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Get the eerie soaring
sounds of science fiction
You wave your hands
around mysteriously
to play it
16 Silicon Chip
Producing sounds with the slightest of hand
gestures, the Theremin is the forerunner of
modern synthesised music. You can create
eerie science fiction movie sounds or make
melodious music simply by hand movements.
By JOHN CLARKE
T
HE THEREMIN WAS invented
by Leon Theremin of Lenin
grad, Russia in 1924 and it
represented a revolutionary change
in thinking about how music could
be produced, challenging traditional
stringed, brass and percussion musical
instruments.
Comprising electronic oscillators, it
allowed control over both pitch and
amplitude by moving the hands over
sensor plates. Its design eventually
led to the development of the Moog
Synthe
siser and electronically synthesised music in general.
But the invention was not only
instrumental in the development of
electronic music, it also had an impact on a free-form style of playing
music. The free gesture hand control
afforded by the Theremin pre-empted
the modern Sensor Chair synthesiser
controller where the whole body is a
part of the musical generation process.
Before this, Jimi Hendrix was
creating new sounds by gener
ating
feedback between his guitar and the
amplified sound and then moving
his body to modulate the amplitude.
It freed him from the restriction of
generating music solely by plucking
the guitar strings.
The Theremin was commercially
manufactured by the Radio-victor
Corporation of America (RCA) around
1929. It comprised a large box to
which were attached an antenna and
wire loop. The antenna provided the
control for the pitch while the loop
enabled the volume to be adjusted.
Moving the right hand toward the
antenna would reduce the pitch while
moving the hand away from the antenna would increase the pitch.
In some ways this is similar to playing a trombone whereby the slide is
moved back and forth to vary the pitch.
The left hand would reduce the
volume as it was brought near the
sensor loop. As you would expect, the
original Theremin circuit used valves.
The pitch control antenna stood
vertically, while the volume loop sat
horizontally. This meant that there
was little interaction between the two
controls especially since the pitch
control hand would be held side on
to the antenna and the volume control
hand would be horizontal. Thus the
interaction between the two controls
would be minimised.
In recent times there has been quite
a renewal of interest in the Theremin
and there is a fair amount of information on the Internet. However, none of
it is really helpful if you want to build
your own Theremin.
And this is where this completely
new Theremin circuit comes into the
picture. It has exactly the same operational characteristics as the original
RCA Theremin but does not use valves
(perish the thought). Instead it uses
just three low cost ICs and a handful
of other components.
Not only that, our new Theremin is
considerably smaller than the original
design although you could build a
large one if that’s what you fancy.
We built ours into a medium sized
plastic Jiffy box with the antenna and
sensor loop mounted on opposite
sides of the box. It includes a small
Fig.1: the Theremin circuit involves three virtually identical oscillators, two to generate the tone and one to
control the volume.
August 2000 17
Fig.2: the reference oscillator and pitch oscillator are
mixed together in the product detector (IC1) to produce
an audible tone which is amplified by IC2a and IC3.
18 Silicon Chip
Fig.3: these digital scope waveforms show the reference
and pitch oscillators locked together at 441kHz and the
difference result is no output tone. Normally, the ref
erence oscillator remains fixed while the pitch oscillator
is varied by hand capacitance.
loudspeaker for practice sessions and
a line output for connection to a sound
system. The only manual controls are
an On/Off switch and volume control.
The unit is powered by a 9V or 12V
DC plugpack.
Circuit principles
The block diagram of Fig.1 shows
the basic arrangement of the Theremin
circuit. It comprises three oscillators
which all operate at about 455kHz.
The reference and pitch oscillators
are mixed together to generate a beat
signal which becomes the audible tone
while the volume oscillator is used to
change the level of the tone output.
The reference oscillator operates at
a fixed frequency and is mixed with
the pitch oscillator in the product
detector (IC1). The pitch oscillator
changes in frequency depending upon
the amount of capacitance to earth
presented by your hand when it is
near the antenna. As noted above, the
frequency will fall when the hand is
brought near to the antenna and rises
when it is further away.
