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Can YOU master the THEREMIN? If you play video games, you’d be aware that some can be played with hand gestures – you’re not actually touching the game itself. Similarly, some phones and tablets can be controlled by gestures. But there’s a musical instrument which also plays with hand movements – and it pre-dates games and phones by nearly a century. It’s called the Theremin (pronounced ther-er-min) which produces some really eerie, almost spooky, sounds. And you can build one yourself. Whether you can master it . . . well, that’s another story! By JOHN CLARKE 24 Silicon Chip Celebrating 30 Years siliconchip.com.au T he eerie sounds of this almost mystical instrument have featured in many recordings and movie sound tracks right up to the present – despite being invented by Léon Theremin in 1919! SILICON CHIP has published six Theremin designs over the years but this is the first which uses transistors rather than ICs. Nor does it have any surface mount devices, so it is really easy to build and getting it to work is simply a matter of adjusting a couple of thumbwheel knobs. While all our previous designs have been quite popular, some of our readers have hankered for a simple, discrete design and have asked us to revise the Theremin published in Electronics Australia in June 1969, in an article by some bloke called Leo Simpson. Simple it was . . . but that design did not have a PCB and it required carpentry and other skills to put it together. Accordingly, while we have changed the fifty-year-old circuit very little, we have brought the presentation up to date. As long as you can solder components to a PCB, you will find it easy to put it together. The revised design uses slightly different transistors because some of those originally specified are now unavailable. In addition, you can run it from a 9VAC plugpack or even a 12V battery. Unlike some commercial Theremins with a bewildering array of controls, there are just two on our Theremin, just like the original invention. One is a vertical “antenna”, which is the pitch control. You vary the pitch by moving one hand near the antenna. The man himself, playing the instrument he invented in 1919. Theremin wowed audiences on three continents. As well as merely changing the pitch, you can add vibrato effects by fluttering your hand or fingers near the antenna. Moving your hand from one position to another by a very small amount will produce a gliding tone or glissando effect; you cannot easily play discrete notes. (Incidentally, we have retained the traditional name, “antenna”, for the Theremin’s pitch control, even though it doesn’t really transmit or receive anything. In addition, it resembles a whip antenna on a portable radio). The other control is a horizontal plate and it is used to Inside our all-transistor Theremin, essentially an updated version of the one Leo Simpson designed back in June, 1969. It’s really easy to build but not quite so easy to play well! The specially-shaped PCB, with its integral volume plate, screws onto the underside of the box lid. Not shown here is the Pitch Antenna (which you can see in the photo at left). It passes through the lid and the PCB, connecting to the circuit via a pair of fuse clips (bottom right of photo) acting as spring contacts. siliconchip.com.au Celebrating 30 Years January 2018  25 vary the volume. As well, you can add tremolo effects (similar but not the same as vibrato) by fluttering your hand or fingers above the volume plate. All this waving and fluttering of your hands near the controls is merely using capacitance effects to vary the circuit performance but the fact that you don’t actually touch anything makes the process seem all the more clever to an audience. Playing a Theremin is not particularly easy but if you have a good musical “ear” and you can play a stringed instrument like a violin or cello, or perhaps a trombone, you will have a head start in making music. Heterodyning Basically, the audio tone or musical note is produced by heterodyning (or 26 Silicon Chip mixing) two radio frequency oscillators to produce an audible beat or difference frequency. Some readers may have heard a similar kind of whistle, produced when a shortwave radio receiver is tuned across the dial. By carefully manipulating the receiver dial, it is possible to produce a beat ranging from a high frequency whistle to a low frequency growl. The two oscillators in a Theremin, used to produce the audible beat, must be set up so that they can operate very close together in frequency and without too obvious a tendency to lock at the same frequency. One oscillator must be designed so that its frequency will change readily when a hand is brought close to the pitch antenna. The other oscillator remains fixed in frequency. Celebrating 30 Years With both oscillators on the same frequency, there is a zero beat and no audible note