CCwosp

a low-cost synthesiser

selektor

clavitar <K. Grasruck) .
This is a new musical instrur
athirst sight, because it looks i

keyboard.

page 3-1 7 ™

Although it may look, at first sight, like a toy piano
or organ, the Chorosynth is actually a real instrument
that has been used with success at live performances.
As the 'hands down' winner of our Eurotronics
competition, it certainly merits a printed circuit
board. a

a piano that sounds like a piano

Good news for those who have built the
few simple changes and one or two addi

chorosynth ij.d. Mitchtin
The Chorosynth is an inexpensi
anyone can play, a mini synt
keyboard by the working music
The Chorosynth is a real instrur

sive keyboard

sposal. The winning design of
ime suggests, a synthesiser with
rototype was designed with a

our competition, it is. as tl
chorus effect. Although tl
keyboard, we have made

dual slide fader

Enthusiastic amateur photographers and professionals
only interested in making good pictures: they also
present them properly. For a slide show, the prof
’trick' of using two projectors alternatively is be
increasingly popular - the ’black’ gap between su
slides can be eliminated in this way. For the finishint
each following slide should be gradually blended i
existing picture. This is where the dual slide fader cc
as the brightness of one projector is smoothly increa
other is gradually faded out.

As microprocessor systems becomeeveHnore like
their big brothers - commercial computers — one of
the features that must be added is a 'micro paper
printout'. The printer for microprocessors described
in this issue is a good example.

coming soon

printer for microprocessors

It can be useful to print the information output froi
processor system on paper. For small pP systems
hexadecimal output, a so-called matrix printer
choice. This is connected to the microcomputer

SC/MP syste

software.

remote control .

Remote control uni
accessory on colour
controlled ’remotely"
is no longer science

almost standar

in the April issue of Elektor ... 'Zjy
electronics in cars in the Eighties!
with designs for electronic ignition systems,
intelligent windscreen wiper delay, instant fuel
consumption meter, various indicator circuits, and
many more.

market

UK 10 — etektor n

Britain’s first comp

A complete personal computer
fora third of the price of a
bare board.

Also available ready
assembled for£99I§

The Sinclair ZX80.

Includin g VAT.
Includin g post and
packing.

Including all leads
, and components *

i '

I '•

1

Sinclair -

zxso

Science of Cambridge Ltd

1980 - 3-01

Accidental destruction of
recorded data on magnetic tape

Many myths surround the accidental
erasure of magnetic tape. This article,
by the manufacturers of Scotch Instru-
mentation Tape, shows the results of an
extensive test program to discover just
how much tape will stand before loss of
information occurs. Although the
purpose of the tests was primarily with
digital data, as used in the computer
world, similar effects will also occur
when storing analogue information on
magnetic tape.

Magnetic fields

The obvious threat to magnetic media is
the permanent magnet which can

produce strong, steady magnetic erasing
fields and needs no external power
source for producing or maintaining its
field.

A variety of tests were carried out using
permanent magnets and it was found
that a signal level loss of less than 50%
of the original value did not result in a
loss of data, because the data at this
level can be still recovered during
normal operations.

For instance, when a horseshoe magnet
strong enough to lift 40 lbs (18 kg) was
placed directly against the edge flange
of the reel of tape, a signal reduction of
80% resulted at the beginning of the
tape (See Fig. 1). This loss of signal
decreased steadily along the tape from
that maximum of 80% until a point 350
feet (107 metres) into the tape where
the signal level was 50% of the original
recorded level. At this point a tape can
still be read.

The decrease in signal loss from 80% to
50% over 350 feet can be explained by
the fact that the further into the tape
the progressively weaker becomes the
magnetic field and thus the erasing
effects.

Since the intensity of a magnetic field

falls off approximately as the cube of
the distance from the magnet (at 10"
away from the magnet its field is roughly
1/1 ,000 as strong as when it is 1" away) ,
it was seen that when the horseshoe
magnet was held 1" (25 mm) from the
edge of the reel, a signal loss of only
22% occured: this reduction of signal
did not result in any data loss at all. It
would therefore be impossible for any
concealable magnet to cause data loss
if there is a distance of at least 6"
between the magnetic source and the
tape.

Even when a large scrap metal 'Electro'
magnet was placed 1 .3 feet (0.41 metre)
above recorded tapes no data loss
occured.

However, it must be pointed out that
any small permanent magnet could
cause a loss of data if placed directly
next to or on to, the surface of the
magnetic media.

As the magnetic media read/write head
is also an electro-magnet, it is also quite
capable of becoming permanently
magnetized and thus cause some erasure
and consequent data loss. This problem
can be overcome by regular use of a
head demagnetizing or degaussing device.
As far as equipment such as transformers
and electric motors is concerned, both
of which produce magnetic fields, if this
type of electrical equipment is enclosed
within a cabinet allowing two or three
inches of spacing between the magnetic
tape and the electrical component there
will be no danger of erasure.

Very often magnetic tapes are trans-
ported within areas where radar antenna
are transmitting, for instance within
airport locations, defence areas, aboard
ships etc.

To gauge what effects, if any, radar
signals had on magnetic tape, tests were
carried out using irradiation by C-Band,
L-Band and X-Band radar systems,
ranging from 200,000 watts for the C-
and L-Band and 500,000 watts for the
X-Band, of peak power. The tape was
placed at 100' (30.5 metres), 50' (15.2
metres) and 10' (3 metres) in the direct
signal path of stationary antenna. No
signal loss or data erasure was observed,
and would probably not occur unless
the tape was in actual direct contact
with the antenna.

Metal detectors

Although there was, and probably still is,
much concern over airport metal

magnetic fields.

3 02 - 1

detectors and the effect these instru-
ments had on magnetic media, it has
been shown that this concern is no
longer necessary.

Many experiments were carried out
using both the 'walk-through' type of
detectors and the 'hand-held' units.
There was no instance where data loss
occured. Even when the highest intensity
type of detectors using a rating of
398 A/m (5 Oe) was tried, no significant
signal loss was seen as shown in figure 2.
Today the maximum strength of a walk
through metal detector is recommended
at approx. 95 A/m (1.19 Oe), a level
much too small to cause erasure of
magnetic media.

X-Ray contamination

On occasions, magnetic media can be
subjected to X-ray energy, especially by
airport surveillance systems.

Various recorded magnetic media have
been subjected to extremely high (lethal
to humans) X-ray doses and no erasure
or data loss occurred. This was the same
result for magnetic tapes subjected to
airport X-ray techniques.

High electrical charges

High voltages were generated by auto-
mobile ignition coils and they were
applied directly across recorded
magnetic tape. More than 1 5,000 volts
were generated, and the ignition coils
produced arcs which struck directly on
to the tape. No erasure or data loss took
place. Only if the voltage was so high
that excessive arcing actually physically
damaged the tape would data loss occur.

Micro-waves

Recorded cassettes were subjected to
the maximum microwave energy, in fact
until they became warm, when placed

2

inside a normal microwave oven. When
the cassettes wer re-read, there was no
measurable data loss. This would include
any leakage areas existing around ovens.

Televisions

Because television receivers are sources
of magnetic fields, high voltage and
X-rays, various tests were performed by
placing recorded magnetic tape both
inside and outside of different receivers,
including colour sets. Despite the high
voltage potential and X-ray sources, no
signal or data loss was recorded.
However, as receivers have many
magnetic field producing components,
and many receivers were not tested, it is
advised not to store magnetic media on
top of these devices.

Nuclear radiation

A gamma ray dose of approximately 3.0
megarads was directed at a recorded
digital tape cassette while in a gamma
ray pool for at least 1 VS hours. No data
loss was recorded at this level.

However, as a general statement on the
effect that nuclear radiation will have
on magnetic tape, we can say that no
measurable effect will take place until
the dosage approaches a level 200,000
times greater than that which would
cause death in 50% of exposed humans.
Radiation of this level (100 megarad)
would tend to increase the layer -to-layer
signal transfer or 'print-through' by
about 4 db. This is slight enough not to
be termed serious and would not prevent
information retrieval.

This amount will also have some
physical effect on the tape coating and
backing. The backing will show signifi-
cant embrittlement and it is expected
that the tape life could be reduced by as
much as 60%. It is reasoned that
whatever Electro-Magnetic Field might
result from a nuclear detonation would
not be of sufficient intensity to adversely
affect the tape; therefore, the threat of
signal erasure is virtually non-existent.
The effect of Neutron bombardment
would no doubt be limited to activation
of the iron-oxide in the coating. This
would produce a radioactive isotope
that itself might become a source of
further radiation, but it is theorized that
such activation would not produce a
change in the overall magnetic properties
of the coating.

Radioactive dust or fallout is not capable
of producing the dosage necessary to
adversely affect magnetic tape.

: igure 2. Electromagnetic fields.

elektor march 1 980 - 3-03

Laser beams are sometimes used as a
recording medium, i.e. heat or
'thermoremanent' recording and the
'medium burning mode'.

However, to prove that data destruction
from a distance is not practical, a
recorded magnetic tape was subjected to
a laser beam spread over a wide area of
the tape. No data loss occurred. Only if
a very high intensity beam were to be
directed at the medium would there be
sufficient heat to destroy the actual
physical properties of the tape.

Effect of heat

For a substance to burn, there must be a
breakdown of the organic materials
contained in it. The organic materials in
magnetic tape are the plastic backing
and the binder. To burn, these must
first vaporize - thus increasing their
exposure to the oxygen in the atmos-
phere — and then rapidly oxidize to
form light and heat. An ample supply of
oxygen is required to sustain burning.
Since magnetic tapes contain no
'built-in' oxidizer, it cannot burn in the
absence of air. Simply stated, its
behaviour can be closely compared to
the way in which a tightly wound roll of
paper would burn.

While the 'self-ignition' temperature of
polyester backed tape is in the neigh-
bourhood of 1000°F, temperatures
below that point can still cause damage.
Polyester film will shrink 1 %% at 300° F ,
and 25% at 325°F. If a roll of tape is
heated to the approximate temperatures
presented below, certain effects would
be noted when the roll had cooled.

250°F — Backing distortion.

320° F — Softening of both the
backing and binder with
some 'blocking' or adhesion
of adjacent layers.

550° F — Darkening and embrittle-
ment of the backing and
binder.

1000 F — Charring of the backing and
binder.

When charring occurs, the tape cannot
be unwound from the reel as it will
flake when touched. The temperature
limitation of present day tapes is a
function of the gamma ferric oxide
pigment.

Two reels of computer magnetic tape
functioned adequately after exposure to
210°F (99°C) and then refused to load
into the tape transport after they had
been subjected to 230°F (110°C). At

first, this appeared to be a permanent
condition since these tapes had not
recovered after six weeks in storage.
However, after two months, they began
to run again; the data could be re-
covered, but the oxide appeared to be
shedding badly. In such cases, if possible,
the data should be copied as soon as
possible onto new tapes and the
shedding tape discarded.

Theoretically, gamma ferric oxide
particles which are the prevalent
magnetic constituents used in most
magnetic storage media, are capable of
retaining data up to a temperature of
approximately 1247°F (675°C). This
temperature is known as the Curie
temperature. On the other hand,
chromium dioxide particles, used in
audio and video tapes, have a Curie
temperature of approximately 275°F
(135°C) and are therefore far more
susceptible to a heat related data loss.
Winding and storing magnetic tape
properly will lessen the possibility of
damage in the event of fire as tape is a
poor conductor of heat. It is sometimes
possible to recover information from a
tape receiving slight fire damage by
carefully rewinding it at minimum
tension. The information it contains
should be transferred immediately to
another reel of undamaged tape.

We recommend the C0 2 type of fire
extinguisher for combating burning
magnetic tape. C0 2 is clean and contains
no chemicals that could harm the tape.
If water reaches the tape it will probably
not cause complete failure but there
may be some evidence of 'cupping' or
transverse curvature. The amount of
'cupping' would depend on the quality
of the wind and the length of time the
reel was exposed. If the wind is loose or
uneven the water can more easily reach
the oxide surface and the cupping
would be more pronounced. The tape
should be removed from the water as
soon as possible, and certainly within
24 hours.

After removal, the reels should be
allowed to dry on the outside at normal
room temperature and then rewound a
minimum of twotimes. This will aid the
drying operation and will also help the
reels to return to equilibrium faster.

If a temperature increase is also incurred
while the tape is water soaked, steam or
at least high humidity will be present.
This is more likely to cause damage than
water alone. A temperature in excess of
130°F, with a relative humidity above
85% may cause layer to layer adhesion
as well as some physical distortion.

Effect of low temperatures

Recorded computer tapes which were
kept at a temperature of —60 F
(— 51 °C) for 24 hours, showed no
data loss after being dried and relaxed —
a process of careful, slow unwinding and
rewinding.

