Fullscreen HTML5Med Res (??MB)HTML4

Create eerie musical sounds with the: Opto-Theremin, Create your own electronicallysynthesised music or produce eerie science fiction sounds with our new “Opto-Theremin”. This completely new design uses an optical proximity sensor to provide a more effective volume control plate which adds the possibility of rapid tremolo, while vibrato can be applied in the normal way with the vertical pitch antenna. By JOHN CLARKE Unlike conventional Theremins, the new Opto-Theremin uses an optical distance sensor to control the volume, making the unit easier to build and adjust. A metal antenna rod is used for pitch control. 20  Silicon Chip T HIS LATEST THEREMIN from SILICON CHIP merges the traditional with the modern. As well as the optical proximity control plate, it includes a touch of ‘bling’ in the form of blue LEDs and polished aluminium tubes. Even the top of the pitch antenna is illuminated with blue light. For those who don’t know what a Theremin is, it is an electronic musical instrument designed by Leon Theremin in the early 1900s. Pitch and volume are varied by moving your hands near two antennas and a wide range of tones covering several octaves can be produced. Just do a Google search for Theremin to see a selection of YouTube videos of Theremin performances. All of those demonstrations involve Theremins of largely traditional format. The Theremin owes its popularity to its extreme versatility and to its unique sound compared to conventional instruments. Even a simple combination of hand movements can lead to interesting effects. Theremin passages can comprise a smooth gliding tone (glissandi) or can be separate notes (staccato), or a combination of both. It really is a versatile instrument, limited only by the skill of the player. Our Opto-Theremin operates in a radically different manner to traditional Theremin designs. The ‘Opto’ prefix refers to its use of an optical volume control and to the blue LEDs which add visual interest – the ‘bling’. Before anyone starts worrying that our new Opto-Theremin may have lost its heritage, be assured that it sounds just like a traditional Theremin and is played in exactly the same way. For example, the Opto-Theremin still has a vertical antenna for pitch control just siliconchip.com.au Pt.1 like a traditional Theremin, whereby the right hand is moved horizontally to change pitch. The big difference compared to a conventional Theremin is the volume control. As with the original, the left hand is moved vertically to control the volume but this movement is sensed using an optical proximity sensor rather than the traditional horizontal loop-shaped antenna. Why use optical sensing? This solves a number of problems. Traditional Theremins use RF (radio frequency) oscillators to feed the antennas for both pitch and volume control. Without careful tuning, there can be all sorts of interactions between the volume and pitch oscillators, leading to unwanted ‘squawks’ in the sound or pitch changes when the player is only trying to adjust the volume. By using optical sensing for the volume control instead, there’s no chance of any interaction with the pitch control circuitry. Additionally, the volume action is always predictable and does not drift with temperature changes. Plus it makes the set-up procedure much easier. We’re still mixing two high-frequency oscillators to produce the audio signal as this results in sounds with the required waveform to imitate musical instruments, such as a cello. So although this new Opto-Theremin has a different method for volume control, it still uses RF techniques to generate the pitch, allowing it to produce the classic Theremin sound. Features In order to play a Theremin, the siliconchip.com.au musician must be able to accurately position one hand near the antenna, to produce the required pitch. The generated tone has to be set ‘by ear’, just as for a violin or a trombone. This is because the Theremin does not have a fixed set of notes but instead deliv- ers a continuous range of tones over several octaves. Correct linearity of pitch variation in response to hand movement is a critical feature of the design. In this case, ‘linearity’ means that there is a similar range of hand