is heard from the loudspeaker. When a hand is brought near the antenna, the frequency of the variable oscillator changes and a beat note is produced. Circuit details The two oscillators controlling pitch utilise NPN PN2222 transistors (Q1 & Q2). These are connected in a Colpitts configuration with an operating frequency of around 470kHz. By the way, a Colpitts oscillator is a type of LC oscillator which lends itself very nicely to this type of circuit. You can find a lot more information on line. The pitch antenna is connected to the collector of Q2, so bringing a hand close to the antenna will alter its casiliconchip.com.au Fig.1: two radio frequency signals, generated by oscillators based on Q1 and Q2, are heterodyned (or mixed), to produce an audio frequency note which can be varied by the distance of the hand from the pitch antenna. A slighty different arrangement, but also based on hand/plate capacitance, varies the volume fed to a conventional audio amplifier and small loudspeaker. pacitance and therefore will vary its frequency. The other pitch oscillator involving Q1 is tuned with a 140pF adjustable trimmer capacitor VC1. This trimmer is a standard plastic dielectric tuning capacitor normally used in small AM radios but only one section is used. A similar circuit arrangement is used for the volume control. Both trimmer capacitors are fitted with thumbwheels so that they can be easily adjusted. The waveform from both oscillators is very clean and as a result, the basic beat note would normally be fairly pure. If the two oscillators were run from the same supply they would tend to lock to the same frequency when they came within a few hundred Hertz of each other. This would mean that siliconchip.com.au the beat frequency would not range smoothly down to the low bass region. For this reason, the supply rail for each oscillator is decoupled via a 1kΩ resistor and 100nF ceramic capacitor. As a result, the two oscillators will not lock until the beat frequency is just a few Hz; a very low growl. It is desirable that the oscillators do eventually lock though, otherwise it would be too difficult to adjust VC1’s thumbwheel for a zero beat. In the original Theremin circuit, the oscillator transistors were Philips BF115 RF devices but these are now obsolete. So we are using cheap PN2222 general purpose transistors which have a very respectable frequency gain (fT) product of 250MHz so they have no trouble oscillating at 470kHz. The output from each oscillator is Celebrating 30 Years fed via 560Ω resistors to a mixer stage consisting of a general purpose BC547 NPN transistor, Q3, connected in common-emitter configuration. The mixer has four output frequencies: the two oscillator frequencies at around 470kHz, the sum of the two frequencies (around 940kHz) and the difference between the two frequencies, which is the audible output. The BC547 does not have a lot of RF gain and the 2.2nF capacitor shunting the collector load resistor further attenuates the RF components, leaving the wanted audible output. The mixer stage is slightly over-driven to add harmonics, so that the sound will be subjectively more interesting. A small change we made to the original circuit is to include the option of coupling between the pitch and reference oscillators using C1, which provides for “voicing”. When the pitch oscillator frequency differs from the reference oscillator so we get an output tone, the difference in frequency between the two oscillators tends to pull or distort the beat frequency wave shape so that it is not a sinewave. Typically, for a Theremin we want a sound that resembles a cello at low frequencies, morphing to something more like a flute as the frequency rises. Adding capacitor C1 allows you to experiment to obtain a different sound – try values from about 220pF to 470pF. Voltage controlled attenuator The output from the collector of Q3 is fed to a voltage divider consisting of a 100kΩ resistor and the drain-source resistance of N-channel JFET, Q6. The resistance of Q6 is dependent on the gate source bias which is provided by the volume control circuitry, involving an oscillator using Q4, the capacitance plate and DC amplifier, Q5. Q4 is another PN2222 NPN transistor and the volume oscillator is also a Colpitts type, running at around 900kHz. The volume oscillator also has its supply decoupled via a 1kΩ resistor and 100nF capacitor. The output of the volume oscillator is fed, via a 4.7pF ceramic capacitor, to a parallel tuned circuit consisting of a 330µH RF choke and the capacitance of the volume plate. A portion of the signal across the tuned circuit is coupled to schottky diode D1, via an 18pF capacitor. January 2018  27 Scope 1: this signal is the output of the “pitch” reference oscillator (based on Q1) which is adjusted in frequency by the pitch thumbwheel capacitor, VC1. Note that the output is quite clean. The resulting DC voltage is amplified by PNP