In theory, no loss of magnetization
should occur with magnetic oxide
particles even in temperatures which
are well betow their Curie temperature
although in most cases they will become
stiff in nature and wet with condensed
moisture.

It is important that after tapes have
been kept in very low temperatures
for long periods of time that they
should be dried and relaxed over a
period of days, at gradually increasing
temperatures. This will relieve some
of the stress which will have built up
and will minimise the shrinkage that
will have taken place.

Tapes that are cycled between tempera-
ture and humidity extremes can develop
severe stresses within the reel due to the
pressures caused by the plastic flow of
the tape material. These stresses can
lead to damaged regions on the tapes.

Conclusions

Failure of magnetic tapes as a reliable
storage media is almost always due to
some form of physical deterioration
of the tape and not to the deterio-
ration of the recorded data.

Magnetic fields were found to be the
only type of energy that could damage
the recorded data without actually
physically affecting the magnetic media.
Only a few inches of spacing is quite
sufficient to prevent erasure of data by
any concealable magnet which could
feasibly be found in a normal environ-
ment. It has been determined that
normally there is no need to shield the
stored data against-X-rays, high voltage
fields, nuclear radiation, high fre-
quency fields or light energy.

Instrumentation Talk volume 2.

(529 S)

3-04 -elektor march 1980

This is a new musical instrument. You might not notice it, at first sight,
because it looks a bit like an electric guitar. You might not notice it at
first 'hearing', either, because it can sound a bit like an electronic organ.
It is played in approximately the same way as a guitar, so that guitarists
should be able to get the hang of it quite quickly. They can then
produce the full, rich sounds of an organ by using a finger-board instead
of a keyboard.

Why bother?

Obviously, a normal guitar — with
strings - is a highly popular musical
instrument. And quite rightly so. How-
ever, replacing the strings by a set of
keys has its advantages. It makes it a lot
easier to produce a range of sounds that
is also in popular demand: those of an
electronic organ. The most significant
difference between an electric guitar
and the 'no-string' version described
here is that, with a normal guitar, you
have to 'keep it going'; with the keyed
version, notes can be 'held' for as long
as you like — as with an organ. On the
other hand, the percussive sound of a
normal guitar can easily be made,
electronically, when using the no-string
version. Since this 'decay' option can be
switched on or off as desired, the result
is a highly flexible instrument: from

dmilnr

A guitar has six strings — normally
speaking. Each string is tuned to a
specific note which can be considered
as the 'basic note' for that particular
string.

When playing it, the fingers of one hand
press one or more of the strings down
onto the 'frets' along the neck of the
guitar. This reduces the effective length
of the string, so that it produces a higher
note when it is struck. Guitars are con-
structed in such a way that each success-
ive fret corresponds to a half-note
increase. By depressing several strings at
once it is possible to play complete
chords. There are a large number of
standard chords for guitars, each with
its own characteristic finger positions.

No strings attached

Now for something completely differ-
ent. Instead of pushing down (or 'stop-
ping') the strings at the right places,
push down on keys! Each key can be
arranged to correspond to a specific
note; then, provided they are positioned
correctly 'between the frets’, the normal
guitar finger positions will select the
same chords.

If a further small group of keys is used
to 'pluck the strings', there is no further
need for the original strings; they only
get in the way. Take them off, and
you're left with the instrument de-
scribed here.

when is a guitar not a guitar?
when it has keys!

organ to guitar at the touch of a switch.
The guitar-with-keys can even be played
single-handed — literally. After all,
only one hand determines the chords to
be played, and that's all that the elec-
tronics need to know. If the left-hand'
keys are bridged, a note will sound as
soon as it is selected. The instrument
can then be played with one hand!

What it looks like

For reasons of cost and for ease of con-
struction, some of the possibilities of a
'real' guitar were omitted. Quite apart
from the fact that you can't 'pluck' a
pushbutton, there are three further
simplifications:

• Instead of six strings along the 'neck'
of the instrument, there are only

four. The two lower strings are omitted,
leaving E', B', G and D.

• Somewhat surprisingly, the other
hand can 'strike' five strings. The

fifth is added electronically. It is lower
than the others, corresponding to an
A string; it produces a note that is one
octave lower than that selected for
either the B' or the G string. Normally
the musician must deliberately select
this note by pressing down on the fifth
string at the correct point between the

• By far the most drastic simplification
is the reduced number of frets: only

five are used. A normal guitar will have
more than twelve — but in practice, you
can do quite a lot with only five.
Basically, it means that you are limited
to playing in the lowest positions. How-
ever, it also means that only twenty

keys are needed along the 'neck' of the
instrument — instead of the large num-
ber that would be required for a full-
scale imitation of a guitar.

How it works . . .

A block diagram is given in figure 1 ; at
the same time, this drawing gives some
idea of the shape and key positioning
for the instrument. The twenty keys
that determine the notes for the four
'strings' are mounted at the end of the
'neck'. These we can refer to as the
'note keys'. In the circuits, they will be
labelled according to the note that they
produce: 'Sd', say, corresponds to a D.
In some cases, where more than one key
produces the same note, these will be
indicated as Sc'i and Sc' 2 . The lowest C
is given as c, one octave higher isc' and
two octaves higher is c".

Sa, Sq, Sq, Sb' and Se' are the five
keys for 'striking the strings'; we'll call
them the 'string keys'. Operating Sb'.
for instance, produces the note selected
(by S e 'i • • • S c 'j) for the second string.
Sa produces the note for the 'fifth
string', as determined by the note keys
of either the B' or G string. If none of
the note keys are operated, the string
keys will produce the open note for the
corresponding string — as with a normal

For 'single-handed' playing this function
can be switched off, so that no tone can
sound until one or more of the note
keys is operated. The string keys can
then be bypassed. The instrument will
now play in the same way as if all five
string keys were held down continu-

ously: a note will sound as soon as
'note key' is operated.

. . . electronically

The basic principles of the circuit can be
derived from the same block diagram.
The note keys are arranged in four
groups of five keys; each group deter-
mines the frequency of a corresponding
oscillator.

As required, the outputs from these
oscillators can be passed through fre-
quency divider stages, bringing the note
down one or more octaves. This helps
to counteract some of the disadvantages
of the simplifications described earlier.
The next step is a multiple mixer stage;
among other things, this derives the
note for the simulated fifth string from
those produced by the second and third
strings.

The string keys Sa . . . Se', each drive
an 'envelope generator'; these determine
the output levels (including attack and
decay) for each string, by controlling
associated 'voltage controlled amplifiers'
(VCAs).

The circuit

The complete circuit is quite a compli-
cated affair — not the kind of thing that
you knock together in an hour or two.
A large number of (inexpensive) com-
ponents are needed; there is a lot of
wiring to and from switches; and there's
quite a bit of tuning to do.

For clarity, the circuit has been broken
up into six partial circuits. The note
keys and the four oscillators are given in

be produced.

figure 2. Two different types of divider
stage are shown in figures 3a and 3b,
and figure 4 shows how these are used
in the complete circuit. Figure 5 is the
envelope generator; five of these are
combined as shown in figure 6. Finally,
figure 7 is the circuit of a vibrato oscil-
lator.

Obviously, there is quite a lot of wiring
between the various partial circuits. The
corresponding signals are all clearly
shown, and references to other figures
are given at most of the connections. As
a further constructional aid, the inter-
connections are listed in Table 1.

A special group of signals are labelled
E', B', G, D and A. These correspond
to opto-couplers: five LEDs (D45
in figures 5 and 6) and five LDRs
(RE' . . . Ra ^ figure 4).

Now, let's take a closer look at the
various circuits.

The note keys

The circuit that produces the basic
notes for the four strings is given in

Interconnections between the various

signal

from

to

Ob'

*1

fig. 6

fig. 2

9d

e'

9

d

fig. 2

fig. 4

« r e'

« r b'

fig. 2

fig. 6

« r d

CA

fig. 2

fig. 4

vibrato

fig. 2

E'

B'

fig. 5

fig. 4

D

(opto-c

ouplerll

A

The oscillators are very simple, each
using a single unijunction transistor, or I
UJT: T1, T3, T5 and T7. They produce
a sawtooth output, the frequency of
which is determined by a capacitor I
(C3, C6, C9 and Cl 2) and a resistor. In 1
this circuit, the resistor is replaced by a
series chain of preset potentiometers. 1
The number of presets actually in use at
any time depends on which note key is
operated: the positive supply voltage is
fed down from that key to the corre-
sponding tap in the chain. If none of the
keys are operated the 'open note' for
that string is produced, provided SI is in
position 'a'. With this switch in position |
'b', the oscillator cannot run unless one
of the note keys is pressed.

The output from each oscillator is taken
via a transistor (T2, T4, T6 and T8); this
is 'gated' by the 'q' signals from figure 6.
The 'tr' signals go the other way, from
figure 2 to figure 6. The circuit given in
figure 6 detects the release of a note key
(as will be explained), and then rapidly
damps the tone output from that string.
The circuit for the note keys that corre-
spond to the B' string is more compli-
cated than the other three, since it must

also provide the signal CA. This signal is
used to control the generation of the
sound for the fifth string (in figure 4).
The note for this fifth (A) string is
derived from the B' string when the CA
signal is low, and from the G string
when CA is high. In this way, the rather
monotonous effect is avoided that
would occur if it was always derived
from the same basic signal.

Divider stages

Two different types of divider stage are
used. The outer strings (E' and D) use
the simple version shown in figure 3a;
the inner strings (B - and G) need the
more extensive circuit shown in fig-
ure 3b. The reason for this is that the
inner strings also provide the signals for
the fifth string. The complete divider
section is shown in figure 4.

Frequency division — to provide the
notes for lower octaves - is done by
flip-flops: FF1 and FF2 in figure 3a,
and FF3 . . . FF5 in figure 3b. So-called
D flip-flops are used, with the Q output
connected to the D input as shown in
figure 3a. The CMOS 1C used (type
CD 401 3) contains two flip-flops, so
that five ICs are required in all.

This divider section is included in order
to extend the tonal range of the instru-
ment. Three octaves are available
(similar to 8', 4' and 2’ stops on an
organ); each 'octave' will only appear in
the final output if the corresponding
control input (oct 1 . . . oct 3) is left
floating. As can be seen in figure 4, one
or more of these inputs can be connec-
ted to supply common by means of
switches S2 . . . S4. Opening S2 gives
the lowest octave, S3 is for the next and
S4 for the highest octave.

In figure 4, the control signal CA from
figure 2 comes in at the left. CMOS
inverters N1, N2 and N3 pass this signal
to the 'control A’ inputs of two of the
divider stages (the more complicated
version given in figure 3b), to determine
which of these two strings is to be used
for deriving the signal for the A string.
If switch S5 is closed, the G string is
permanently selected (so that the A
string will always sound one octave
lower).

The outputs from the divider stages go
to a 'light-controlled' mixer stage. Each
output (including the two A outputs
for the fifth string) is fed through a
resistor network that includes an LDR
RE' - . . Ra)- The amount of light on
the LDR determines the output level for
that string. This means that each LDR,
with an associated LED in figure 5, is
basically equivalent to a very low-cost
VCA.

The output from figure 4 is the final
audio output from the instrument. In
principle, it can be fed direct to a
(power) amplifier; however, it will
usually be far more useful to include
a control amplifier with more-or-less
extensive filtering capabilities - similar
to those used in electronic organs.

/elope ger

jr. These units (five in all) determine the 'attack' and 'decay'

Figure 6. The complete envelope generator section. Four different ways of playing the
instrument can be selected by means of S6 and S7.

3-10 - elektor

Attack and decay

'Attack' is the rapid increase in
amplitude of a tone, initially; 'decay'
refers to the gradual level reduction
towards the end of each note. These
effects must be produced electronically,
if this instrument is to sound anything
like a guitar.

The output level for each string is
determined by the amount of light on
the corresponding LDR in figure 4.
When a string key is pressed briefly, a
short pulse of light must fall on the
LDR, with the correct attack-decay
characteristic.

The light pulse is produced by the
circuit given in figure 5. The actual light
source is the LED, D45. Five of these
envelope generators, as they are called,
are used in the complete instrument; the
combined circuit is given in figure 6.
One envelope generator is used for each
string. Obviously, the LEDs must be
mounted close to the corresponding
LDRs (LED 'A' beside LDR 'A', and so
on), and inside a reasonably 'light-tight'
box of some kind — so that the only
light falling on the LDR comes from the
LED.