movement for September 2014  21 FOR HAND PITCH CONTROL ANTENNA L1 EQUALISING COIL VOICING VC1 HAND VOLUME SIGNAL BUFFER • • • • • • • • VOLUME VR1 INVERTER & LEVEL SHIFTER (IC2a, VR4) External pitch adjustment control Linear pitch change with hand movement over four octaves Linear volume control with hand movement Adjustable hand volume range Voicing adjustment (internal) Integral loudspeaker with volume control Minimal pitch drift during warm-up No volume control drift during warm-up 9VAC or 12V DC operation <at> 250mA (eg, from AC plugpack or 12V battery) Line output level: 250mV RMS Frequency range: <40Hz to >5kHz 22  Silicon Chip Vref Q4 Main Features • LOUDSPEAKER CARRIER each octave. It’s important that no octave is compressed into a very small hand movement range, as this would make the instrument difficult to play. The Opto-Theremin is designed to avoid this and it includes a test circuit to assist in correctly adjusting the linearity. An adjustment is also included to modify the tonal quality or ‘voice’ of the Opto-Theremin. This allows it to be adjusted from producing a sinusuoidal (or pure) tone through to a sound that’s reminiscent of a cello at low frequencies and a soprano voice at higher frequencies. In addition, an externally adjustable pitch control provides compensation for changes in pitch due to the unit’s location and its surroundings and/or due to temperature variations. The unit contains an in-built ampli- • • LINE OUTPUT CON3 MIXER IC1 VR2 ADJUST PITCH DISTANCE SENSOR (SENSOR1) BUFFER IC2b Q3 REFERENCE OSCILLATOR (T2, Q2) V+ LOW PASS FILTER BUFFER PITCH OSCILLATOR (T1, Q1) IC3 AUDIO AMPLIFIER Fig.1: block diagram of the Opto-Theremin. The pitch and reference oscillators are mixed together in mixer IC1 and filtered to provide the tone, while Sensor1 controls the volume by adjusting the output level from the mixer. fier and loudspeaker but it also has a “Line Out” socket on the front panel so it can be connected to an external amplifier and loudspeaker system. The loudspeaker volume is independently adjustable so it can be silenced when using an external amplifier or alternatively, used as a monitor speaker during on-stage performances. Appearance & controls As shown in the photos, the SILICON CHIP Opto-Theremin is housed in two plastic cases, one to accommodate the main PCB (and support the pitch antenna) and a smaller one to house the distance sensor PCB for the volume control. They are connected by threaded rods housed within aluminium tubes and the whole assembly mounts on a timber pedestal via another set of aluminium tubes and rods. The vertical pitch antenna is also made from aluminium tube and is easily detached for transportation. The volume control box is translucent and lights up during operation to make it look ‘cool’. A translucent dome at the base of the pitch antenna is also lit using blue LEDs, while a separate blue LED illuminates the transparent cap at the top. These blue LEDs not only give the Opto-Theremin an impressive appearance but also reflect from the player’s hands when the instrument is being played, for even greater visual effect. The three external controls (power, volume and pitch) are arranged along one side of the case, together with the line output socket. Power can come from a 9VAC supply or from a mainsderived 12VDC supply or battery. Note that a switchmode DC supply (eg, a switchmode DC plugpack) is not suitable for use with the OptoTheremin. That’s because noise from a switchmode supply would find its way into the two onboard oscillators and upset the operation. Operating principles Fig.1 shows the block diagram of the Opto-Theremin. It uses two oscillators: (1) a pitch oscillator and (2) a reference oscillator. Both oscillators are set to run at close to 455kHz. The reference oscillator includes pitch adjustment VR2, to precisely trim the frequency. While the reference oscillator basically runs at a fixed frequency, the pitch oscillator is varied via the attached antenna. Any hand movement adjacent to the pitch antenna alters its coupling to ground and this changes the frequency of oscillation. Both oscillator outputs are buffered to isolate them from the following mixer stage, an MC1496 balanced modulator (IC1). As