transistor Q5 and applied to the gate of the FET after filtering with a 2.2nF capacitor. The level of the audio tone being reproduced should decrease when a hand is brought near the volume plate. Initially, the volume oscillator is adjusted, by means of 140pF rotary trimmer capacitor VC2, to give a minimum loudness of the audio tone when the hand is near the volume plate. This involves tuning the oscillator so that its frequency coincides with the resonant frequency of the tuned circuit. As a result, the voltage derived from the diode will be at a maximum so that Q5 is forward biased and consequently, turned on. The gate of the FET is taken toward the positive supply rail and its drain to source resistance is held to a low value. This shunts a large portion of the beat note signal to the positive supply. When you move your hand away from the volume plate, the capacitance in the tuned circuit changes the resonant frequency so that the DC derived from the diode decreases. This progressively carries Q5 toward cut-off so that the drain-source resistance of the FET increases. Thus more of the audio tone signal is fed to the following amplifier. At this point, a particular characteristic of the FET becomes apparent. For small voltages of either polarity (or AC) applied between the drain and source, the FET behaves as 28 Silicon Chip Scope 2: similarly, the output of the “volume” oscillator based on Q4. This is adjusted by VC2. Both these measurements are difficult to make because of loading by the scope probe. a resistor which can be varied in linear fashion by a voltage applied between source and gate. With the gate voltage varying between zero to about 4V below the source, the relationship between gate to source voltage and drain to source resistance may be relatively linear but this is no longer true as the gate to source voltage approaches the pinchoff voltage of the FET. In this region, the relationship becomes very non-linear, with a small increase in gate to source voltage resulting in very large change of drain to source resistance and so the FET is turned off over a small voltage range. It means that, in a certain region near the volume plate, a small hand movement will result in a large change in loudness so that it tends to act almost as a switch. To reduce this effect, a 33kΩ resistor is connected between collector and emitter of Q5. When the transistor is turned off, the 33kΩ and the 10kΩ collector load resistor form a voltage divider which limits the FET gate to source voltage to about minus six volts. This has the effect of making the volume control action more progressive but it does reduce the available range of the control. Note that it is not possible for the volume control circuitry to give zero sound output, since the minimum resistance of the FET is typically 100Ω and it cannot shunt all the signal to the positive supply. To sum up, the pitch of the TherCelebrating 30 Years Fig.2: PCB component overlay for the Theremin showing were everything goes. All components, with the exception of the speaker, mount on this PCB. Immediately below is a same-size photo of the PCB, this time installed on the lid of the UB1 Jiffy box we used. If you were really keen, you could make a timber case, just like Theremin’s original and, indeed, most of the early commercial Theremins sold. Incidentally, there are two minor differences between the photo of the prototype at right (PCB Rev “A”) and the final PCB/ component overlay above (Rev “B”). The value of VR1 has been changed to 50kΩ (it was 100kΩ) and a 2.2nF capacitor has been added near Q7. Always follow the component overlay when assembling. siliconchip.com.au emin is controlled by beating two RF oscillators running at about 470kHz together, one of which is sensitive to hand capacitance. The resulting beat note can be varied over the whole of the audible range. The loudness of the beat note is controlled by a third oscillator running at about 900kHz and feeding a tuned circuit which has its resonant frequency shifted by hand capacitance. A DC voltage, derived from the tuned circuit, is used to vary the drain to source resistance of a FET, which is part of a voltage divider to which the beat note signal is applied. Having grasped this, the rest of the siliconchip.com.au Theremin is easy to understand. The signal from the FET attenuator is fed to a 50kΩ potentiometer and then to an audio amplifier and loudspeaker. The 4-transistor amplifier is a conventional direct-coupled design with the two output transistors connected in the complementary symmetry mode but operating in pure class-B mode, ie, there is no quiescent current to reduce crossover distortion. We are not concerned with crossover distortion in this design, partly since providing a quiescent current would increase overall current drain which is not desirable if operating the Theremin from a battery. Celebrating 30 Years As it turns out, as you can see