With the various switches in figure 6 in
the positions shown, the main control
signal for each envelope generator is
derived from the corresponding string
key (Se'.-.Sa). Before a key is
operated, input 2 in figure 5 is connected
to supply common through S6. Til
(figure 5) conducts; Cl 5 is discharged at
this point, so T12 is cut off; the LED
will not light. Operating a string key
pulls input connection 3 up to positive
supply, cutting off Til. Cl 5 charges
fairly rapidly, so that the voltage at the
C15/R66 junction falls. When the key is
now released. Til again starts to
conduct; via D42, it supplies current to
T12 and, as Cl 5 discharges through R66
and the base-emitter junction of T12,
this transistor conducts and the LED
lights. Cl 5 and C16 provide the desired
gradual attack and decay characteristics.
The tone is therefore produced after the
key is released.

In figure 6, PI sets a DC bias at input
connections 4 of all envelope generators.
As can be seen in figure 5, this bias
determines the lowest voltage to which
Cl 5 can be charged when T1 1 is cut off;
it also determines the initial discharge
characteristic of this capacitor - and
with it, the attack/decay.

Switches S6 and S7 in figure 6 are used
to select other types of 'play'. With S6
in position 'b' and S7 left in the (open)
position shown, 'percussive' play is
obtained: Til is initially cut off and
Cl 5 is initially charged, so that the tone
is produced as soon as a key is operated.
Even with the key held down, the tone
will then decay. Alternatively, with S7
switched to its other position (closed)
and S6 in position 'a' a more organ-like
effect is obtained: the note sounds as
long as the string key is held down; after
the key is released, the tone decays
gradually ('sustain'). Finally, with S6 in

7

8

Figure 8. A suggested layout for the controls and keys.

(Hz)

784.0

740.0
698.5

659.3

622.3

587.3

1046.5

987.8

932.3

1 760.0
1661.2

1569.0

1480.0
1396.9
1318.5

position 'b' and S7 closed, a tone will
sound as soon as a note key is operated:
the string keys need not be operated
(actually, they will interrupt the tone -
but the intention is to leave them
alone . . . ).

Four flip-flops, FF6 . . . FF9 in figure 6,
ensure that the tone dies out rapidly
when the corresponding note key is
released — once again, as in a normal
guitar. As mentioned earlier, they block
the output in figure 2; furthermore, the
Q output is used to discharge C15
(figure 5) rapidly. A 'damper' key is also
provided: operating S8 mutes all strings
rapidly - the same effect as placing the
flat of the hand across the strings on a
real guitar.

Vibrato

So far, nothing has been said about the
'vibrato' input to figure 2. It is connected
to the circuit given in figure 7: a con-
ventional very low frequency oscillator.
Its output modulates the output
frequency of the 'string' oscillators. The
modulation depth — i.e. the extent to
which the effect is audible — is varied
by means of P3. The vibrato frequency
is set by P2.

The circuit is designed so that the
oscillator will stop when P2 is set to
maximum resistance. P3 can then be
used to introduce 'manual' vibrato —
like the vibrato 'handle' on an electric

Construction and tuning

Figure 8 gives a suggested layout for the
various keys and controls. Obviously,
the positions can be modified according
to personal taste: in particular, the
'damper' button (S8) might be mounted
some way to the left of the string keys,
so that it can be operated by the palm
of the hand.

Good quality pushbutton keys should
be used - they must operate easily,
reliably and very often ... As shown in
the photo of the prototype (figure 9)
the original author used 'Digitast'
switches. He also extended the principle,
building a complete six-string version.

As far as the electrical construction is
concerned, the only 'fiddly bit' is the
LED-LDR combination. Each pair must
be adequately 'screened' from the other
pairs and from ambient light.

A 9 V stabilised supply should be used.
The total current consumption is only
150 mA.

Calibration is quite a job — as with most
musical instruments. First: figure 6. The
wipers of presets P9 ... PI 2 are first
turned down to supply common. They
are then turned up to the point where a
tone is just heard when the correspond-
ing string key is operated (with S6 and
S7 in the positions shown) .

The presets in figure 4 (P4 . . . P8) are
used to set the output levels of the
strings. The best way is to first set them

all to maximum. The softest output is
then taken as a reference, and all others
are turned down until they produce the
same output level.

Then, finally, the presets in figure 2 . . .
These determine the frequencies of the
notes, so each one must be set accu-
rately. Whatever system you use, one
thing is essential: in each chain, the
presets must be adjusted from left to
right (first the highest note, i.e. the
preset closest to T1). The easiest is to
start with the highest note of all:
P a ", at the lower left in figure 2. With
S a ", and Se' both operated, an 'A'
should be produced. This can be checked
against some other (correctly tuned)
instrument, or with a tuning fork.
Obviously, it's a rather clumsy business
to have to operate two keys and adjust a
preset - all at the same time - so it is
advisable to switch to the 'organ' mode,
where the string keys are not used. P a "
is then adjusted so that the correct note
is produced when S a " is operated.

Each following preset in the chain is
adjusted until the corresponding note
key produces a note that is exactly one
half-tone lower: A, G# G, F# and so on.
This can be done 'by ear', if you have
the knack, or else by comparing the
note to that on some other instrument.
Proceeding from the E' string to the
(next lower) B’ string is not too difficult:
the tone produced by the 'open' E'
string (SI in position 'a', no note key
operated) is the same as that produced
by the B' string when the highest note
key (S e ') is operated. After P e ' has been
correctly adjusted, that is. The other

presets in the B' chain can then be
adjusted in half-note steps, as before.
The fisrt preset in the G string chain
(P c '1 ) is adjusted to give the same note
as the last key in the B' string (S C '2>-
Once again, the remaining presets are
tuned to half-note intervals. The open
note of the G string is then used as
reference for tuning the first preset in
the D chain.

Along the way, some further checks are
possible. This is illustrated in figure 10.
Two keys with the same symbols should
produce the same note; the open note
for each string is shown to the right of
the keys. Some octave relationships are
also shown: two symbols in one key
indicates that the note is one octave
higher than that produced by the key
with only one symbol. For example, the
extreme left-hand note key on the upper
string (Sgi) should be one octave lower
than the centre key on the lower string
(Sg').

Those who have access to a frequency
counter may benefit from Table 2. This
lists the correct frequencies for all keys,
with the instrument set to the highest
octave (S2 and S3 closed, S4 open). As
before, the presets in each chain must
be adjusted from 'high' to 'low': from
top to bottom in the Table, in other

Play

Those who can play the guitar should
have no difficulty 'picking up' this
instrument: it is meant for them! As an

that sounds like .

rch 1980-3-13

There's no accounting for taste, so they
say. The Elektor piano seems to be an
exception: several readers have called
us to account for the fact that it didn't
sound the way it should. Perhaps this
was only to be expected: we were
quite satisfied when it sounded like
other electronic pianos - but most
people want it to sound like the real
thing.

We have listened to our readers' com-
ments; we have listened to the piano
— modified it — and listened to it again.
To be honest, we tried several modifi-
cations and rejected most of them
because they were either too extensive

R4, R5, RIO, R11, R16, R17, R22,
R23, R28 and R29 are deleted.
Modifying the boards is a fairly simple
matter, especially since all changes
are clearly shown in figures 3 and 4.
First, the octave board (figure 3). PI
and R37 can be removed (or left in, if
you like — it doesn't make any differ-
ence) and a wire link is added between
the wiper connection of PI and the
right-hand end of R37. This connects
the original output rail (and the original
output connection) to supply common.
The output is now taken from the -J1
connection at the upper right-hand edge
in the component layout in figure 3.

a piano that
sounds like a piano

a minor change for a major improvement

Good news for those who have
built the Elektor piano! A few
simple changes and one or two
additional components make it
sound much more 'realistic'. Less
like an electronic piano and more
like the real thing.

or just not worth the effort. Now,
finally, we've found what we were
looking for: a few simple changes that
make all the difference.

Operation: piano

The proposed modifications affect both
the octave boards (figure 7 in the
original article, modified as shown in
figure 1 here) and the filter circuit
(figure 13 in the original, figure 2 here).
On comparing the new version of the
keyboard circuit with the original, the
differences will be obvious. The original
output (the common rail linking R25 to
R36) is now connected to supply
common; this makes PI and R37
redundant, so they can be omitted.
Obviously, it is a good idea to have
an output somewhere. This is achieved
by adding a preset potentiometer
between the commoned collectors of
T1 . . .T12 and the -13 V rail (Ui).
Effectively, in other words, the output
is moved from one end of the re-
sistor/electronic switch/transistor groups
to the other. That's all for the octave
circuits.

In each of the five input lines to the
filter circuit (figure 2), electrolytic
capacitors are added (C47...C51).
Furthermore, four capacitor values are
changed: Cl now becomes 6n8, C2
becomes 27 n, C3 is now 47 n and C4
is also changed to 47 n. At the outputs
of opamps A1 . . . A5, several com-
ponents are removed: diodes D1 . . . D5
are replaced by wire links, and resistors

The left-hand '— UI' connection retains
its original function. For this modifi-
cation, the copper track beside pin 7 of
IC3 must be cut. The easiest way is to
make two parallel cuts in the track,
using a sharp knife, the track between
these cuts is then heated with a soldering
iron, after which it can be lifted off the
board with the tip of the knife. Further-
more, the wire link immediately to the
right of IC1 and that to the left of IC2
are removed. The IC1 wire link is
replaced by preset potentiometer P9;
note that the wiper is connected to one
end. Finally, the — UI connection to
IC2 and IC3 must be restored. Using
insulated wire, the -UI connection
is linked to pin 7 of IC2 and pin 7 of
IC3, as shown. Note that the
connection to the next board may also
have to be rewired - if it was taken
from the upper right-hand edge of this
board.

The filter board is even easier. Diodes
D1 . . . D5 are replaced (or bridged) by
wire links. Resistors R4, R5, RIO, R11,
R16, R17, R22, R23, R28 and R29
are removed. Capacitors Cl . . . C4 are
replaced, using the new values given
above. Finally, electrolytics are included
in series with the inputs — the neatest
way to do this is by breaking the copper
tracks near the inputs, as shown, and
mounting the capacitors across the gap.
That just about wraps it up. The only
remaining point is the adjustment
procedure for the five new potentio-
meters (P9). Initially, they are set to
minimum resistance; then they are
turned up to the point where even

hitting eight or ten keys within the same
octave, hard, still doesn't cause audible
distortion. Not that it will sound 'nice',
obviously, but the difference between a
dischord and distortion should be
obvious. Having adjusted each octave
board in this way, the 'softest' of them
is taken as a reference, and all others
are turned back to the same level.
If one has access to an oscilloscope, a
simpler adjustment procedure can be
used: each preset is set so that the cor-
responding output of opamps A1 . . . A5
is 500 mV peak to peak when one key
in each octave is hit.

Finally, if necessary, the filter circuits
can be readjusted by means of PI . . . P4.

In conclusion

Is everybody happy? We hope so.
Should you feel that there aren't enough
even harmonics, let us know. We've got
a solution for that one, too — even
though no-one has complained yet.
Maybe we'll include it in the Summer
Circuits issue. Be warned: it involves a

'Master tone generator' and ' Electronic
piano', Elektor, September 1978.
'Extending the Elektor piano', Elektor,
December 1978.

'Missing Link', Elektor, October and
December 1978.

elektc

1-3-17

or march 1 980

inrofipdi

the Chorosynth is . . .

an inexpensive keyboard instrument that anyone can play, a mini
synthesiser for use as a second keyboard by the working musician, or
anything in between.

The Chorosynth is a real instrument with an amazing variety of sound
possibilities at its disposal. The winning design of our competition, it is,
as the name suggests, a synthesiser with chorus effect. Although the
prototype was designed with a keyboard, we have made it possible to
'play now and buy your keyboard later'.

The dynamic range of the instrument
needs to be between 1 50 and 3000 Hz
at least, before it can be played. This
corresponds to the length of organ pipes
ranging from 16' to 2' (feet). Further-
more, the bass notes must be rich in
harmonic tones which can be filtered as
required. Now a tune played on only a
few notes, however rich in harmony,
sounds as though something is missing.
However, when the same melody is
played with fifths a much richer and
therefore more musical sound is
produced. If the tune is played using
several sound sources in unison (tuned
to virtually the same frequency), it
sounds quite different and this in fact
produces the chorus effect. A low
frequency modulation (vibrato) is also
particularly useful when simulating the
sound of stringed instruments.

What does the Chorosynth sound like?
As always the description of any specific
sound is extremely difficult, however
we are sure that the reader will be more
than satisfied with the overall sound
quality of the Chorosynth. It is signifi-
cant that the prototype has been used at
live performances with great success.

march 1980

the chorosyr

The Chorosynth uses a 2 Vi octave
keyboard and has a dynamic range of
6 octaves. Because of this broad fre-
quency range, instruments ranging from
violin to cello and flute to clarinet can
be imitated. It is also possible to adapt
the dynamics of the output signal with
an envelope generator that can be
switched to AR (attack -release) or ASR
(attack-sustain-release) envelopes.