shown, the signals are fed to the SIGNAL and CARRIER inputs of IC1. Its output comprises several frequencies, including the sum and difference frequencies of the reference and pitch oscillators. If the two oscillators are almost at the same frequency, eg, 455kHz and 454kHz, then the sum of the two frequencies will be 909kHz while the difference frequency will be 1kHz. The low-pass filter on the mixer’s output removes all frequencies above 3.3kHz, leaving only the difference frequency; in this case, 1kHz. The resulting 1kHz audible tone is then fed to unity gain op amp stage IC2b which buffers it and provides the siliconchip.com.au Volume control As mentioned, we use an optical distance sensor (made by Sharp) for the volume control. It comprises an siliconchip.com.au OBJECT AT ~ 300mm LE C TE D LIG HT SCATTERED LIGHT RE F line output signal. This also drives a small internal power amplifier (IC3) and loudspeaker. So far, we haven’t mentioned the equalising coil that’s connected between the pitch antenna and the pitch oscillator. This vastly improves the linearity of the pitch oscillator’s response as it changes frequency due to hand movements near the antenna. Without it, relatively small hand movements would cause large frequency changes at the higher octaves. The equalising coil works by forming a tuned circuit in conjunction with the capacitance of the antenna. Its resonant frequency is set to just below the pitch oscillator’s frequency by its 9mH inductance and the antenna’s ~14pF capacitance. Moving a hand closer to the antenna increases this capacitance, thereby reducing L1’s resonant frequency. In practice, changes to the equalising coil’s resonant frequency will be much greater than any corresponding frequency changes in the pitch oscillator. This is because hand capacitance effects of just few picofarads will have a far greater effect on the antenna’s 14pF capacitance (and hence the resonant frequency of the equalising coil) than on the much larger 220pF capacitor that’s in parallel with the 560µH pitch oscillator coil (both contained within a 455kHz IF transformer). So, with the equalising coil, hand capacitance changes have a greater effect on the pitch oscillator for hand movements further away from the antenna than closer in. This nonlinearity counteracts the non-linearity of the pitch oscillator’s sensitivity to capacitance changes and results in the required linear response. For further information on this, see www.element14.com/community/ thread/1802/l/theremin-linearity Trimmer capacitor VC1 adjusts the coupling between the pitch and reference oscillators. This is the ‘Voicing’ adjustment and it affects the waveshape of both oscillators due to intercoupling, thus also affecting the output waveform shape. In practice, it’s just a matter of setting VC1 to obtain the required sound from the Opto-Theremin. OBJECT AT ~ 40mm RE LENS C FLE TE D LIG HT SCATTERED LIGHT PULSED INFRARED LED CHARGE-COUPLED IR SENSOR ARRAY DISTANCE SENSOR Fig.2: how the optical distance sensor works. As the object moves away from the pulsed infrared LED, the angle of the reflected light passing through the lens changes and this changes the position of the light spot focussed onto a charge-coupled sensor array (or CCD). infrared transmitting LED, a receiving lens and a sensor array. The LED and the receiving lens are spaced about 20mm apart, while the sensor array is a Charge Coupled Device (CCD) consisting of numerous light sensors arranged in a single row. In operation, the LED is pulsed so that it produces high-intensity flashes of infrared light focused as a small dot. If an object is within the sensor’s range of measurement, the infrared light will be reflected and some of it focused by the lens. If the reflecting object has an uneven surface, the infrared light will tend to be scattered – see Fig.2. However, part of the light will be reflected back to the lens which then focuses it on the CCD. The exact position of the light spot on the CCD will depend on several things: (1) the spacing between the IR LED and the lens, (2) the distance between the focal point of the lens and the CCD’s light-sensitive surface, and (3) the distance from the reflecting object to the sensor. The first two distances