The product detector essentially
multiplies the reference oscillator (f1)
with the pitch oscillator (f2) to produce
sum (f1 + f2) and difference (f2 - f1)
frequencies.
The sum (f1 + f2) signal is at around
900kHz which is easily filtered out
with a low pass filter and we are left
with the difference signal of f2 - f1
which comprises audio frequen
cies
from 1.4kHz down to below 10Hz.
So if the pitch oscillator frequency is
Fig.4: this is the line output signal when the pitch osc
illator is 833Hz lower than the reference oscillator. Note
the relatively clean sinewave output.
456kHz and the reference oscillator
is at 455kHz, we will obtain a 1kHz
audio output from the low pass filter.
The audio output from the low pass
filter is applied to a power amplifier
which can drive a loudspeaker. The
overall volume from the amplifier is
set by the volume control VR1.
The volume oscillator is controlled
by the sensor loop which is also affected by hand capacitance. As you
bring your hand closer to the loop,
the frequency of the volume oscillator
decreases. This is fed to a bandpass
filter which has a centre frequency
(fc) which is higher than the volume
oscillator frequency. So if the volume
oscillator is operating at frequency f1
the level will be low. As the frequency
increases, the level will increase as it
approaches the centre frequency of
the filter.
This signal level is detected using
a diode and filtered to produce a DC
voltage. The following amplifier increases the DC voltage and the level
shifter sets the voltage so that it can
control the product detector output
level over a suitable range via its
transconductance input.
Circuit details
The circuit for the Theremin uses
three JFETs (Junction Field Effect
Transistors), four prewound IF (Intermediate Frequen
cy) coils, three
ICs, one detector diode, a 3-terminal
regulator and associated resistors and
capacitors.
As you can see, all three oscillators
are identical with the exception of the
100Ω drain resistor for Q3. Each oscillator comprises a junction FET (JFET)
Q1 and a standard IF transformer, as
used in low-cost AM radio receivers.
The transformer comprises a tapped
winding which has a parallel-connected capacitor to form a tuned circuit. The secondary winding couples
the oscillator signal to the following
circuitry.
Each JFET drives a portion of the
primary winding (ie, between the tap
connection pin 2 and ground) while
the signal across the full winding is
applied back to the gate via a 68pF
capacitor. This is the positive feedback
which ensures oscillation.
To make them controllable by hand
capacitance, the Pitch and Volume
oscillators have the antenna and disc
connected to the top of the tuned coil
where they will have the most effect.
The reference oscillator and pitch
oscillator outputs are applied to an
MC1496 balanced mixer, IC1. Resistors
between the +6V supply and ground
set the bias voltages for the inputs of
the balanced mixer, while the 1kΩ
resistor between pins 2 & 3 sets the
gain of the circuit.
IC1 provides a balanced output with
signals at pin 6 and pin 12. These
outputs are filtered with a 3.3kΩ
pullup resistor and .047µF capacitor
to produce rolloff above about 1kHz.
This heavily attenuates frequencies
at 455kHz.
The output from pin 12 is AC-coupled to op amp IC2a which simply
August 2000 19
Fig.5: the component overlay
for the PC board. Note that
IF transformers T1, T2 &
T3 should all be fitted with
white slugs, while T4 should
have a black slug.
buffers the signal before it is applied
to volume control VR1. IC2a’s output
signal also goes to the line output
terminal. The signal from IC2a is
AC-coupled to both VR1 and the line
output to prevent DC voltage flowing
through the pot and the line output.
IC3 is an LM386 1W amplifier which
The loudspeaker
and on/off switch
are mounted on
the lid of the case
and connected to
the PC board using
light-duty hookup
wire.
20 Silicon Chip
drives the loudspeaker via a 470µF
electrolytic capacitor. The .047µF
capacitor and series 10Ω resistor form
a Zobel network to prevent spurious
oscillation from the amplifier.
