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n i m e r e h T 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.