Block diagram

The block diagram of the Chorosynth is
shown in figure 1 . The 2 Vi octave
keyboard produces a KOV (keyboard
output voltage) which controls four
VCOs (voltage controlled oscillators).
VCOs 1 . . . 3 have the same voltage to
frequency characteristics and are tuned
to the same pitch. The fourth oscillator
(VC0 4) is tuned to a frequency 1%
times higher, in other words it produces
a note which is a fifth higher than that
of the other VCOs. A fifth oscillator
(marked vibrato in the block diagram)
produces a relatively low frequency
signal which is fed to the modulation
inputs of the VCOs. The VCO output
signals will then be slightly frequency

modulated to produce a vibrato effect.
The frequency as well as the modulation
depth can be varied.

The output of the VCOs are divided by
2, 4, 8 and 16. The dividers' outputs are
exactly one octave apart and can be
selected with the aid of 'stops' (or
switches if you are not into organs!).

The output signals of the dividers and
pulse shapers of VCOs 1 and 2 are
added together and mixed with those of
VCO 3 via the chorus switch. In this
way the chorus effect can be switched
on or off as required. The pulse shapers
are simply a set of NAND gates which
produce a square-wave with a duty-cycle
of 25% from the divider outputs. It is
this type of waveform that is particu-
larly suitable for simulating the sound
of stringed instruments. Of course, some
filtering of the signal is required before
it is acceptable to the human ear. The
Chorosynth has two filters, one for
strings and one for woodwinds.

A gate-pulse is derived from the KOV
which is used to trigger the envelope
generator. This can provide one of two
envelope patterns - attack -release or
attack-sustain-release. This envelope
controls the VCA (voltage controlled

amplifier) before being fed to an exter-
nal amplifier.

Circuit diagram

The current source and voltage divider
chain for the keyboard (shown in figure
2) provide each key with a specific
voltage level. This keyboard output
voltage (KOV) is then passed to the non
inverting input of op-amp A5 in the
main circuit diagram (figures 3a and 3b).
This can be done by means of the
'printed circuit keyboard' (figure 4) and
a stylus; alternatively, a conventional
keyboard can be used.

The FET Til functions as a switch to
'sample-and-hold’ the keyboard voltage
(in Cl). The rate of change of the
voltage level at the non-inverting input
of IC1 is controlled by P2 and Cl
allowing a glissando effect between

The four VCOs in the block diagram are
the circuits around IC4 to IC7. They are
555 timers which have been wired as
astable multivibrators. ICs 4, 5 and 6
(VCOs 1, 2 and 3 in figure 1) are tuned
to the same frequency and therefore
have similar component values. IC7

(VC0 4) is tuned to a frequency V/i of the 4520 have been used. Thus it is The woodwind filters are active lowpass

times greater (that is a fifth higher) only divided by 2 or 4; in other words, elements with a turnover frequency of

which accounts for C5 having a lower only a T or 2' organpipe can be ob- 2 kHz (for 16', 8' and 4') and 4.5 kHz

value. Each VCO has a modulation tained. The outputs of the two dividers (for the top three registers, 2', 2 2/3',

input (pin 5) controlled by the vibrato in IC9 are coupled and then mixed with and 1 1/3'). The lower registers thus

oscillator circuit - ICs 2 and 3. The the outputs of the divider for VCO 3 have a greater proportion of higher

frequency of the vibrato is varied by P3 (IC6) via the 'chorus' switches SI to S4. harmonics, which improves the musical

and the modulation depth is controlled This provides a seperate chorus effect tone.

by P4. The potentiometer P5 introduces for each register.

an offset to all the VCOs which in Switches S5 to SI 4 serve as stops.

practice allows the pitch of the Depending on which of the stops is Gate pulse

Chorosynth to be fine tuned. closed, the signal reaches either a The gate pulse is derived from the KOV

The output of VCOs 1 , 2 and 3 are each woodwind or a strings filter. The filter via A5, A7 and A8 and its purpose is to
divided by a 4520 producing frequencies circuits are comparatively simple. Passive trigger the envelope (AR/ASR) circuit,
which are 2, 4, 8 and 16 times lower highpass filters whose top-end response The type of envelope contour is selected
than the VCO frequencies. As far as is slightly rolled off by capacitors C28 by switch SI 5. With this switch in the
VCO 4 is concerned, only two outputs and C30 provide the voicing for strings. AR position, the positive going edge of

the gate signal triggers the flip-flop
formed by NAND gates N10/N11,
turning on T7 and charging capacitor
C36 via the attack control, P10. As soon
as the voltage on C36 reaches approxi-
mately 13.5 V, T10 turns off and the
flip-flop is reset. The capacitor then
starts to discharge via the release control,
P1 1, and transistor T8.

With the ASR envelope selected, the
flip-flop remains set as long as the gate
signal is present, that is as long as a note
is held (sustained) on the keyboard.
Only when the key is released can T10
reset the flip-flop and C36 discharge
(release).

The output of the envelope shaper

circuit controls a simple VCA (voltage
controlled amplifier), which in turn
determines the dynamic amplitude
characteristics of the output signal. The
VCA consists of an op-amp (A4), with a
FET (voltage controlled resistor) con-
nected in the feedback loop.

Construction and setting up

The printed circuit board for the
Chorosynth (figure 4) includes all the
components and a keyboard layout. The
keyboard has been included for those
readers who wish to keep the cost down
to a minimum. It is a simple matter to
connect a conventional keyboard, one

set of make contacts per key is all that
is required.

It should be noted that since the
oscillators have a linear voltage-
frequency characteristic, the keyboard
tuning resistors must form a logarithmic
potential divider. The appropriate values
(for example R86a and R86b) are all
made up using resistors from the E24
series. With 1% tolerance resistors, a
tuning accuracy of 1% of a semitone is
obtained, however (with the exception
of R115a and R1 15b where 1% must be
used) 5% resistors will also prove
suitable, since the chorus effect by and
large obscures any slight mistuning.
Inductors LI to L4 are ferrite beads

5

1980-3-23

Resistors:

R1 = 10 M

R2,R3,R6.R10.R21 . . . R43,
R64.R68.R74.R75.R82,
R84 - 10k
R4.R73 = 100 k
R5.R72 ■ 5k6
R7.R77.R78 * 1 M
R8,R9,R19,R20.R66,R70.
R80 - 47 k

R11.R53... R62 = 22 k

R12 . . . R15 = 18k

R16 = 15k

R17-4k7

R18 = 82 k

R44 . . . R52 - 1k8

R63.R67 - 39 k

R65.R69 = 6k8

R71 ,R97B,R1 128 * 330 ft

R76 ■ 100 !1

R85.R105B '220S7
R86A.R90A = 6fl2
R86B.R101B > 560 U
R87A = 6S28
R87B = 150 n
R88A.R89A = 10 n
R88B.R108 = 22 SI
R89B.R1 10A.R1 1 1 A,

R1 13A = 27 SI
R90B.R94B.R106B = 1S25
R91.R94A.R109B = 802
R92A = 901
R92B.R103B = 180 O
R93A.R96A.R97A.R99A.

R100A= 12 0
R93B = 39 O
R95A.R98A.R101A,

R109A = 15 0
R95B.R1 14A = 33 O
R96B = 1200
R98B = 68 O
R99B = 1 O
R100B = 108
R102A = 16 0

R102B.R104B.R114B = 470 0
R103A.R104A.R106A,

R107A = 180
R105A.R1 12A = 20 0
R107B = 207
R110B » 270 0
R 1 1 1 8 = 680 O
R113B ■ 202
R1158 - 120k
R1 15A = 1k2 1%

Potentiometers:

PI = 500 k (470 k) preset

P2 = 1 M lin

P3 = 22 k (in

P4.P5- 10 k lin

P6 . . . P9 = 25 k presets

P10.P11 = 500 k (470 k) log

Capacitors:

Cl = 470 n
C2.D3.C4 = 3n3
C5 ■= 2n2
C6 . . . C9.

C18 . . . C26 “ 10 p/35 V
tantalum

CIO . . . C13.C17.C38 . . . C45,
C50.C51 .C53 = 100 n
C14 = 33 n

C15.C36 = 4p7/35 V tantalum

Cl 6= 2p2/35V tantalum

C27.C28.C31 ,C52 ■ 10 n

C29.C30.C35.C37 = 47 n

C32 ■ 12 n

C33 - 22 n

C34 = 27 n

C46 ■ 1000 p/35 V

C47 = 330 m/35 V

C48.C49 - 330 n

Semiconductors:

D1.02.D3.D4 = DUS
D5.D6 = 1N4001
T1 . . . T5.T8.T10.

T13 = BC177B. BC557
T6.T7.T12 = BC 107B, BC 547
T9.T1 1 = 2N3819, BF 2568
IC1 = CA 3140, LF 356
IC2.IC4 . . . IC7 = NE 555
IC3 - 741

IC8.IC9 = CD 4520. MC 14520
IC10 . . . IC12.IC18 = CD 401 1
IC13.IC14 = TL084
IC15 = 78L15
IC16= 78L12
IC17 = 79L15

Miscellaneous:

Trl = 16 V/150 mA transformer
S1.S5 . . S15= SPDT
S2 . . . S4 * DPDT
LI . . . L4 = 5 turns of 0.2 mm

Technical details:

Range: C to c 1

Keyboard: 214 octaves

Registers: Cello (16')

Bassoon (16')

Viola ( 8’)

Clarinet ( 8')

Violin ( 4')

Clarinet ( 4’)

Violina ( 2')

Flute ( 2')

Twelfth (2 2/3')
Larigot (1 1/3')
Effects: Chorus (16')

Chorus ( 8')

Chorus ( 4')

Chorus ( 2')

65.41 Hz to
4184 Hz

510
S14
S9
S13
S8
S12
S7

511

S4

S3

S2

SI

Additional controls:

Glissando: (Portamento) P2

Vibrato: Depth and rate P8 and P9

shaper: Attack-release or attack-sustain

release selectable by SI 5; attack
and release times independently
variable between 1 mSand 10 S
(P10 and P1 1)

with five turns of 0.2 mm enamelled
copper wire. All other components are
readily obtainable.

Since the Chorosynth has a large number
of potentiometers, controlling it may at
first be a little difficult. It is advisable to
set all the controls to the following
positions at the beginning of the setting
up procedure:

Pi
P2
P3

P6

P7 . . P10
P11

P12...P13

SI . . . S4
S5.S6

57

58 . . . S14

51 5

The VCA has two adjustment points,
PI 2 and PI 3. PI 3 determines the
minimum gain and is adjusted such that
no output signal is audible. A note is
then 'struck' and held, whilst PI 2 is
adjusted until a slowly rising signal
appears at the output of A4. PI is then
adjusted so that no change in frequency
occurs when a key is struck.

In order to trim the keyboard, PI 4 is
adjusted to give 8.43 V across R 1 1 5.
This should be measured with a universal
meter of at least 10 kii/V. With the aid
of P8, the third VCO (IC6) is adjusted,
so that when the key to the far left of
the keyboard is operated, a C2 note can
be heard at the output. The pitch can be
compared with that of another instru-
ment, or the frequency can be measured
(523.2 Hz).

52 is then closed and, by using P7, the
frequency of IC5 is trimmed to a value
which should correspond as much as
possible with that of IC6 — minimum
beat note in the output signal. At the
same time P6 must be trimmed so that
there is very little vibrato at the output.
It should now sound like a chorus.
Finally, P9 is adjusted so that the
output frequency of IC7 (VCO 4) is 1 Vi
times that of the other VCOs. The
Chorosynth should now be ready to
play. Under 'technical details' the stops
and their values are indicated.

Although this may be the end of the
Chorosynth article it certainly is not the
end of the possibilities for this instru-
ment. It will be apparent to many
readers that the Chorosynth offers
considerable scope for many modifi-
cations. If you find one that you would
like to share, we would like to know
about it. M

esistance, fully
esistance, fully

antl-clockwisa.
mid -position

minimum resistance, fully

wiper to earth, fully
clockwise.

switched to ground
switched to ground

for smooth, continuous projection

Enthusiastic amateur
photographers and professionals
are not only interested in making
good pictures: they also want to
present them properly. For a slide
show, the professional 'trick' of
using two projectors alternately is
becoming increasingly popular —
the 'black' gap between successive
slides can be eliminated in this
way. For the finishing touch, each
following slide should be gradually
blended into the existing picture.
This is where the dual slide fader
comes in: as the brightness of one
projector is smoothly increased,
the other is gradually faded out.