from the Scope 4 waveform, crossover distortion is not noticeable in the output. The total current drain is mostly due to the collector current of Q8, the class-A voltage gain stage of the amplifier. Maximum power output is about 400mW into an 8-ohm speaker. One interesting point to note about the amplifier is that we are using a standard arrangement whereby the loading on the collector of Q8 is reduced by “boot-strapping” from the output. Instead of connecting the 470Ω collector load for Q8 to the 0V rail, we have connected to the speaker active terminal, ie, at the negative January 2018  29 Scope 3: this scope grab shows the signal at the output of the mixer, Q3, measured at its collector. Its amplitude is varied by JFET Q6 before being fed to the volume control, VR1, and the audio amplifier. Scope 4: the output from the audio amplifier, across the loudspeaker. Note that there is no visible crossover distortion despite the fact that there is no quiescent current in the output transistors: this is operating in pure class B. electrode of the 470µF output coupling capacitor. By dint of the emitter-follower action of output transistors Q9 & Q10, the AC load impedance “seen” by the collector of Q8 is a great deal higher than 470Ω. In effect, because of the emitter-follower action, the AC voltage (ie, the audio signal voltage) is virtually the same at either end of the 470Ω resistor and therefore the AC current is greatly reduced. Note that the small DC load current of Q8 flows through the voice coil of the loudspeaker to the 0V rail. This improves the gain, linearity and output voltage swing of Q8. The only potential drawback of this circuit is that if the loudspeaker is disconnected, Q8 has no current path and therefore the amplifier latches up, drawing negligible current. By the way, we should also note that running the Theremin from battery power will have a drawback, since the virtual earth effect provided by those two 470nF capacitors. Therefore, the effects of hand capacitance may be reduced to some extent. Power supply Power for the circuit comes from a 9VAC plugpack. A 12V battery can also be used but may not give the performance of an AC supply. Note that a switchmode 12V DC supply is not suitable for this project due to the large amounts of harmonics and noise they normally emit – it’s a fair bet that would either interfere with the oscillators, get into the audio amplifier . . . or both. Switch S1 applies power to the circuit. The 470nF capacitors on each side of the input supply ground the AC connections and swamp any capacitance effects of the plugpack to ground. This ensures there are no spurious sounds from the Theremin due to the plugpack. As a side benefit, the 470nF power supply capacitors provide a virtual earth effect so that the hand capacitance is more effective for the pitch and volume controls. The 9VAC is rectified by bridge rectifier BR1 and then filtered with a 1000µF capacitor to provide a relatively smooth ~12VDC supply for REG1, a 9V regulator that delivers a stable 9V DC to the circuit. A 470µF capacitor close to the regulator output ensures stability of the regulator and can provide any short term peak current for the amplifier. LED1 shows that the power is on. 30 Silicon Chip Construction All of the circuit components are accommodated on a relatively compact PCB which also provides the volume control plate – making it easy to build. The pitch antenna is a 400mm length of 10mm aluminium tube, inserted into a hole in the front panel and PCB and making contact with the circuitry via two springy contacts, which are actually the contacts from a standard 3AG PCB fuseholder. The two tuning capacitors are mounted directly on the PCB and their thumbwheels protrude slightly from each side of the box, in our case a standard plastic UB1 Jiffy box. Begin construction by installing the resistors. You can check the colour code for each resistor value by referring to the table of resistor values later in this article. However, whether or not you are familiar with the resistor colour code, we strongly suggest that you check each resistor value with a digital multimeter before it is inserted and soldered into place (some colour bands are notoriously similar to others). Resistors are not polarised and can be inserted either way into the board. But it is good practice to install them so that their colour codes all align in the same direction (eg, tolerance band at the bottom or on the right). This makes it so much easier to check their values later on. The four 330µH inductors can be placed now. Next, install the capacitors. There are three types used in this circuit. One type is MKT polyester, recognised by their small “block” shape. The second type is disc-shaped ceramics. Neither polyester nor ceramic capacitors are polarised – they can be inserted either way around. They are usually marked with a code (shown in the small capacitor code table) to