are fixed by the sensor itself, leaving the distance between the sensor and the reflecting object as the variable. If the object is close to the sensor, the reflected light will be focussed towards the outside edge of the CCD. However, as the object moves further away, the reflected light angle becomes progressively shallower. As a result, the reflected light progressively moves towards the other end the CCD. The sensor includes circuitry to detect where the light is focussed on the CCD and processes this information to produce a voltage output that varies with distance. Note that the object does not need to be perfectly flat or parallel to the sensor. The sensor will detect the object as long as there is sufficient scattered light from the object to reach the lens. Sharp makes several different versions of the distance sensor, each with different optics that set the range of distance measurements. The OptoTheremin uses the GP2Y0A41SK0F sensor which has a range of 40-300mm. For further information on this device, refer to the data sheet at www. sharp.co.jp/products/device/doc/opto/ gp2y0a41sk_e.pdf The output from the distance sensor drives IC2a which inverts and level shifts the signal. IC2a’s output then supplies bias current to mixer stage IC1, to control the volume. Inverter September 2014  23 Background To The Theremin In 1919, Russian Physicist Lev Termen (or Leon Theremin as he is called in the western world) invented an electronic musical instrument called the “Theremin”. At that time, the Theremin was innovative and unique in the musical world and was essentially the first electronic instrument of its kind. Playing it relied solely on hand movements in the vicinity of two antennas to control two electronic oscillators – one antenna to vary the pitch of the sound and the other to change the volume. The Theremin was subsequently further developed and manufactured by the Radio Corporation of America (RCA) around 1929. General Electric (GE) and Westinghouse also made Theremins in the 1920s. However, the number of units produced was quite modest, totalling about 500. Today, the Theremin is hailed as the forerunner to modern synthesised music and was instrumental in the development of the famous Moog synthesisers. There is also a website devoted to Theremins (www.thereminworld.com). Because of its unique sound, it has been popular with music producers for both film and live performances. The sound is ideal for setting the scene for supernatural events and for close encounters with extraterrestrial beings in science fiction movies. A Theremin was used to produce background music in the feature film “The Ten Commandments” by Cecil B DeMille (1956). Its eerie sounds have also made it ideal for science fiction movies such as “The Day the Earth Stood Still” (1951), “Forbidden Planet” (1956) and “Mars Attacks!” (1996). The Beach Boys also used an instrument similar to the Theremin – called an Electro-Theremin (also named a Tannerin) – in their 1966 hit, “Good Vibrations”. More information on Theremins is available at www.thereminworld.com/ Theremin-Models Finally, SILICON CHIP has produced four previous designs for home construction: a basic Theremin in August 2000, a MIDI Theremin in April/May 2005, the Mini Theremin in July/August 2006 and the Mk2 Theremin in March 2009. stage IC2a is necessary because the output voltage from the sensor reduces with distance but we want the volume to increase as the hand is moved further away (ie, upwards). Circuit details Fig.3 shows the full circuit details of the Opto-Theremin. As well as the distance sensor (SENSOR1), it uses three low-cost ICs (IC1-IC3), four JFETs (Q1-Q4), several coils and sundry other parts. Both the pitch and reference oscillators utilise pre-wound 455kHz IF (intermediate frequency) transformers (T1 & T2), as commonly used in AM radio tuners. Each of these stages is connected as a common drain Hartley oscillator, with T1 & Q1 forming the pitch oscillator and T2 & Q2 making up the reference oscillator. T1 has a tapped primary winding with a parallel-connected capacitor to form a tuned circuit. Its resonant frequency can be varied using a ferrite slug which screws into the core. Q1 drives a portion of the tuned circuit