Volume oscillator
The output from the volume oscil-
lator at the secondary winding of T3
is AC-coupled to the base of transistor
Q4. This is connected as a common
emitter amplifier with the collector
load being a parallel-tuned circuit
comprising an IF coil with internal
capacitor. T4 and the associated capacitor are tuned to a frequency just
above the maximum available from the
volume oscillator. The emitter resistor
is bypassed with a 560pF capacitor
which provides rolloff below about
400kHz.
The output level from transformer
T4 will vary in proportion to the frequency from the volume oscillator.
This is because the filter provides a
sharp rolloff below its tuning frequency and small changes in frequency
which are below the centre frequency
will cause large changes in the filter
response. The action of this circuit is
a simple frequency modulation (FM)
detector.
The high frequency signal from T4
is rectified by diode D1 and filtered to
provide a DC signal which is amplified
by op amp IC2b by up to 1000, depending on the setting of VR2. IC2b’s output
is then fed to pin 5 of IC1 to vary the
level of the audio signal.
Oscilloscope waveforms
The oscilloscope waveforms of
Fig.3 & Fig.4 demonstrate the circuit
operation. Fig.3 shows the reference
and pitch oscillators locked together
at 441kHz and the result of that is no
output tone. Normally, the reference
oscillator remains fixed while the
pitch oscillator is varied by hand
capacitance.
Fig.4 shows the line output signal
when the pitch oscillator is 833Hz
lower than the reference oscillator.
Note the relatively clean sinewave
output. The output level is a healthy
539mV RMS.
Power for the circuit comes from
a DC plugpack which is regulated
by REG1, a 5V regulator. The output
The PC board fits inside a standard plastic case, with the volume plate protrud
ing through a slot at one end. Alternatively, the plate could be permanently
fixed to the lid of the case or mounted so that it can swivel over the lid for easy
storage.
This view shows the mounting details for the telescopic antenna, which
functions as the pitch control.
August 2000 21
is “jacked up” by a nominal 0.7V by
diode D2 to give a nominal +6V which
will actually be around +5.7V. The input and output terminals of REG1 are
decoupled with electrolytic capacitors
to aid in supply filtering and to prevent
instability in the regulator.
Construction
All the parts for the Theremin are
assembled onto a PC board coded
01207001 and measuring 133 x 88mm.
It is mounted in a plastic Jiffy box
which measures 158 x 95 x 53mm.
While our assembly description
revolves around the plastic case with
its small speaker, there is no reason
why you couldn’t build it into a much
larger case in keeping with a musical
in
strument. A larger amplifier and
loudspeaker would also be a considerable benefit in the overall sound
quality.
You can begin construction by
checking the PC board for any defects
such as shorts between tracks, breaks
in the copper tracks and incorrectly
drilled holes. You will need slightly
larger than the standard 1mm sized
hole for the coil earth pins on the sides
of the shielding cans, while holes for
The Theremin will give more
consistent results if it is mounted
on a raised non-metallic stand so
that it is at least 60mm from any
surfaces. This stand was made
from a discarded plastic panel
and some plastic tubing.
Table 2: Capacitor Codes
The completed Theremin, together with its plastic stand. You use your right
hand to control the pitch by waving it near the antenna, while your left hand
and the circular metal plate control the volume.
Value
IEC Code EIA Code
0.1µF 100n 104
.047µF 47n 473
560pF 560p 561
220pF 220p 221
68pF 68p 68
Table 1: Resistor Colour Codes
No.