indicate their capacitance. Celebrating 30 Years siliconchip.com.au The third type of capacitors are the electrolytics. These are (usually) cylindrical in shape and with rare exception (and none in this circuit) are polarised – they must be inserted the right way around, as shown on the PCB overlay. They have a polarity marking of “–” symbols along one side which indicates the negative lead. Next to go in are the semiconductors, all of which are polarised. Install diode D1 and the bridge rectifier, BR1, followed by the transistors. Make sure you put the correct transistor in each position – some look identical. Note that the PCB is designed for the PN2222A transistors for Q1, Q2 and Q4. If using 2N2222A transistors, they will require insertion at 180° to that shown on the PCB overlay, with the base lead bent back to fit the PCB hole position. Transistors Q9 and Q10 are mounted horizontally with the metal face toward the PCB. Their leads are bent down 90° to insert into the PCB holes. As well as soldering, these transistors are attached to the PCB with M3 x 10mm screws and nuts with the screw placed from the solder side of the PCB and the nut on the transistor. Attach the screw and nut before soldering to ensure they fit in the right position. REG1 is mounted horizontally, similarly to Q9 and Q10 but is mounted on a small heatsink that is sandwiched between it and the PCB. Bend the leads down 90° before inserting into the PCB, secure the tab to the heatsink and PCB using an M3 x 10mm screw and nut and then solder the leads in place. CON1 and S1 can be installed now. Make sure these two parts are mounted hard up against the PCB before soldering. The two fuse clips which make contact with the pitch antenna can then be soldered in. The clips may require opening out a little to ensure a good contact with the 10mm aluminium tube antenna. CON2 is for making connection to the loudspeaker. Install the 2-pin header on the PCB. The 2-pin socket is wired to ~100mm lengths of hookup wire by crimping the wire ends to the crimp connectors first (you can solder these too for a secure joint) and then inserting into the socket shell. The other ends of the wire are soldered to the loudspeaker terminals. LED1 mounts horizontally inside the cutout in the PCB, with the leads bent to insert into its holes in the PCB. Make sure the polarity is correct – the longer lead is its anode. The two plastic dielectric tuning capacitors (VC1 and VC2) are secured to the PCB by two short M3 screws (they should be supplied with the capacitors). Their three tag leads need to be bent at right angles to insert into the holes on the PCB. They are then soldered in place. Cut the potentiometer shaft to 12mm in length from its end to where the threaded boss starts. Snap off the location spigot and install onto the PCB. Testing Check your construction carefully to make sure there are no mistakes – especially the orientation of all polarised components (electrolytic capacitors, diode, transistors and regulator) and the right components are in the right places. If you are satisfied that all is correct, plug in your 9VAC plugpack (or 12V DC battery – positive to centre pin) and switch on. LED1 should light up. We have included quite a few test points on the PCB. These are labelled from one siliconchip.com.au Parts List – Theremin 1 PCB coded 23112171, 226 x 85mm (includes integral volume plate) 1 UB1 Jiffy box 158 x 95 x 53mm 1 9VAC 350mA plugpack 1 ~400mm length of 10mm diameter aluminium tube (for pitch antenna) 2 Mini tuning gang capacitors (includes thumbwheel and mounting screws) (VC1,VC2) [Jaycar RV-5728] 4 330µH chokes (L1-L4) [Jaycar LF-1106 Altronics L 7040] 1 3” loudspeaker (4Ω or 8Ω) 1 knob to suit pot 1 PCB mount SPDT toggle switch (S1) [Altronics S 1421] 1 PCB mount DC socket (2.1 or 2.5mm) (CON1) 1 mini heatsink 19 x 19 x 9.5mm 2 M205 fuse clips 4 M3 tapped x 9mm standoffs 11 M3 x 10mm screws (4 are optional. See text) 3 M3 nuts 1 2-way pin header socket 1 2-way pin header plug (CON2) 4 stick-on rubber feet (the taller the better!) 1 PC stake for TP GND 1 15mm length of 10mm diameter heatshrink tubing Semiconductors 3 PN2222 NPN transistors (Q1,Q2,Q4) [or 2N2222A (see text) Jaycar ZT-2298; Altronics Z 1166] 2 BC547 NPN transistors (Q3,Q7) 2 BC327 PNP transistors (Q5,Q8) 1 2N5484 JFET (Q6) 1 BD139 NPN transistor (Q9) 1 BD140 PNP transistor (Q10) 1 BAT46 schottky diode (D1) 1 7809 9V regulator (REG1) 1 W04 1A bridge rectifier (BR1) 1 3mm high intensity blue LED (LED1) Capacitors 1 1000µF 25V PC electrolytic 3 470µF 16V PC electrolytic 1 220µF 16V PC electrolytic 1 22µF 16V PC electrolytic 1 10µF PC electrolytic 2 470nF MKT polyester 2 100nF MKT polyester 2 10nF MKT polyester 2 2.2nF MKT polyester 8 100nF ceramic 1 10nF NP0 (COG) ceramic 2 330pF NP0 (COG) ceramic 1 100pF ceramic 1 47pF NP0 (COG) ceramic 1 18pF NP0 (COG) ceramic 1 4.7pF NP0 (COG) ceramic Resistors (0.25W, 1%) 1 820kΩ 3 