winding via the tapping at pin 2, while the signal at the top of the tuned wind24  Silicon Chip ing is coupled to the self-biased gate of Q1 via a 68pF capacitor. This arrangement provides positive feedback to maintain oscillation at the tuned frequency. The second winding inside T1, at pins 4 & 6, provides a low-impedance output signal. This signal is fed to the gate of JFET Q3 via a 330pF capacitor. Q3 is wired as a source follower stage, buffering the signal from T1 and feeding it to pin 1 (SIG IN+) of mixer IC1. Current is fed to Q1’s drain via a 680Ω resistor connected to the 9V DC supply rail, while Q3’s drain current is set by a 100Ω resistor to ground. The reference oscillator is very similar to the pitch oscillator, the difference being that JFET Q2 is powered via 1kΩ potentiometer VR2 and a 220Ω resistor. VR2 varies Q2’s drain-source current to provide pitch adjustment since altering this current affects Q2’s gate-source capacitance. This in turn alters the reference oscillator’s tuned frequency. Q4 buffers the signal from the reference oscillator, feeding it to pin 8 (CARRIER IN+) of IC1. Equalising coil L1 is connected directly to pin 1 of T1 by placing jumper link LK1 in its NORMAL position. Moving LK1 to the TEST position means that the equalising coil is in series with a 100kΩ resistor. Diode D1 connects to the junction of the equalising coil and the 100kΩ resistor, while its cathode goes to test point TP1. In test mode, the equalising coil is sufficiently isolated from the pitch oscillator to allow the resonance of the coil and antenna to be monitored by a DMM set to read DC volts, connected between TP1 & TP GND. In operation, the DMM filters the rectified RF signal from D1 due to both lead capacitance and internal capacitance, and it discharges this stray capacitance via its own loading. Once the DMM is in place (and LK1 set to TEST), the slug in T1 is adjusted to alter the frequency of the pitch oscillator to give the lowest voltage reading. This sets the pitch oscillator to the resonant frequency of the equalising coil and antenna. The frequency is then adjusted slightly away from this resonance point. Mixer stage As mentioned, the signals from JFET buffer stages Q3 & Q4 are applied to pins 1 & 8 of mixer IC1 via 1nF capacitors. The signal level applied to pin 1 is around 180mV, while the level applied to the carrier input at pin 8 is reduced to around 50mV by the resistive divider at Q4’s source, preventing carrier overload. The signal inputs at pins 1 & 4 and the carrier inputs at pins 8 & 10 are all DC biased from a voltage divider connected across the 9V supply. This divider comprises the 1.2kΩ, 820Ω and 1kΩ resistors and each input is connect to the divider via a 1kΩ resistor. Note that the SIG IN- and CARRIER IN- inputs (pins 4 & 10) are only DC biased, with any AC shunted to ground via 100nF capacitors. The 680Ω resistor between pins 2 & 3 of IC1 sets the gain of the mixer, while the bias voltage applied to pin 5 (from IC2a) sets the signal level at the two output pins (6 & 12). As shown, these outputs are biased using 2.2kΩ pull-up resistors (to the 9V rail) and filtered using 22nF capacitors to remove ultrasonic signals. Unity gain op amp stage IC2b buffers the low-pass filtered audio signal from pin 6 of IC1. The signal is AC-coupled via a 100nF capacitor to IC2b’s noninverting input (pin 5), while a resissiliconchip.com.au siliconchip.com.au September 2014  25 68pF 10 µF 100k G 2 3 1 IN 470Ω LED6 GND OUT K K λ A 4 6 4 6 K A 470Ω λ LED7 A 1000 µF 25V K MAX NORMAL 39Ω 100Ω G +9V 1.2k 100Ω G CON5 CON2 100k 330pF 100k 330pF TP GND TP1 D3 1N4004 T2 (WHITE) T1 (WHITE) 100k A D1 1N4148 THE OPTO THEREMIN DISTANCE SENSOR GND Vcc GP2Y0A41SK0F Vout 3 2 1 VC1 2-10pF 100pF 3 2 1 NRML LK1 TEST REG2 7805 68pF 220Ω VOICING PITCH ADJUST 100k G SENSOR1 S D S D ~9mH L1 EQ. COIL +9V S D S D A D1 TPS LK2 47k 1nF Q4 2N5485 1k 820Ω 1nF Q3 2N5485 K OUT– IC1 MC1496 VOLUME SPAN VR4 10k 1 100k 100nF +9V A 2.2k 1k IC2: TL072 K VR1 1k K K A 100nF K A S1 D 2N5485 S 470nF – + 3 1 VP 100nF GND IN 8 +~~– OUT K 560Ω LED2 λ λ K 560Ω λ LED4 A K λ LED3 A 1 2 3 GP2Y0A41SK0F A K A 10 µF 100nF 8Ω LOUDSPEAKER +9V CON4 GND OUT GND W04 IN REG1 7809 –OUT 5 10 µF CON3 LINE OUT 10 µF LED1 150Ω 100k +OUT 7809, 7805 6 PWR GND 100nF 10 µF 470 µF 25V ~ 470nF SIG G ND POWER G 7 IC3 TDA7052A 2 INPUT ~ 4 +9V 4 DC VC +11V 8 IC2b 100nF 6 5 BR1 W04 LEDS1–7 CON1 9VAC OR 12V DC INPUT D2 1N5819 (BODY) VOLUME VR3 5k 100k MAX VOLUME SET 10k 100k 100nF 2.2k 22nF 22nF D2, D3 A LED5 λ 12 6 6.8k 8 CARRIER IN+ GND BIAS 5 14 CARRIER IN– 4 SIG IN– 10 IC2a 100nF 3 2 82Ω 100nF 1k 1k 100nF 100nF 1k 1k 3 OUT+ 680Ω 2 GAIN 1 SIG IN+ +9V Fig.3: the complete circuit diagram for the Opto-Theremin. JFET Q1 & transformer T1 form the pitch oscillator, while Q2 & T2 form the reference oscillator. Their outputs are buffered by Q3 & Q4 and mixed in IC1. Pin 6 of IC1 then drives the line output socket via buffer stage IC2b, while IC2b drives audio amplifier stage IC3. Sensor 1 is the optical distance sensor. Its output is buffered and inverted by IC2a which then drives the BIAS input of IC1 to control the volume. SC 20 1 4 HAND VOLUME VOLUME CONTROL BOARD 100nF Q2 2N5485 (BODY) VR2 1k Q1 2N5485 680Ω ANTENNA SHARP LED6 A Vcc GND A Vo LED7 470Ω 3 2 1 10 µF REG2 7805 RANGE SENSOR SENSOR1 D3 1N4004 C 2014 23108142 470Ω Fig.4: install the parts on the two PCBs as shown in this parts layout diagram, starting with the main PCB assembly as shown below. If you are using SMD ICs for any of IC1-IC3, then these should be installed on the back of the main PCB as shown in Fig.5. Note that equalising coil L1 must be secured to the PCB using an M4 x 25mm Nylon or polycarbonate screw and nut (do not use a metal screw). GP2Y0A41SK0F 1000µF 25V CON5 24180132 Vo GND V+ 82Ω ~ – BR1 + ~ 100nF TP GND VR4 10k 100nF100nF A The main PCB has been designed to accept either DIP or SOIC (surface-mount) ICs (IC1 & IC3 are SOICs on this assembly, while IC2 is a DIP IC). D1 4148 10 µF + 1 (SMD under) 1 100k THEREMIN C 2014 TEST 23108141 A LED1 LED3 A A A LED4 tive divider consisting of two 100kΩ resistors across the 9V supply biases this input to 4.5V. IC2b’s output appears at pin 7 and is fed to the Line Out socket (CON3) via PITCH ANTENNA 560Ω LED2 100nF 1nF 2N5485 330pF Q4 T2 REFERENCE OSCILLATOR 68pF 2N5485 Q2 39Ω TP1 1k 1k 100nF 100Ω 14180132 L1 1 (SMD under) 1 100nF M4 x 25MM NYLON OR POLYCARBONATE SCREW 1k 820Ω PITCH ADJ VR2 1k LIN 100k 100k Right: a 3-pin header is soldered to the distance sensor’s output terminals before installing it on its PCB – see Fig.6 for the mounting details. LK1 2.2k Normal 680Ω T1 2.2k Q1 100k 1k 560Ω 68pF 22nF 22nF 100pF 1nF 100nF VOICE VC1 2-10pF 1k 330pF 2N5485 GND PITCH OSCILLATOR 2N5485 Q3 100nF 100k 150Ω CON3 (WIRE TO VR2 BODY) 100k IC1 MC1496 26  Silicon Chip 680Ω 100k 100k 6.8k 1.2k 220Ω 470nF TPS 100nF IC2 TL072 SPAN 100Ω CON4 10 µF SPEAKER VR3 5k RANGE LINE OUT LED5 100nF 1k (SMD under) 5819 10k 100nF 1 1 D2 VR1 1k LIN IC3 470nF 470 µF 25V LK2 MAX. Normal 10 µF 100nF GND SPKR VOL. 100k 47k TDA7052 10 µF CON2 POWER S1 VOL. REG1 7809 GND V+ CON1 Vo Above: the completed volume control PCB. Note how the two electrolytic capacitors are bent over so that they later clear the case lid. a 10µF coupling capacitor (to remove the 4.5V DC bias voltage) and a 150Ω resistor. The 150Ω resistor isolates the op amp from any capacitive loads, preventing oscillation. IC2b’s output also feeds power amplifier IC3, a 1W bridge-tied load (BTL) amplifier. Its volume is controlled by a DC voltage at pin 4, with a range of about -70dB to +35dB for 0.4-1.2V. siliconchip.com.au Volume control pot VR1 is connected in series with trimpot VR3 and a 10kΩ resistor from the 9V supply, with VR1 being the volume control and VR3 being the maximum volume preset. VR3 allows the top of VR1 to be adjusted from 0.75-1.0V, giving a maximum gain between about -20dB and +20dB. In practice, VR3 is set so that the loudspeaker produces sufficient volume without gross distortion. The bottom end of VR1 connects to ground via Schottky diode D2. This provides a fixed bias of approximately 0.2V at the bottom of VR1 and is necessary to set the minimum volume level. Optical volume control The Sharp GP2Y0A41SK0F distance sensor (SENSOR1) forms the heart of the optical volume control circuit. Its output at pin 1 varies from about 0.4V when the hand is 300mm above the sensor, to about 2.8V at 40mm. The sensor’s output is non-linear and must be inverted and