1
1
6
2
1
2
1
1
8
1
1
1
2
1
22 Silicon Chip
Value
1MΩ
270kΩ
100kΩ
10kΩ
6.8kΩ
3.3kΩ
2.2kΩ
1.2kΩ
1kΩ
820Ω
680Ω
150Ω
100Ω
10Ω
4-Band Code (1%)
brown black green brown
red violet yellow brown
brown black yellow brown
brown black orange brown
blue grey red brown
orange orange red brown
red red red brown
brown red red brown
brown black red brown
grey red brown brown
blue grey brown brown
brown green brown brown
brown black brown brown
brown black black brown
5-Band Code (1%)
brown black black yellow brown
red violet black orange brown
brown black black orange brown
brown black black red brown
blue grey black brown brown
orange orange black brown brown
red red black brown brown
brown red black brown brown
brown black black brown brown
grey red black black brown
blue grey black black brown
brown green black black brown
brown black black black brown
brown black black gold brown
the PC stakes should be sized to suit
their diameter; they should be a tight
fit.
Check that the PC board clips neatly
into the plastic case’s integral side
pillars. It may need to be filed down
to make a snug fit. The component
overlay and wiring details are shown
in Fig.5.
Insert the two links and then the
resistors. Use Table 1 as a guide to
selecting each value. Alternatively,
use a digital multimeter to measure
each resistor.
The three ICs can be mounted next,
taking care with their orientation.
Make sure that IC2 & IC3 are placed
in the correct positions.
Next, the capacitors can be mount
ed, noting that the electrolytic types
are polarised and must be oriented
with the correct polarity, as shown.
The MKT and ceramic types usually
are coded and you can check this
against the values shown in Table 2.
The coils are prewound types each
with an integral tuning capacitor and
are mounted as shown. Be sure to
place the ones with the white slugs
(the threaded ferrite core) in the T1-T3
positions and the coil with the black
slug in the T4 position.
Now mount the JFETs (2N5484),
transistor Q4 (BC548) and the 3-terminal regulator. Diodes D1-D3 can be
mounted next, taking care with their
orientation.
Finally, mount potentiometer VR1
and trimpot VR2. VR1 is mounted using five PC stakes as shown. Three PC
stakes connect the pot terminals to the
PC board while the remaining two PC
stakes are soldered to the pot body to
hold it in place. Scrape the passivated
coating off the pot body adjacent to the
PC stakes for easy soldering.
With the PC board complete, you
are ready to work on the case. You
will need to drill holes in the sides
of the box for the DC panel socket,
the RCA socket and for the antenna
securing screw.
Volume plate
The volume plate is made from light
gauge aluminium 70mm in diameter
and is connected via a lead and solder
lug to the PC board.
We mounted the volume plate on the
prototype so that it could slide into the
case when not in use. This requires a
narrow slot to be cut into the side of
the case. We made the slot by drilling
Parts List
1 PC board, code 01207001, 133
x 88mm
1 plastic Jiffy box, 158 x 95 x
53mm
1 front panel label, 155 x 92mm
1 9VDC or 12VDC 300mA
plugpack
1 telescopic antenna (pitch
antenna)
1 70mm diameter (or 70mm
square) 1mm aluminium plate
(volume)
1 DC socket (2.5mm diameter
pin)
1 panel-mount RCA socket
1 50mm 8Ω loudspeaker
3 2nd IF coils (DSE L0260 white)
(T1-T3)
1 3rd IF coil (DSE L0260 black)
(T4)
1 SPST rocker switch (S1)
1 knob
2 crimp eyelets
1 M3 x 15mm screw
1 M3 x 10mm screw
3 M3 nuts
1 200mm length of yellow
medium-duty hookup wire
1 200mm length of blue
medium-duty hookup wire
1 50mm length of 0.8mm tinned
copper wire
1 short cable tie
13 PC stakes
1 10kΩ log pot (VR1)
1 2kΩ horizontal trimpot (VR2)
a series of small holes and then filing
it to the correct size.
If you don’t want to go to the trouble
of making the slot you can permanently attach the volume plate to the lid
of the case or mount it so that it can
swivel over the lid for easy storage.
Attach the front panel label to the
lid and drill the holes for the switch
and some holes for the loudspeaker.
We mounted the speaker by smearing
super glue around its perimeter and
then placing it inside the lid.
Follow the wiring details of Fig.5.