560kΩ 1 330kΩ 3 150kΩ 4 33kΩ 3 10kΩ 1 5.6kΩ 1 1.5kΩ 1 470Ω 1 220Ω 1 100Ω 1 22Ω 1 50kΩ 16mm log pot (VR1) Celebrating 30 Years 2 100kΩ 7 1kΩ 2 1Ω 5% January 2018  31 This close-up of the PCB shows how the two variable capacitors (actually mini AM radio tuning gangs) are fastened in place. If you find the knob catches or binds on the board or case, you may need to adjust the position or deepen the slot. through to twelve, with additional test points labelled TP GND, 9V, 9V’, 9V1’, 9V’2 and 9V’4. Connect the negative lead of your multimeter to TP GND. TP 9V should measure close to 9V (but can range from 8.85 to 9.15V). Test point 9V’ should be around 8.6V and test points 9V’1’, 9V’2’ and 9V’4’ should be around 8V to 8.6V. Test points 1, 3 & 5, should be about 1.0V, although TP5 might be a little lower at around 0.8V instead. TP2, 4 & 6 should be at 0.4V, with TP6 possibly as low as 0.22V. TP7 should be around 1.1V and TP8, 0.6V. Test point 9 will depend on the setting of VC2, but should be in the range of 2V to 8.6V and adjustable with VC2. Test point TP10 should be 6.2V. Connect the loudspeaker for the next readings. You should measure 5.5V at TP11 while TP12 should be around 5.3V. If all the voltages measure correctly, remove power ready for installation in its box. Housing We housed our Theremin in a UB1 Jiffy box (as we believe most constructors will do) but for authenticity, you might like to make your own timber box just like Léon Theremin’s original design (and most early models). That’s up to you. The PCB is mounted upside-down on the lid of the box (so that the component side is facing downward). If you make a timber box, it should have the same or similar arrangement. Four 3mm holes in the lid hold the PCB in place. Three slots need to be cut in the top edge of the box itself. One is to allow the volume plate (part of the PCB) to emerge from the left side, while two others allow the dials attached to VC1 Resistor Colour Codes                32 Qty 1 3 1 3 2 4 3 1 1 7 1 1 1 1 2 Value 820kΩ 560kΩ 330kΩ 150kΩ 100kΩ 33kΩ 10kΩ 5.6kΩ 1.5kΩ 1kΩ 470Ω 220Ω 100Ω 22Ω 1Ω# Silicon Chip 4-Band Code (1%) 5-Band Code (1%) grey red yellow brown grey red black orange brown green blue yellow brown green blue black orange brown orange orange yellow brown orange orange black orange brown brown green yellow brown brown green black orange brown brown black yellow brown brown black black orange brown orange orange orange brown orange orange black red brown brown black orange brown brown black black red brown green blue red brown green blue black brown brown brown green red brown brown green black brown brown brown black red brown brown black black brown brown yellow purple brown brown yellow purple black black brown red red brown brown red red black black brown brown black brown brown brown black black black brown red red black brown red red black gold brown brown black gold gold (#: 5%)     n/a Celebrating 30 Years and VC2 to emerge from the front and back. Other holes required are in the right end (7mm for the volume control pot, 10mm for the power socket; 5mm for the power switch) along with one 10mm hole in the box lid for the Pitch Antenna to pass through (plus the four already mentioned for holding the PCB in place). The base of the box will also need a series of holes to let the sound out for the loudspeaker. We have provided diagrams for all of these holes. You can either measure and mark the hole positions or photocopy the diagrams and use them as templates (or download the diagrams from siliconchip.com.au, print those out and use them as templates). Attach the two thumbwheels to VC1 and VC2 with the supplied M3 screws. Make sure that the thumbwheels do not bind against the PCB when they are rotated. If they do, you may need to file a little off the thumbwheel bush to provide extra clearance above the PCB. Glue the loudspeaker to the base of the box using contact adhesive, silicone sealant or similar. Rubber feet are attached to the underside of the box to raise it for sound to escape. Installation in the box While the PCB can be secured to the box by means of the potentiometer nut, we elected to also secure the PCB to the lid of the box using four 10mm M3 tapped spacers, each with a 5mm M3 screw top and bottom. (Alternatively, you could use 10mm untapped spacers with a 20mm M3 screw and nut, right through from the front panel). This approach does make the installation of the PCB in the box slightly more difficult but it can be done – as our photos prove! Small Capacitor Codes    Qty  2  2  2  2  8  1  2  1  1  1  1 Value/Type 470nF MKT 100nF MKT 10nF MKT 2.2nF MKT 100nF ceramic 10nF ceramic 330pF ceramic 100pF ceramic 47pF ceramic 18pF ceramic 4.7pF ceramic EIA 474 104 103 222 104 103 331 101 47 18 4.7 IEC 470n 100n 10n 2n2 100n 10n 330p 100p 47p 18p 4p7 siliconchip.com.au Learning more about the Theremin (and even learning how to play it!) The internet has thousands