level shifted using op amp IC2a to derive the correct volume control function to drive the bias input (pin 5) of mixer stage IC1. As shown on Fig.3, the sensor’s siliconchip.com.au output is fed to the inverting input of IC2a via LK2. IC2a operates with a gain of just over -2, as set by the ratio of the 100kΩ and 47kΩ feedback resistors. IC2a’s non-inverting input (pin 3) is biased to about 1.7V by trimpot VR4 and this offsets the output by 1.7V x the non-inverting gain, ie 1.7V x (1+ 100kΩ/47kΩ) = 5.3V. VR4 allows the volume control range to be set to suit the degree of hand movement. IC2b is configured in a rather unusual way, with its output driving a red (or green) LED (LED5) and a 1kΩ resistor to ground. The arrangement ensures that the output at LED5’s cathode can swing all the way down to 0V. This is necessary because IC2a’s output can only go down to 1.8V (it’s a TL072) and we need 0V to set the minimum bias on pin 5 of IC1. So why not use an op amp that can swing down to 0V, such as an LMC6482 or LM358? The answer is that these aren’t tolerant of RF signals and produced high-frequency noise in this circuit, even with extra compensation and filtering. The TL072 doesn’t have this problem. In addition, LED5 acts as a volume indicator, displaying full brightness at maximum volume and dimming down as the volume is reduced. The output from LED5 drives the bias input of IC1 via a 6.8kΩ resistor. With 0V output, the lack of bias completely shuts down any signal at IC1’s output to provide full attenuation. The maximum output from IC2a is around 7V. So after taking the LED voltage drop into account, the maximum voltage that can be applied to IC1’s bias input is about 5.2V, sufficient to give full volume. Link LK2 is included so that the distance sensor can be bypassed. When it’s moved to the MAX position, pin 2 of IC2b inverting amplifier is tied to 0V via a 47kΩ input resistor. As a result, IC2b’s output goes high and the distance sensor no longer has any effect, making pitch adjustments easier. Power supply As stated, power for the circuit is derived from a 9VAC plugpack or from a 12V DC linear (non-switchmode) supply. RF is filtered from the incoming AC (or DC) rails by 470nF capacitors, while BR1 full-wave rectifies the AC supply. BR1 also makes the unit insensitive to DC polarity. A 470µF capacitor filters the resulting DC, while regulator REG1 provides the 9V rail to power most of the circuit. A 5V supply rail for the distance sensor is derived via diode D3 and regulator REG2. D3 provides reverse polarity protection, while the following 1000µF filter capacitor is necessary to supply the peak current for the pulsed infrared LED inside the sensor. An 82Ω resistor in series with the 11V supply input limits the peak charging current into the 1000µF capacitor. This prevents unwanted noise in the output due to the pulsing of the IR LED in the sensor. LEDs 5 & 6 illuminate the area adjacent to the volume sensor with blue light when power is applied. A 470Ω resistor in series with each LED provides current limiting. Construction Virtually all the parts for the OptoTheremin are mounted on the two PCBs. The main PCB (code 23108141) is double-sided and measures 147 x 85mm, while the volume control PCB (code 23108142) is single-sided and measures 61 x 47mm. Fig.4 shows the parts layout for both boards. Start by assembling the main PCB. This board has been designed to accept either DIP or SOIC (surface-mount) packages for IC1-IC3. DIP package ICs are installed on top of the PCB, while SOIC package ICs go on the underside, as shown on Fig.5. DIP ICs are somewhat easier to install but many types are now difficult to obtain in this package, especially the MC1496 and TDA7052. An SOIC package is still quite easy to solder though, even though its pins are closer together. If using one or more SOIC packaged (SMD) ICs, then these should be installed first (see Fig.5). Begin by placing a tiny amount of solder on one of the corner pads, then coat the remaining pads with some no-clean flux paste. That done, place the IC in position (with the correct orientation) and hold it in place using tweezers. Now solder the relevant corner pin to its pad, then check that the IC is correctly positioned, with