a n easy access for
the terminals. The cover can
accommodate a PCB of 77 min
X 72 mm from inside & has a I
1 .2 mm deep recess in front to
take an Aluminium plate of 65
mm x 66 mm. for control
indications. The box offered in
Black & Grey colour with
most suitable for small
instruments to be mounted side
by side from the back, like eg.
counters, controllers & timers
PROXIMITY SWITCHES
IEC offers a new line of
I inductive Proximity switches
which are basically
contactless limit switches. In
| addition, they feature
for further Details contact
ADVANCE INDUSTRIES
1 1 . finer ala Bldg
Tribhuvan Road
Near Dreamland Cinema
Bombay 400 004.
for further details contact
COMPONENT TECHNIQUE
8, Orion Appartment
29 -A La/lubhai Park Road.
Andheri /West)
Bombay 400 058
2-72
R.N. No. 3988 1/83
MH/BY WEST -228
LIC No 91
Don’t miss
the
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Lamington Road, Bombay 400 007
Tel: 362421, 353029 Telex: 011-71801 DYNA IN Gram: ELMADEVICE
Primer Publisher - C.R. Chandarana. 2. Koumari. 14th A
tied at Trupti Offset. 103 Vasan Udyog Bhavan. TuLsi Pipe R<
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I. Bombay 400 013.