A complete dual projection installation
consists of two slide projectors and a
control unit. Complete, integrated units
also exist — everything built into one
case — but it is more common to use a
separate control unit with two standard
projectors. Most commercially available
units have one disadvantage in common:
they are expensive. The reason for this
is that they are usually designed for
maximum flexibility, with all kinds of
additional 'features': automatic fading,
tape-slide synchronisation, so-called
'sparkling' effects and so on.

However, if one reduces the number of
applications and features, aiming at a
manually operated dual fader, it is
possible to come up with a simple and
cheap design. The 'fade' from one
projector to the other is done manually,
by means of a stereo potentiometer ;
slide changing is also done by operating
the two projectors in the normal manner,
it is not done automatically by the fader

Figure 1 gives a drastically simplified
block diagram. It should be noted that
there are two types of projection lamp:
one operates at the full mains voltage,
whereas the other (more modern) type
uses a much lower voltage (24 V). In
this article, two circuits will be described

for this reason: however, the block
diagram is equally valid for both.

Mains voltage control

The circuit given in figure 2 works in
the same way as a normal triac mains
dimmer. For older projectors, using a
mains-voltage lamp, this circuit is ideal.
It can also be used for more modern
projectors that use a 24 V lamp, varying
the primary voltage to the lamp
transformer in the projector. This is not
an ideal solution, however, since a mains
transformer is not designed to run on
the type of distorted waveform that a
triac control unit produces. The ratio
between primary and secondary voltage
is not constant over the full control
range, making the control characteristic
rather less smooth than one would like.
To sum it up: if you have older
projectors, using mains-voltage lamps,
read on; if you have modern projectors
with 24 V lamps, skip this section . . .

The circuit itself is quite straightforward.
Two standard triac control circuits are
used, with the control potentiometers
combined into one stereo potentiometer
(P2). Note that a tandem 470 k linear
potentiometer should be used. Both

control circuits can handle any load up
to 400 W. 1

One control circuit can be mounted on
the printed circuit board given in
figure 3. For a complete unit, two of
these boards are therefore required. The
interference suppression coils (LI and
L2) must be able to handle a current of
up to 2 A; this type of coil is available
ready-wound. The two boards, two coils
and the stereo potentiometer can all be
mounted in a suitable plastic case.

Note that the whole circuit is connected
to the mains! This means that all due
care must be taken when constructing
the unit. In particular, an insulating case
must be used and the potentiometer
should have a plastic spindle. The
capacitors must also be suitable for the
relatively high voltages present in the Figure
circuit. are cor

If you have reached this point, you are
apparently interested in the mains
voltage version. You may therefore pass
over the following section (which deals
with the other circuit) and proceed to
'construction and use'!

Low voltage control

The circuit given in figure 4 is intended
for controlling the low-voltage supply to
a 24 V lamp. In this case, the two triacs
are mounted inside the slide projectors
and the two coupled control circuits are
mounted on a single printed circuit
board in the control box.

As before, the circuit consists of two
complete triac control circuits, linked
by means of a stereo potentiometer.
The circuits are rather more sophisticated
than those used in figure 2, since a
much lower voltage must be regulated
smoothly. Since both control circuits in
figure 4 are identical, we will confine
the explanation to the upper half.

Trigger pulses for the triac Tri 1 are
produced by the well-known timer 1C,
type 555. In this circuit, it is used as a
monostable multivibrator; it is triggered
each time the voltage at pin 2 decreases
to below approximately one-third of the
supply voltage. Since this voltage is
actually the full-wave rectified output
from a bridge rectifier (B1), the 1C will
be triggered once during each half-cycle
of the mains waveform.

When IC1 is triggered, its output (pin 3)
goes 'high' - to the full supply voltage
- and pin 7 (the 'discharge output')

goes basically open-circuit. Capacitor C2
now starts to charge through P2a and
R2. When the voltage across C2 reaches
approximately two-thirds of the supply
voltage, the 1C switches back to the
other state: pin 3 goes 'low' and pin 7 is
also connected to supply common —
discharging C2 rapidly. The circuit will
remain in this state until the next trigger
pulse occurs at pin 2.

When the output of IC1 goes low,
transistor T1 cuts off. The voltage at the
collector of T1 swings positive, triggering
the triac (provided SI is in the position
shown). The triac will now start to
conduct, until the following zero-
crossing of the mains waveform. Since
the moment that the output of IC1 goes
'low' depends on the setting of P2a, this
potentiometer effectively controls the
point on the mains waveform where the
triac starts to conduct. This, in turn,
determines the brightness of the
projection lamp, Lai .

The other half of the same poten-
tiometer (P2b) controls the brightness
of the second projector. Provided a
linear potentiometer is used, wired
exactly as shown, the result is a smooth
fade from one projector to the other.

Construction and use

As far as the construction of the unit is
concerned, the main points for the mains
voltage version have already been
mentioned: adequate electrical insu-
lation and sufficiently high voltage
rating for the capacitors.

For the low voltage control unit, there
are a few more points to note. The bulk
of the circuit is mounted on the board
given in figure 5; this, and the mains
transformer (Tri) can be mounted
in a small case. A miniature 12 V
transformer can be used, as the circuit
only draws a few milliamps. The triacs,
switches SI and S2 and resistors R*i
and R x 2 are a " mounted in the
projectors; transformers Tr2 en Tr3 are
the existing transformers in the
projectors.

The switches are included so that the
projectors can also be used on their own
in the normal way. With SI , for instance,
in position 'a' the corresponding triac
will conduct during the full mains cycle
— the projector will continue to work as
if nothing had been modified. When the
switch is in position 'b' the projector
can be controlled by the slide fader unit.
Quite a high current is required for the
lamps (a 250 W lamp draws more than
10 A), and since some 1.2 V remains
across the triac even when it is fully 'on'
the power dissipation can be over 1 2 W.
For this reason, each triac must be
mounted on a fair-sized heatsink; if it is
possible to locate this at some point
where it is cooled by the fan in the
projector, so much the better. When
mounting the triacs, it should be noted
that 'main terminal 2' (mt2, the cathode)
is connected to the metal part of its

When it comes to actually wiring up
either of the control units to a given
pair of projectors, some electrical

.R3.R6 = 12 k
,R5 ■ 560 12
,R7,R x1 ,R x2 = 330n

niconductors:

,IC2 - 555
T2 = TUN, BC 107
= 1 N4002
= 4 x 1N4002
1,Tri2 “ TIC 226

- 1 2 V/100 mA mains
ansformer

S2 - single-pole switch

peculiarities are likely to be found.
Since virtually every projector is
different, it is impossible to discuss all
possibilities in detail. However, the
following points may be a help:

• Nearly all projectors contain a fan. In
most cases, this is connected to the

mains; sometimes it runs off the 110 V
tap on the lamp transformer. It may
also be found that the fan motor
actually doubles as mains transformer!

• A remote-control slide-change mech-
anism is usually driven (via belt or

gear wheels) by the fan motor, although
more expensive projectors may have a
separate motor for the job. When a slide
is to be changed, a mechanical coupling
is operated by an electromagnet; the
latter is often powered from the 24 V
winding on the lamp transformer. In
some cases, a simple DC supply may be
used for the magnet.

• Many projectors also incorporate
some kind of 'back-one-step' slide

change. This will also usually involve
some electromagnetic mechanism that
is powered from the low-voltage supply.

• Remote-control or automatic
focussing Cautofocus’) and/or timer

mechanisms usually operate off a full-
wave rectified 24 V supply. In some
cases, an additional 12 V supply may be
found for these features.

• In some slide projectors, it is possible
to switch the lamp to a lower voltage

- giving less light, but extending the
lamp life considerably. Usually, a lower-
voltage tap on the transformer is used

It will be apparent from the points
listed above that installing the mains
voltage control unit, in particular, may
call for quite extensive rewiring of the
projector. If the lamp runs off the raw
mains voltage, well and good — that's
what this unit is designed for. If the
projector uses a low-voltage lamp,
however, things get complicated. Every-
thing except the lamp must be discon-
nected from the transformer and
powered from a separate low-voltage
supply. After all, the idea is that slides
are to be changed when the lamp is off
— and at that time, the transformer is
virtually disconnected from the mains!
Installing the low-voltage control unit
will rarely be a problem. In nearly all
cases, at least one wire will be found
that runs direct from the transformer to
one side of the lamp (and to nothing
else) - the other lamp lead may run
through some switches, making it more
difficult to trace. The triac can be
connected in series with the direct
connection to the lamp. H

microprocessors

The matrix printer shown in the photo
the Matsushita EUY-IOE-IOT. This is
so-called electro-sensitive printer, that
prints the characters (letters, numerals
symbols) on a special type of paper.
This paper is coated with a very thin
(0.1 micron) layer of aluminium; im-
mediately below this is an almost black
surface. If the aluminium is scratched
off, this dark layer becomes visible; it
is possible to 'write' on it in this way,
using a sharp point.

The matrix printer does its job in a
more elegant way. A miniature motor
moves a 'print head' across the paper, as
each line is printed. The same motor
also takes care of the paper transport
for 'line feed'. The print head contains
a vertical row of seven little 'pins';
these can be connected (via electronic
switches) to supply common. The pins
are in contact with the aluminium
coating on the paper. This electrically
conducting layer is connected to a
positive voltage (27 V). When one of
the pins is connected to supply common,
a current of a few hundred milliamps
flows through the aluminium coating
and the printer pin. At the point where
the latter touches the paper, a high
current density results. The heat pro-
duced is sufficient to burn away the
aluminium layer at that point, leaving
a small black dot. Since more than
one of the pins can be connected to
supply common simultaneously, it is
possible to write up to seven dots in
a vertical row at any time.

If voltage is now applied to the motor,
the print head will move across the
paper - writing up to seven horizontal
lines, depending on the number of
pins that are activated. At the end of
the line, the printer head is automati-
cally moved back to the beginning of
the next line: 'line feed' and 'carriage
return'.

a black-and-white display

It can be useful to print the information output from a microprocessor
system on paper. For small pP systems that give a hexadecimal output,
a so-called matrix printer is a good choice. This is connected to the
microcomputer system via a suitable interface.

In this article, a printer and interface for the SC/MP system are
discussed, as well as the necessary software. A 'disassembler' program
is also included - an invaluable aid when analysing existing programs.
The printer and interface are suitable for other microprocessor systems
as well; however, the software must then be modified, of course.

As the head moves across the paper, it
is obviously possible to switch the pins
on and off, producing dotted lines. In
this way, all sorts of characters can be
printed as groups of dots: the trick is
to switch the pins on and off at exactly
the right moments to produce the
the desired character. As shown in
figure 1, each character is built up in
a 35-dot matrix: five successive vertical
columns (T1...T5), each consisting
of seven dot positions (N1...N7,
corresponding to the seven pins).

A character generator switches the
printer pins on and off at the correct
moments. This unit is part of the
electronics that link the SC/MP system
to the printer: the printer interface.
This interface circuit will be discussed
later.

The printer contains a reed switch to

1980-3-29

Figure 1. Using a 5 x 7 dot m

indicate the end of a line, and the
moment that the print head has returned
to the beginning of the next line.
Furthermore, a 'sync generator' is
included, intended for synchronising the
character generator to the position of
the printer head on the line. This
'generator' consists of a gear-wheel,
driven by the motor in the printer and
moving past the core of a coil. The core
is magnetised, by means of a permanent
magnet; as the teeth of the gear-wheel
pass through the magnetic field, a
sinusoidal voltage is included in the coil.
The interface described here doesn't do
the actual synchronisation itself; instead,
the software is designed to ensure that
the column spacing is maintained

proportional to the speed of the printer
head.

So much for the printer itself. On its
own, however, without the interface
circuit and corresponding software, it is
as useless as a typewriter without a
typist. Which brings us to the next
section:

The interface

A block diagram of the interface circuit
is given in figure 2. The buffer memory
(8-bit latch) is basically equivalent to a
single RAM location. When the address
decoder provides a 'store' pulse, the
information present on the data bus is
stored in this latch and passed (via
suitable power drivers) to the printer

pins. Each 'store' pulse therefore enters
the data for a single column of dots.

As mentioned earlier, the characters are
printed one column at a time - five for
each character. For each column, seven
data bits are required ('stored' at an
address in the field F200 . . . F3FF);the
eighth bit is used for motor control, via
a separate buffer stage: it ensures that
the motor starts at the correct moment
at the beginning of a line, and stops
immediately after returning to the
beginning of the next line.

The address decoder is designed for use
with 'A K address fields. The EPROM
with the firmware for SC/MP system
control and character generation is
located between addresses F000 and
F1FF. The following 14 K is used for
addressing the 8-bit latch; then another
'A K (from F400 to F5FF) correspond
to a tri-state buffer that outputs the
data from the reed switch and sync
generator in the printer to the data bus.
The fourth address field (F600 . . . F7FF)
can be used for an additional EPROM.
This could be used, for instance, to
store firmware that makes the printer
routine independent of Elbug in the
original SC/MP system — the push and
pull routines, in particular. Alternatively,
a special printer program could be
located here. All in all, the complete
interface uses a 2 K address field - half
a page, in other words.