You can use hookup wire to connect
the sockets, switch S1 and the loudspeaker to the PC board.
The antenna is attached with an M3
x 15 screw plus an M3 nut which is
secured to the case with another M3
nut. The eyelet lug is held beneath the
Semiconductors
1 MC1496 balanced modulator
(IC1)
1 LM358 dual op amp (IC2)
1 LM386N-1 1W amplifier (IC3)
1 7805 3-terminal regulator
(REG1)
3 2N5484 N-channel JFETs
(Q1-Q3)
1 BC548 NPN transistor (Q4)
1 1N914, 1N4148 signal diode
(D1)
2 1N4004 1A diodes (D2,D3)
Capacitors
2 470µF 16VW PC electrolytic
1 100µF 16VW PC electrolytic
6 10µF 25VW PC electrolytic
1 2.2µF 16VW PC electrolytic
8 0.1µF MKT polyester
3 .047µF MKT polyester
1 560pF ceramic
3 220pF ceramic
3 68pF ceramic
Resistors (0.25W, 1%)
1 1MΩ
1 1.2kΩ
1 270kΩ
8 1kΩ
6 100kΩ
1 820Ω
2 10kΩ
1 680Ω
1 6.8kΩ
1 150Ω
2 3.3kΩ
2 100Ω
1 2.2kΩ
1 10Ω
Miscellaneous
Solder, super glue.
nut and the wire connects to the PC
board as shown. A connection is made
to the volume plate via an eyelet lug
using an M3 x 10mm screw through a
hole which is held using a nut.
Setting up
When your Theremin is complete,
check your work carefully. Apply power and check that there is a nominal
+6V between the case of one of the
transformer coils T1-T4 and pin 8 of
IC2 & pin 6 of IC3. The voltage should
be between +5.6V and +5.8V.
Adjust VR2 so that pin 1 of IC2 goes
to about +4.3V and wind VR1 slightly
clockwise from its fully anticlockwise
position.
Use a plastic alignment tool to rotate
the slug in transformer T2 slightly until a tone is heard in the loudspeaker.
August 2000 23
Fig.6 actual size
artwork for the
front panel.
Fig.7: actual size
artwork for the
PC board.
Then adjust it to obtain a
good frequency range when
your hand is brought near
to the extended antenna.
The note should be at its
highest when your hand is
away from the antenna and
should fall to a very low
frequency (just a growl)
when your hand is very
close to the antenna.
If the effect is the reverse
of this (higher frequency as
your hand is brought close
to the antenna) then adjust
the slug in the opposite
direction until the effect
is correct.
Note that you must do
this adjustment away from
the effects of metallic objects or the Theremin will
require retuning when removed from these grounding sources. In fact, the Theremin
will give more consistent results if it
is mounted on a raised stand which
keeps the unit at least 60mm from
any surfaces. The stand should be
made from a non-metallic material.
Volume plate alignment
The volume operation is set by adjusting the slug in T4 until the voltage
at the cathode of diode D1 is at +1.7V.
Then carefully adjust VR2 so that the
volume is at its maximum when your
hand is away from the volume plate.
24 Silicon Chip
Bringing your hand close to the plate
should reduce the volume.
You may need to set VR2 so that
the volume just goes to its minimum
level when rotated anticlockwise.
You then slowly adjust it clockwise
until the volume just snaps into full
level. If the volume does not reduce
with your hand approaching the plate
and the level remains essentially
constant or if the level rises, then
T4 is adjusted with the slug too far
clockwise.
This means that the circuit is op-
erating with the volume oscillator
equal to or higher than the tuned
frequency. Adjust the slug of T4
anticlock
wise so that the volume
plate operates correctly.
Note that when the lid is fitted to
the case, the tuning will change. We
drilled a hole in the lid to allow VR2
to be adjusted with the lid in place.
Also, the adjustment of VR2 will set
the sensitivity of the Theremin volume plate to hand movement. The
more precisely VR2 is adjusted, the
SC
greater will be the sensitivity.
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