of examples of Theremin exponents. (just Google “Theremin”). Many of them are brilliant musicians and they really know how make this instrument literally “sing”. One of the best is actually Léon Theremin’s grand-niece – Lydia Kavina’s demonstration at www.bbc.com/news/magazine -17340257 is only a couple of minutes long but is well worth watching. On the same page is an interesting article by Martin Vennard, of the BBC World Service, about Léon Theremin and the instrument he invented. Lydia Kavina demonstrates the instrument her greatuncle invented. Kavina’s Theremin rendition of Debussy’s Clair de Lune is simply enchanting. Search online for her other music. Another masterful example of Theremin playing is in the nearly 17-minute long video at https://youtu.be/MJACNHHuGp0, where Carolina Eyck, a German composer and Theremin player (reputed to be one of the world’s best) not only demonstrates her prowess on the instrument but as she does, she explains in some detail just how she plays it. Admittedly, the Theremin she plays is considerably more complex (and expensive!) than our simple model and offers a range of user controls which would scare off all but the most expert of players. But this video will help you gain a real understanding of the intricacies of the Theremin – especially if you want to get more from it than just the usual howls and squeals of a novice player! Carolina Eyck explains what the Theremin can do! Slide the box lid/PCB assembly into the box with the switch lever and potentiometer shaft emerging through the holes in the right end. Then secure the potentiometer with its nut. Install the antenna before making adjustments. The antenna is inserted 24mm into the top lid. We placed a 10mm diameter length of heatshrink tubing at the lower end of the Aluminium tube to mark when to stop any further insertion of the tube into the box. Ensure that the two thumbwheel knobs for VC1 and VC2 can move freely within the box when the lid is in place. If they bind, you may need to deepen the slots they sit in. If all The UB1 Jiffy box with the speaker glued in, plus the three slots and three holes required in the sides and ends. You will also need to drill a circular pattern of holes in the base of the box to let the sound out. siliconchip.com.au Celebrating 30 Years is OK, secure the lid to the box with its screws. Adjusting for pitch and volume Set VR1 at mid position, plug your power supply in and turn it on. Adjust the volume thumbwheel and pitch thumbwheel till a sound can be heard, then set the volume thumbwheel so that sound can be heard even when the hand is near the plate. Adjust the pitch thumbwheel with left hand index finger and hand over the volume plate. That is so the hand is kept away from the pitch antenna. Adjust the pitch thumbwheel trimmer for a zero beat with your hand away from the pitch antenna. Frequency should rise as your hand is brought near to the antenna. With your hand close to the volume plate, adjust the volume control trimmer for a minimum loudness. Note that it is not possible for the volume trimmer to completely turn off the sound, for the reason already explained. These adjustments will have to be repeated each time the unit is set up in a different position. You will find the Theremin is capable of an endless variety of sounds. January 2018  33 This photo shows the PCB mounted on the box lid, ready for installation. The PCB “hangs” from the box lid with the components underneath. The pitch antenna goes through the lid, through a matching hole in the PCB and is held in place with the spring fuse clips you can see near the power switch (left end). Low grunts and growls can be produced by a quick, sweeping motion of the hands. Similarly, one can obtain wails and squeaks in the high range. To produce a vibrato effect, hold the volume hand in a fixed position and flutter the pitch hand near to the antenna at the desired rate. Finer changes can be made by moving the fingers while the hand remains still. Similarly, to create a tremolo effect, hold the pitch hand in a fixed position and flutter the volume hand. (You will see the two ladies playing the Theremin in our examples [see panel] make extensive use of their fingers). As we mentioned earlier, if you are interested in altering the voicing, you can add in capacitance between the emitters of Q1 and Q2, shown on both the circuit and PCB overlay as C1. Somewhere around 220pF to 470pF is a good starting point when experimenting but you could go higher or lower than this without risking anything. SC (Above): this drilling/ cutting diagram for the UB1 Jiffy Box is reproduced half size so you will need to enlarge it 200% if using as a template. The front panel we glued to the box lid for a really professional finish. This can also be downloaded from siliconchip.com.au if you want to print it on heavier or glossy stock. 34 Silicon Chip Celebrating 30 Years siliconchip.com.au