As mentioned earlier, the firmware
takes care of character generation,
including correct column spacing as
detertnined by the speed of the printer
head. Furthermore, it ensures that the
text is printed in lines, each containing
8, 16 or 32 characters, as required. The
SC/MP system must supply the data to
be printed, one complete line at a time.
To this end, the text must be loaded in
ASCII code in a specified RAM area.
Any available area can be used: as long
as the interface is told where to look for
it!

The character generator, stored as
firmware, can generate 64 different
characters. Only the lower 6 bits of the
ASCII code in each RAM location are
needed to select the correct character.
The two remaining (upper) bits are used
to define the desired character size,
corresponding to 8, 16 or 24 characters
per line. As an example, the letter A in
6-bit ASCII code corresponds to 01.
When the two upper bits are both 0
(complete code: 01) the smallest
character size will be printed, corre-
sponding to 32 characters per line.
However, if one of the two upper bits is
'1', larger characters are obtained: '41'
for an A corresponds to 16 characters
per line, and '81' makes the A even
bigger, for 8 characters per line.

When the firmware routine is started, it
will want to know where to look in
RAM for the text buffer that contains
the data for one line of text. It will then
scan the (hexadecimal) codes for the
characters, as found in the text buffer,
and convert them (by means of a

2

Figure 2. Block diagram of the printer interface.

3-30 — elektor march 1980

'look-up table') from ASCII code to
printer pin control data. The routine
transfers the data for the five columns,
in turn, to the latch; the character is
printed.

Each character is automatically followed
by a two-column space, after which the
next character is retrieved from RAM
and printed. This process is repeated
until the end of the line (up to 32
characters). As each line is printed, the
reed switch is 'open'; via N6, data bus
line 00 is therefore 'high'. At the end of
the line, the reed switch closes and the
corresponding data line goes 'low'. This
causes the program to load the text
buffer, with the data for the next line;
however, the printer head will not have
got back to the beginning of the new
line yet. The program continues to scan
the data line, until the head is correctly
positioned and the pins are again in
contact with the paper. At that point,
the reed switch opens and the next line
can be printed.

Further details on the use of the text
buffer and printer routine will be given
when we come to the software.

The circuit

The complete circuit is given in figure 3.
IC1 is the address decoder. It is connec-
ted to the upper seven address lines
(making FFFF the highest address) and
to the NWDS + NRDS line. Basically,
this 1C is a 3-to-8 decoder, but only half
of it is used - as a 3-to-4 decoder.
Output YO selects an EPROM, IC2,
from address F000. This EPROM is also
connected to the lower nine address
lines, so that 'A K addresses are available.
The output from the EPROM goes
straight onto the data bus.

Outputs Y1 . . . Y3 of IC1 correspond
to three further 'A K address fields:
F200 to F400, F400 to F 600 and F600
to F800. When output Y1 is active (and
NWDS), the character information on
the data bus is passed via IC6 to IC7.

printer for microprocessors

The latter contains seven power drivers
that can easily supply the current
required for the printer pins. R8 and C6
are included to ensure that IC6 is
cleared when power is first applied
('power-on reset'). The eighth output
from IC6 is used to start and stop the
motor, via T1...T4. This circuit is
designed to introduce a slight voltage
drop, since the motor is intended to run
on 24 V. When the motor is to be
stopped, it is actually shorted by T4,
making for a more rapid stop.

Output Y2 of IC1 enables the tri-state
buffers, N5 and N6, when the third
address field is selected (from F400).
These buffers pass the information from
the reed switch and sync coil in the
printer to the data bus.

The power supply for the printer is a
standard circuit, as shown in figure 4.
The output voltage can be adjusted
(by means of PI) in the range from
20 ... 30 V. This gives some measure of
'copy darkness' control. A slightly

3-32 - elektc

Eurocard size, and fitted with a 64-pin
connector so that it can be plugged
straight into the main bus in the existing
SC/MP system.

A 14-pin DIL connector at the other
short edge of the board is used for the
connections to the printer. The printer
itself uses a 15-pin connector. The
connections between the latter and the
14-pin connector on the p.c. board are
listed in Table 1; for a neat appearance,
it is a good idea to use so-called flat
cable.

Since the power supply circuit is on the
p.c. board, two connections for the
mains transformer (24 V secondary) are
also provided.

The complete unit can be plugged into
the SC/MP system, without affecting its
operation in any way. There is, however,
one important point to note: the metal
case of the printer and the paper are
both connected to the +27 V supply, so
they must be kept well away from
supply common. That includes, for
instance, the metal case of the existing
SC/MP system . . .

The software

Without software, the printer is useless.
The basic principle of the printer routine
was mentioned earlier; a complete
listing is given in Table 2. An extremely
useful extension is given in Table 4: a
disassembler program. An interesting
point: both of these Tables were actu-
ally printed with the aid of these two
routinesl

The 'instructions for use' of this
software are best broken down into a
few groups.

Motor control

The latch in the interface can be ad-
dressed as F200. Storing data there
(by means of the instruction MOD F200)
with a value between 80 and FF starts
the motor and causes up to seven
horizontal lines to be printed.

The motor can be stopped by means of
the NRST key, or by storing data
between 00 and 7F in the latch.

Printer routine

As mentioned earlier, the text is printed
one complete line at a time; all data for
the line must be available in a text
buffer (somewhere in RAM) before
starting the printer. The location of this
text buffer can be specified as required,
provided the printer routine is started at
address F000 (see Table 2). Alterna-
tively, the routine can be started at
address F00F; in this case, address 0F00
is automatically selected as the first
address of the text buffer — in other
words the ASCII character codes must
be stored from 0F00 on, so that the
interface can retrieve them from there
as the line is being printed.

If the text buffer is to be located
elsewhere in RAM, the first address
must be specified in the TEXTAD
locations: 0FF4 + 0FF5. The printer

Table 1. Interconnections between the DIL connector (on the p.c. board) and the printer.

Table 2. Listing of the printer routine, as stored in EPROM.

ch 1980-3-33

routine is then started at address F000.
It should be noted that the printer
routine uses a further five RAM bytes as
scratch pad for counting; these bytes
correspond to the five locations im-
mediately preceeding the first address of
the text buffer.

Short texts, less than one line in length,
should be concluded by storing FF in
the text buffer. This corresponds to a
space, so that no further (unwanted)
characters are printec. For that matter,
it is good practice to conclude even
complete lines with FF. as otherwise the
printer may add part of a 33rd character
at the end of the line
After loading the text buffer, and the
corresponding first address if necessary,
the next step is to jump to the printer
routine. This can only be done via the
LIFO stack in the Elbug routine. This
has the advantage that Elbug stores the
CPU status in the stack before executing
the printer routine. At the end of this
routine, Elbug returns to the main
program after restoring the original CPU
status'.

The printer routine is started as follows.
The start address of the printer routine
minus one (i.e. FOO0 - 1 = FFFF — same
page! — or F0OF 1 = F00E) is stored
in ROUTAD (Routine address:
0FFC + 0FFD). Then a JS3 (PUSH)
= 0056 - 1 instruction is given. An
example is listed in Table 3; the

* Further details of the Elbug monitor
routines are contained in Elektor,
February 1979, and in the SC /MP book
that will be appearing shortly.

comments given there should give
sufficient explanation. In this example,
0E30 . . . 0E40 is the text buffer; it
contains the text '.SC/MP . . PRINTER.',
where each '.' corresponds to a space.

If the same text buffer locations are
used for the next line, the address
specified in TEXT AD can be left.
Obviously this data must be modified,
however, if a different RAM area is to
be used for the following line. Further-
more, it is possible to initiate further
jumps to the printer routine by means
of the instruction XPPC3, provided the
contents of pointer register P3 are not
modified by the main program.

SC/MP disassembler

A disassembler routine can be used to
analyse an existing program, and print a
'listing' by means of the printer routine.
As mentioned, the 'raw' machine-code
programs for the printer routine and
disassembler routine were converted
into Tables 2 and 4 in this way.

The disassembler first locates PC-relative
jump instructions in the program to be
analysed, calculates the initial addresses
of the corresponding routines and marks
these with a S symbol and a number.
This, in itself, breaks the program up
into small sections — a great help when
studying and trying to analyse an
'unknown' program. The complete
program is printed as a series of addresses
and mnemonics, as shown in the Tables.
The disassembler routine starts at
address 2600; the display then reads
da-d-t-s. There are now three ways of
executing the program: operating the
D key (on the hex keyboard) initiates a
print-out via the printer described above.

elektor marc

TheT key (block transfer) is used for an
output to the Elekterminal (1200 BD at
flag 0). In this case, the halt LED will
light every 16 lines; operating the halt/
reset key initiates the print-out of the
next 16 lines, and so on. Finally,
operating the S (subtract) key gives a
serial output at 110 baud via flag 0; in
this case, the print-out is continuous: it
doesn't stop after each group of 1 6 lines.
In some cases, determined by the
characteristics of the peripheral equip-
ment, the print-out may appear on
alternate lines - in other words, every
other line is left unused. In this case, the
data in address 2516 can be modified
from 3F to 08.

After the desired key (D, T or S) has
been operated, the display will indicate
the type of output that has been
selected. The next step is to enter the
first and last address of the program
that is to be 'disassembled'. If the first
address is x000, the last address should
not be xFFF (due to a peculiarity in the
program . . .); it must be xFFE or less.

If there are tables in the program, key T
(block transfer) is operated, 'ta' now
appears in the display, and the first and
last address of the table can be entered.
More than one table (up to 15) can be
specified in this way, provided they are
entered in the order in which they occur
in the program, A single-byte table can
be specified; furthermore, two or more
tables that occur in a consecutive series
of addresses may be specified separately.
All tables are printed in hex matrix
format. It is even possible to print out a
complete program in this way.

If a program is to be examined in which
the addresses of the tables are unknown,
the disassembler can still be used for an
initial listing. Where tables occur, two
question marks will be printed at the
majority of the addresses, since the data
will rarely correspond to a valid instruc-

Once the tables — if any - have been
specified, the disassembler can be
started by operating any key except T.
It then scans the specified program
(section) twice, locating the various
subroutines on the first pass and printing
during the second.

The disassembler can only handle
programs that are contained on a single
page. Larger programs must be dealt
with one page at a time.

If certain locations are used as 'scratch-
pad' memory, they will be indicated as
'byte' provided the data '55' is stored
there before running the disassembler.
PC-relative jump instructions that
specify an address outside the program
range (lower than the start address, say)
are not permitted.

The disassembler uses memory locations
from 2840 up to 2A40 as scratch pad
for 'lable' addresses: room enough for
255 PC-relative jump instructions.

A useful tip

The EPROM that contains the printer
routine is not strictly necessary. The

printer for

licro processors

elektor march 1980- 3-;

program given in Table 2 can be stored
in RAM in the SC/MP system, say from
address 0C00 to address 0DFF. In this
case, the instruction at address F05A
must be modified accordingly: to C40C
in this example, instead of C4F0. In the
demonstration program given in Table 3,
the routine address will also have to be
modified in this case: at address 0E07,
the data becomes C40B instead of C4FF.
This is because the address of the
printer routine minus 1 is stored at
addresses 24EB and 24EF.

With these modifications, the three
routines can be entered into the SC/MP
and transferred from there to tape. If
the printer routine is to be located
elsewhere, the modifications must
obviously be modified accordingly . . .
Admittedly, this suggestion involves a
lot of laborious load ing-by -hand; but it
saves an EPROM. It is the intention to
include these routines on an ESS
software record — at some future date.

M

3-36 - elektor ■

remote

contra

Remote control units are becoming an almost standard accessory on
colour TV sets. Hifi equipment that can be controlled 'remotely' is
perhaps not commonplace - but it is no longer science fiction. Even
modern slide projectors are going 'wireless'.

Obviously, tripping over wires is to become a thing of the past. Remote
control systems - of the type described in this article - will become
increasingly popular.

elektor march 1980 - 3-37

There may be all kinds of reasons for
wanting to control some item of equip-
ment from a distance. In domestic
surroundings the most important reason,
nine times out of ten, is a desire for
increased 'ease'. There are also cases
where remote control can be quite
useful: adjusting the balance control on
an audio system, for instance — the
optimum setting can only be judged
when you're in the listening position.
From there, you can rarely reach the
amplifier . . . There are even cases where
some kind of remote control is essential.
Model railway locomotives, for instance,
will have to be controlled from a dis-
tance until somebody succeeds in
manufacturing sufficiently miniaturised
engine drivers.

Among remote control systems, the
'wireless' variety are the most useful.
After all, there's not much fun involved
in tripping over cables all the time —
particularly when the other end of the
cable is connected to some expensive
item of equipment.

Whatever the reasons may be for want-
ing a wireless remote control system,
this article gives some suggestions. The
circuits are all intended for indoor use:
they use an ultrasonic link, so that the
range is too limited for model boats and
aeroplanes. Remote control of television
sets, hifi equipment, doors, lights, slide
projectors - that is the kind of appli-
cation where these systems can prove
useful. A further application is in com-
bination with the single-button control
unit described in Elektor, June 1979:
the Monoselektor.

Two systems

We will describe two different systems,
both using an ultrasonic link. The first
system was designed primarily for trans-
mitting sixteen independent 'on/off
type' commands. These command sig-
^ nals may be used to control all kinds of
things: channel selection in a TV set,
switching a lamp on or off, operating an
electric door-opener, and so on. The
sixteen command signals may all be
received and decoded by a single re-
ceiver, but several receivers may also be
^ used. For instance, it might prove useful
to have one receiver for command
signals 1 ... 3, another for signal 4, a
third receiver for signals 5 and 6, and so
on. It is also possible to 'tune' several
receivers to the same command signal.
In the example given above, a fourth
might be added that will
respond to commands 1, 4 and 6. It is
not a good idea, however, to use more

than one transmitter in this system — it
would almost certainly cause confusion
at the receiver end.

The electronics required for this sixteen-
channel system is all of the 'available-
off-the-shelf' variety: CMOS ICs, 555

Not so for the other system: it uses two
special Plessey ICs, one for the trans-
mitter and one for the receiver. This
simplifies the circuits, quite drastically.
The ICs are intended for remote control
of colour TV receivers, but they can
prove equally useful in other appli-
cations. In principle, this system can
transmit 32 'on/off' signals; however,
some of these may also be combined to

1

The first system

First, let's discuss the system that uses
standard components. After all, it was
designed with the home constructor in
mind!

Digital signals are easier to transmit
reliably. 'Digital' means that they can
represent only two conditions: on or
off, yes or no, 'nought' or 'one'. These
digital commands must be transmitted
to the (remote) receiver; as with radio,
this is done by modulating a high-
frequency carrier signal. Two likely
possibilities are illustrated in figure 1 :
Phase Shift Keying (PSK) and Frequency
Shift Keying (FSK). Both have their

Figure 1 . Two possible weys of modulating a digital signal on an ultrasonic carrier, 'e is th<
original digital signal; V illustrates the principle o* Phase Shift Keying (PSK); 'c' is an exar
o* Frequency Shift Keying (FSK). For PSK, the cerrier frequency is unaltered - but its ph
shifted 180 at every zero-crossing in the digitel signal; FSK, on the other hand, changes th
carrier frequency.

j* j imiu @H

Figure 2. Block diegram of the transmitter.

transmit 'analogue' control signals. The
latter can be useful for volume or
balance control, brightness or contrast
on a TV set, and so on. Perhaps it
should be noted that 'on/off' type con-
trol signals can also be used for analogue
functions, by converting them — at the
receiver end - into 'more/less' type
control. Opening or closing curtains, for
example: by switching a motor on and
off at the correct moments, the curtains
can be opened as far as you like.

own advantages and disadvantages, but
tests have shown that PSK has more
disadvantages than FSK. In particular,
PSK is more 'vulnerable' to the Doppler
effect; an FSK system, on the other
hand, can be made virtually immune to
this kind of problem.

Having chosen FSK, we can proceed to
the block diagram of a suitable trans-
mitter: see figure 2. This contains two
square wave generators, the firet running
at a much lower frequency than the

second. The frequency of the first
squarewave generator depends on which
of the sixteen control signals is to
be transmitted. This implies that only
one command can be given at a time: it
would be asking rather a lot for the
generator to run at two different fre-
quencies simultaneously! The sixteen
different command frequencies are in
the 6 Hz to 90 Hz range.

The second square wave generator is
controlled by the first: its output fre-
quency is 38.5 kHz when a 'high' volt-
age is applied to its input, and 40.9 kHz
for a 'low' input. The output from this
second generator is transmitted as an
ultrasonic wave, by the US transducer.
To achieve a reasonable control range,
a sufficiently high signal level must be
applied to this transducer. This is
achieved by amplifying the output from
the second squarewave generator, and
usina an LC resonant circuit. In this

way, voltages of up to 1 50 V can be
produced across the transducer, without
the need for high supply voltages. The
LC resonant circuit has a further advan-
tage: it converts the square wave into
something that resembles a sinewave —
and most US transducers prefer it that

In practice

The transmitter circuit is given in fig-
ure 3. As mentioned earlier, sixteen con-
trol inputs are provided. When a control
signal is to be transmitted, the corre-
sponding control input to the trans-
mitter must be connected to supply
common — by means of a pushbutton,
, say. Only one control input should be
selected in this way at any given
moment. It may be noted, in passing,
that the outputs of the Monoselektor
mentioned earlier are also 'active low'.
This means that they can be connected
direct to the inputs of this transmitter
circuit.

When one of the control inputs is con-
nected to supply common, the corre-
sponding transistor (T 1 ... T 16) starts
to conduct. One of the sixteen. resistors
R17 . . . R32 then goes into action as a
frequency-determining element for the
first squarewave generator, IC1. This 1C

elektor march

i - 3-39

starts to produce an output signal with
a frequency that is determined by the
v selected control input (the squarewave
5 I frequency varies between 1 2 Hz and
180 Hz for the resistor values given in
' the Table). The following flipflop, FF,
halves this frequency; of more import-
1 ance is the fact that it produces a nice,
ie ' symmetrical squarewave output. This
n signal goes to the modulation input of a
3 second 555 (IC2), that produces the
)r ' modulated carrier signal (at approxi-
mately 40 kHz). A transistor, T19, is
used as output amplifier. The various
!r inductors are included to boost the out-
put signal even further and to clean it
i- up — providing a sinewave at some

b- 150 V peak-to-peak across the trans-

ts ducer.

rs The three ICs and the output stage are
a not connected directly to the positive
ie supply rail; an electronic switch, con-
C sisting of T17 and T18. is used. Sixteen

diodes (D1 . . . D16) are used in an OR
gate configuration, switching on the
electronic switch as soon as one of the
control inputs is activated. When no
control signal is to be transmitted, the
electronic switch opens; the circuit then
draws virtually no current. A useful
feature for a circuit that will almost
certainly be battery-powered!

The receiver

The transmitter circuit described above
is built into the remote control unit. It
transmits one of sixteen commands as a
modulated ultrasonic signal. This signal
must be received and 'decoded' at the
other end, to control the equipment as
required.

The block diagram of the receiver is
given in figure 4. The first sections
- trigger, monoflop, low-pass filter and
another trigger - amplify the signal
pciked up by the ultrasonic transducer

(US) and recover the low-frequency
squarewave from it (the 6 Hz ... 90 Hz
control signal). This signal is then
applied to one or more (up to sixteen)
digital band-pass filters that serve to
distinguish between the sixteen chan-
nels. When a command is transmitted,
only the digital filter that is 'tuned' to
the corresponding low-frequency square-
wave will produce an output. This signal
toggles a flip-flop, FF. Giving a com-
mand therefore results in this flip-flop
changing state; if it, in turn, is used
to control a lamp, the lamp can be
switched on and off by means of the
remote control unit. The whole object
of the exercise!

The circuit of the receiver is divided
into two sections. The first part (fig-
ure 5) receives the ultrasonic signal and
retrieves the low-frequency squarewave.
This signal is then passed to one or more
digital filters; the circuit of one of these

Table 2.

filters is given in figure 6.

Depending on the type of transducer
used, either of two possible input cir-
cuits may be required. The input stage
shown at the upper left in figure 5,
using FET T1, is intended for use with
high-impedance transducers. Low-
impedance transducers, on the other
hand, can be connected direct to C2.

The ultrasonic signal is amplified by T2.
The resonant circuit (L1/C3) is tuned to
40 kHz. The signal is then 'squared up',
by A1 and A2, producing a squarewave
that switches between supply common
and the full supply voltage. This signal
goes to a complicated network around
A3. In brief, R12-C7-R14-D1 and R16-
C8-R15-D2 each operate as simple
differentiating networks; they each
provide constant-width output pulses at
the zero-crossings of the input signal.
These two networks are followed by
simple RC low-pass filters (R 13/CIO
and R 15/Cl 1) that serve to retrieve the
original low-frequency component from
the ultrasonic signal. A3 and A4 amplify
this low-frequency signal, producing a
square-wave that swings between 0 V
and full positive supply. This signal is
ideally suited for further processing in
the digital filters.

Digital filter

A digital filter circuit is given in figure 6.
The values for C2 and C3 depend on the
control signal that is to be detected by
the filter; see Table 2.

Cl and R1 are another differentiating
network; together with N1, they
produce pulses with constant width.
ICIa and ICIb are monostable multi-
vibrators. To understand the circuit, it is

Values for C2 and C3 in figure 6.
control C2 C3

signal

1 10 n 470 p

2 15 n 470 p

3 15 n 820 p

4 18 n 820 p

5 22 n 1 n

6 33 n 1 n

7 39 n 1n5

8 47 n 1n5

9 56 n 2n2

10 68 n 3n3

11 82 n 3n3

12 100 n 4n7

13 120 n 4n7

14 150 n 6n8

15 180 n 8n2

16 220 n 10 n

easiest to refer to figure 7: this shows
the various signals that are present at
the indicated points in figure 6, when a
signal is being received at the 'resonant
frequency' of this filter.

Each negative-going edge in the input
signal. A, produces a brief pulse at B.
The negative edge of this signal triggers
ICIa, so that its Q output goes high (D);
the negative-going edge of this latter
signal triggers ICIb, producing signal E.
NAND gate N2 now combines signals B,
C and E; if all three signals are 'high' at
the same time, a short output pulse is
obtained at F. Since the three signals B,
C and E are derived from three consecu-
tive input pulses, with delays deter-
mined by the monostable time
constants, an output signal at F can
only be produced if the frequency of

the input signal is almost exactly right. ci
The brief negative output pulses from t(
N2 are used to clock flip-flop FF1 . This u
flip-flop is actually used as a monostable a:
multivibrator, with a period time that is Cl
longer than the delay between consecu- r <
tive pulses. The result is that the
output of this flip-flop remains 'low' as 31
long as N2 is passing pulses. This signal ai
is used to clock the second flip-flop, h
FF2 - the one shown as 'FF' in the °
block diagram given in figure 4. The ri
complete circuit therefore works as a c
kind of 'selective flip-flop': it is not T
toggled by a single pulse; instead, it ti
responds to a square-wave input at n
exactly the correct frequency.

Preset PI is used for setting the 'resonant "
frequency' of the digital filter, as will be fi
explained later. *

Complete system

Constructing the remote control system
should not present any real difficulties. •

As explained, the transmitter (figure 3) v\
has sixteen control inputs. To give a c<
control signal, the corresponding input n
must be connected to supply common o
— using a pushbutton, for instance. No r<
power on/off switch is required, since s'
this function is performed by the elec- q
tronic switch T 1 7/T 1 8.

Each receiver consists of one circuit as n
shown in figure 5 and one or more of tl
the circuits given in figure 6. If a high- C
impedance transducer is used, the input o
wiring around FET Tl (figure 5) should
be kept as short as possible.

When setting up the transmitter circuit, i u
the signal at the output of A2 in the
receiver ('A' in figure 5) must be moni- e
tored. This signal can be measured, of c

course, but it is just as easy to 'listen'
i to it. A high-impedance headphone is
; used, with a series resistor and capacitor,
. as shown in figure 8. This little circuit is
; connected between point A in the
receiver and supply common. One of
. the control inputs to the transmitter is

s activated, and PI in the transmitter is

| adjusted until a tone is heard in the

, headphone. This tone should be present

> over a small part of the adjustment

3 range of PI ; the preset is now set to the

3 centre of this range,
t The same headphone circuit can be used
t to adjust the digital filters. It is con-
t nected to the output of N2 I'F') in
figure 6. The control input to the trans-
t mitter that is to correspond to this
3 filter is activated, and PI in the filter is
adjusted so that pulses are clearly
audible — corresponding to the low-
frequency squarewave.

The easy way out: two ICs
) With all its advantages, the remote
a control system described above would
t never win an award for low component
i count. Obviously, manufacturers of TV
o receivers would prefer a more compact
e system, and 1C manufacturers have been
quick to meet the demand. One special
1C in the transmitter and one in the
s receiver takes a lot of hard work out of
f the manufacturing process,
i- One manufacturer who supplies a pair
t of ICs for this job is Plessey. The SL490
d (transmitter) and ML920 (receiver)

form the main components in the
:, second remote control system that we
e will describe. These ICs can be used in
i- either infrared or ultrasonic remote
if control systems. For several reasons, an

ultrasonic system is preferable - and so
that is what we will use.

There is little point in going to such
extensive detail on the operation of this
twin-IC system. Discussing the 'innards'
of the Plessey ICs would be of little
practical use - there's no way to modify
them. Instead, we will restrict ourselves
to giving suitable circuits.

As we shall see, one of the advantages of
these ICs is that they can be used for
transmitting 'analogue' signals as well as
the digital kind.

A transmitter circuit using the SL490 is
given in figure 9. As in the previous
system, the control signals can be given
by means of pushbuttons. However, in
this case they are not connected
between a control input and supply
common; instead, they are connected in
a 'matrix' - like in pocket calculators.
Since a 4 x 8 matrix is used, up to
32 pushbuttons can be used.

Of the 32 pushbuttons (assuming that
all positions are used . . .) only one
should be operated at a time. The
position of this button is converted into
a five-bit binary code, in the 1C. The
code numbers run from 00000 (upper
left) to 11111 (lower right); they
increase progressively from left to right
and then down to the next row - like
reading lines on a page. In effect, this
means that the two right-hand bits in
the code are determined by the position
in the row (horizontally), whereas the
other three bits indicate which row is

This five-bit code is transmitted, using a
special type of modulation: pulse
position modulation, or PPM. A series
of six pulses of equal length are tran-

mitted; the five periods
pulses can be either long or short,
depending on the five-bit code. A long
'pause' corresponds to a logic 0, and a
short interval is for a logic 1 . This is
illustrated in figure 10. Preset PI is used
to adjust the correct pulse/pause lengths.
The interval between pulses should be
approximately 20 ms for a logic 1 and
30 ms for a logic 0; the pulse length is
approximately 10 ms. The ultrasonic
carrier is transmitted during the pulse.
The carrier frequency is set by P2; a
good choice is 40 kHz.

A few transistors are added to boost the
current through the transducer - the 1C
itself can supply only 5 mA. The
maximum range of the system is
increased in this way. As in the system
described earlier, this one also contains
an electronic supply switch - included
in the 1C — so that the current con-
sumption from the 9 V battery is only
6 *iA if no command is being trans-
mitted.

The receiver

The ultrasonic commands given by the
SL 490 transmitter can be decoded by
a receiver using the ML 920. This 1C is
intended for use in colour TV receivers,
but there is nothing against using it
for other applications. The 32 com-
mands given by the SL 490 are not all
decoded as such; instead, there are three
analogue outputs as well as several
digital ones.

Before taking a closer look at what the
ML 920 can do — in conjunction with
the SL490 — we should first point out
what is cannot do: amplify and de-

Table 3.

modulate the ultrasonic signal. This 1C
expects to receive a clean series of
pulses, like those shown in figure 10,
without any ultrasonic components. A
separate preamplifier and demodulator
must therefore be included between the
transducer and the 1C. No problem, as
we will see.

The circuit that distills the commands
from the 'clean' pulse signal is given in
figure 11. Not counting a 'monitor'
output for adjustment purposes, there
are thirteen 'true' outputs. These are
divided into three groups: three analogue
outputs < A 1 . . . A3), five digital out-
puts (D1...D5) and five 'channel'
outputs (Cl . . . C5).

The 'channel' outputs are intended
for switching channels on the TV-
receiver. Up to twenty channels can
be selected by these five outputs:
the desired channel number (0 . . . 19)
is available as a binary five-bit code
(00000 ... 10011) at outputs Cl . . .C5.
When one of the commands 00000
...10011 is given from the control
unit, the binary output at the channel
outputs assumes the same value. When-
ever the value at the C outputs changes
(when a channel change is required) a
short pulse also appears at digital output
D4; this can be used for various appli-
cations. If a command with a higher
number than 10011 is given by the
control unit, the channel outputs
remain unchanged and no pulse appears
at the D4 output.

| It is also possible to step through the

10

juinuu luuuuui

Figure 10. The 1C transmits the five-bit code
in a chain of six pulses (two of the pulse

tained in the interval between pulses: a logic 1
gives a short pulse, logic 0 is coded as a longer

same code: 10109.

channels automatically. Control signal
10101 causes the receiver to step
through the channels in ascending order;
code 11101 makes it step through the
channels in the opposite direction. If,
in some application, the twenty channels
are required individually — instead of as
a five-bit code — they can be 'de-
multiplexed' by means of a suitable 1C
(the CMOS type 4514, for instance).
The only point to note, in that case, is
that the ML 920 uses 'negative logic':
logic 0 is positive supply voltage, logic 1
is equivalent to 0 V.

The analogue outputs of the ML 920
are intended for controlling volume,
brightness and colour saturation. The
voltages at these outputs vary in sixteen
steps. Their functions, and the corre-

Functions and control signals for the analogue
outputs in figure 11.

output

A2

A3

11100 colour saturation

11110 volume

11111 brightness

sponding control signals, are listed in
Table 3.

The digital output D2 is intended for
controlling the 'colour killer': it pro-
vides a logic 0 (almost positive supply
level) when the analogue colour output f
A1 is at zero level — this corresponds to
'no colour'. A 'mute' command is also
provided, for temporarily switching off F
the sound: the control signal 11001
switches output D1 alternatively low c
and high. Output D3 provides the i
'on/off' signal for the TV receiver; the r
corresponding control signal is 11000.
More accurately, this should be referred i
to as a 'stand-by' switch — obviously, the ;
TV receiver cannot be switched off ;
altogether: at the very least, the ultra- ;
sonic receiver must be left on.

' ff Finally, a ‘basic setting' command PPM signal. of the TV line frequency (31.3 kHz

91 (11011) causes all three analogue As stated earlier, the transducer cannot and 46.9 kHz).

outputs to assume an intermediate be connected direct to the 1C. A pre- Demodulating the ultrasonic signal is no
he level — approximately one-third of their amplifier is required, that also removes problem — it calls for nothing more
he range. the 40 kHz components from the signal, than filtering out the 40 kHz com-

! 0- There is only one preset adjustment A suitable circuit is given in figure 12. ponent from the signal. This is done by
ed in the ML 920 circuit: PI . This must be The ultrasonic signal is amplified by T1. means of a low-pass filter (IC2).
he adjusted so that a squarewave appears IC1, with the associated components. The output of the circuit given in figure
>ff at the monitor output (pin 9) with a is used as a 40 kHz band-pass filter. The 12 can be connected direct to the input
a " period time that is one-twentieth of main reason for including this is to of that in figure 1 1.
the interval time for a logic 0 in the reject the second and third harmonics

3-44 - eloktor

Wireless: three varieties

For 'wireless' transmission of control
signals or other data, a few different
options are available.

The most common system is radio — in
fact, 'wireless' has become almost
synonymous with this. The control unit
contains all controls, a (miniature) radio
transmitter, and a circuit that codes the
various control signals for transmission.
The equipment that is to be controlled
contains a receiver and a decoding
circuit that retrieves the original control
signals. A radio link of this kind has
several advantages - considerable range
for low power, for instance — but it also
has its disadvantages. The most import-
ant 'disadvantage' is that the Post Office
authorities take a very dim view of
people operating this kind of link
without a licence. This limits its uses to
exactly those fields were it is virtually
the only possibility - controlling model
aeroplanes, for instance.

For use indoors, remote control by
radio is 'not on'. As an alternative, we
could consider using light. A light
source in the control unit, a light
detector in the receiver, and suitable
coding and decoding circuits are again
required. To avoid undesirable 'disco
lights' effects, it is now common prac-
tice to use light that is invisible to the
human eye: 'redder than red'- infrared,
in other words. This has a further
advantage in that it penetrates smoke
and dust haze better than visible light.
However, an infrared remote control
system is not an ideal solution. It is no
easy matter to make the detector in the
receiver sufficiently sensitive. Nor can
the infrared light source in the trans-
mitter be particularly powerful — not,
that is, unless you don't mind changing
the batteries every time you give a new
control signal. Furthermore, infrared is
light; and like any other kind of light it
tends to travel in very straight lines.
Admittedly, it is reflected off all kinds
of objects so that it usually ends up
at the receiver some time - even if
the transmitter isn't aimed like a search-
light -but a lot of its power is absorbed
by all that bouncing around.

There are also a large number of
'spurious infrared transmitters' in any
living room. Heat is also infrared, and
incandescent lamps emit quite a lot of
energy in the infrared region that we are
interested in. What all this means is that
only a little of the desired infrared
signal will reach the receiver, and that
there is a lot of interference. For this
reason, both the transmitter and the
receiver circuits must incorporate all
kinds of sophisticated tricks, if the
receiver is to be able to distinguish the
control signals from the 'noise'.

A sound system

We have seen that neither radio nor
infrared links are ideal for domestic use.
The former is forbidden, and the latter
doesn't work very well — at least, it

takes some doing to make an infrared
system work reliably.

Fortunately, a third option is available:
sound waves. For use indoors, these are
much more suitable. In particular, it is
much easier to fill' a room with sound:
the waves bounce and bend all over the
place. This is still true at the relatively
high frequencies that are of interest for
remote control systems.

When discussing light as a transmission
medium, it was pointed out that
'invisible' light is preferable to the
visible variety, to avoid being dazzled by
the light show put out by the control
unit. For the same reason, ’inaudible'
sound is preferable to the normal
variety in this application. By inaudible,
we mean above the range of human
hearing (maybe a few dogs or bats may
hear these control signals): 'ultrasonic',
as it is called.

Using high frequency sound has a
further advantage: the 'loudspeakers'
and ‘microphones' (both are usually
referred to as 'transducers', actually)
are both small and cheap. They are also
surprisingly efficient- putting out quite
a lot of 'sound' for a fairly modest
electrical power consumption.

So why doesn't everybody use ultra-
sonic transducers for remote control
systems? They must have some dis-
advantages? Rest assured, they have.
They are more bulky than infrared
LEDs; they are more 'breakable'; and
'spurious ultrasonic transmitters' also
exist. Rattling keys, for instance. For
this reason, a sufficiently 'interference-
proof' coding and decoding system is
still required.

There is one further problem:

The Doppler effect

One of the major problems associated
with ultrasonic systems is the Doppler
effect. As most people will know, if an
object or person that is transmitting a
sound wave, for instance, is moving
towards the receiver, the latter will
'hear' a sound wave at a higher fre-
quency than that transmitted. Similarly,
if transmitter and receiver are moving
away from each other the apparent
frequency will be lower. A well-known
example is the sound of a passing
express train: at the moment that it
passes us, the sound seems to suddenly
drop to a much lower frequency.
Obviously, the train doesn't really
’change its tune’ when it sees us. The
frequency that we are hearing is not the
same as that which the train is 'trans-
mitting'.

Indoors, it is unlikely that we will
travel at the speed of an express train.
Even so, the Doppler effect can be
quite a nuisance when using an ultra-
sonic remote control link. (The same
applies to infrared links, by the way,
but to a lesser extent.)

When designing a system, all due care
must be taken to ensure that it is
relatively insensitive to the kind of fre-

quency shifts that can be caused by this
effect. Fortunately, this is not too
difficult. The two designs given in this
article are 'Doppler-proofed'.

The Monoselektor goes remote

The Monoselektor described in Elektor,
June 1979, can be used to control a
large number of different things at the
touch of a single button: radio and/or
television set, lights, doors, curtains,
and so on.

There is no problem in constructing the
unit, as experience has shown, but when
it comes to putting it into operation '
things are apt to get 'ropey'. Or 'wirey',
rather. The curtains and door may be
operated by motors, the radio and
television set may be designed for
remote control; we have already
published designs fora 'solid state relay'
that will switch this type of equipment.
But one problem remains: connecting
the Monoselektor to all these items,
distributed all over the room. This
normally involves yards and yards of
cable. Not very neat, we must admit.
There is a lot to be said for using a
remote control link.

For this application, the first of the two
systems described in this article is
ideally suited. All wiring from the
Monoselektor to the other equipment
can be replaced by a 'wireless' link. The
ultrasonic transmitter (figure 3) is built
into the Monoselektor case; the control
inputs can be connected direct to the
Monoselektor outputs.

Receivers (figures 5 and 6) can be
mounted wherever required in the
room, to control the motors, relays or
whatever. The fact that several receivers
can be used in the same system is an
advantage. In fact, if cost is no problem,
sixteen receivers can be built, each
tuned to its own channel. In practice, of
course, it will normally be more econ-
omical to use one receiver for several
items of equipment that are close to
each other in the room. H

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