all pins centrally located on their pads. If the IC needs adjustment, reheat the soldered pin and slide the IC to its correct position. Once it’s correct, solder the remaining pins but don’t worry about solder bridges between pins during this proSeptember 2014  27 Parts List Main Theremin Section 1 double-sided PCB with platedthrough holes, code 23108141, 147 x 85mm 1 UB1 plastic utility box, 158 x 95 x 53mm 1 front panel label, 149 x 87mm 1 9VAC 250mA plugpack 1 PCB-mount DC socket (inner diameter to suit plugpack) (CON1) 1 3-way PCB-mount screw terminal block with 5.08mm pin spacing (CON2) 1 PCB-mount 3.5mm stereo switched socket (CON3) 1 2-way polarised header, 2.54mm spacing (CON4) 1 SPDT miniature PCB-mount toggle switch (S1) (eg. Altronics S1498) 1 75mm 8Ω loudspeaker 2 1kΩ linear 16mm potentiometers (VR1,VR2) 2 knobs to suit potentiometers 1 5kΩ horizontal trimpot (VR3) 1 10kΩ multi-turn top adjust trimpot (VR4) 2 2nd IF coils (white) (T1,T2) (can be bought in a set of IF coils from Jaycar Cat LF-1050. Two sets required) 1 potcore pair and bobbin (L1) (Jaycar LF-1060 cores/LF-1062 bobbin, Altronics L 5300 cores/L 5305 bobbin) 2 M205 PCB-mount fuse-clips for antenna connection 1 2-way polarised header plug, 2.54mm spacing, with crimp pins 2 3-pin headers with 2.5mm spacing (LK1,LK2) 2 jumper shunts (for LK1 & LK2) 1 M4 x 25mm Nylon or poly­ carbonate screw (to secure L1) 1 M4 x 10mm Nylon or polycarbonate screw (for top of pitch antenna) 2 4mm ID Nylon or polycarbonate washers (spacer for L1) 3 M4 Nylon or polycarbonate nuts (to secure L1 and for top of pitch antenna) 3 M3 x 6mm machine screws 2 M3 x 10mm machine screws 3 M3 nuts 2 M3 x 9mm tapped spacers 1 100mm length of medium duty 28  Silicon Chip hookup wire (to earth VR2) 1 200mm length of medium-duty hookup wire or 100mm of light gauge figure-8 wire (for speaker) 1 12m length 0.25mm enamelled copper wire (L1) 1 400mm length of 0.7mmdiameter tinned copper wire (LED lead extensions) 1 400mm length of 1mm-diameter heatshrink tubing (LED1-LED4 leads) 1 10mm length of 20mm-diameter heatshrink tubing (L1) 7 PC stakes (TP, 3 x GND, TP1, TPS, 2 x L1) Semiconductors 1 MC1496P (DIP) or MC1496D (SOIC) balanced modulator (lC1) 1 TL072CP (DIP) or TL072CD (SOIC) dual op amp (IC2) 1 TDA7052A (DIP) or TDA7052AT (SOIC) BTL amplifier (IC3) 1 7809 3-terminal regulator (REG1) 4 2N5485 N-channel JFETs (preferably from the same manufacturer & batch) (Q1-Q4) 4 3mm blue LEDs (diffused lenses preferable) (LED1-LED4) 1 3mm red or green LED (LED5) 1 W04 bridge rectifier (BR1) 1 1N4148 signal diode (D1) 1 1N5819 Schottky diode (D2) Capacitors 1 470µF 25V PC electrolytic 4 10µF 16V PC electrolytic 2 470nF MKT 12 100nF MKT 2 22nF MKT 2 1nF MKT 2 330pF NP0 ceramic 1 100pF NP0 ceramic 2 68pF NP0 ceramic 1 2-10pF trimmer capacitor (VC1) Resistors (0.25W, 1%) 9 100kΩ 2 680Ω 1 47kΩ 2 560Ω 1 10kΩ 1 220Ω 1 6.8kΩ 1 150Ω 2 2.2kΩ 2 100Ω 1 1.2kΩ 1 82Ω 6 1kΩ 1 39Ω 1 820Ω Volume Control Board 1 single-sided PCB, code 23108142, 61 x 47mm 1 UB5 translucent blue plastic utility box, 83 x 54 x 31mm 1 Sharp GP2Y0A41SK0F 40300mm distance measuring sensor (SENSOR1) (RS Components Cat 666-6568P, Littlebird Electronics DF-SEN0143, Digi-Key 425-2819-ND) 1 3-way PCB-mount screw terminal block, 5.08mm spacing (CON5) 1 3-pin header with 2.5mm spacing (for Sharp sensor) 1 M3 x 6mm machine screw 2 M3 x 10mm machine screws 3 M3 nuts 4 3mm ID washers 1 50mm length of 1mm clear heatshrink tubing (central wire between CON2,CON5) 1 300mm length of 1mm straight steel or aluminium wire (between CON2 & CON5) 1 120mm length of 6mm diameter heatshrink tubing (packing inside aluminium tubing) Semiconductors 1 7805 3-terminal regulator (REG2) 1 1N4004 1A diode (D3) 2 3mm blue LEDs (diffused lenses preferable) (LED6,LED7) Capacitors 1 1000µF 25V PC electrolytic 1 10µF 16V PC electrolytic Resistors 2 470Ω 0.25W 1% Extra hardware 1 800mm length of 10mm-diameter (OD) x 1mm-thick aluminium tubing (cut for 450mm antenna, volume control attachment and tripod stand) 1 350mm length of M5 or 3/16-inch zinc-plated threaded steel rod (cut to 2 x 62mm and 3 x 75mm) 10 M5 or 3/16-inch nuts to suit threaded rod (eg, Nylon lock nuts in preference to half nuts) 1 151 x 90 x 19mm DAR pine timber 1 29mm-OD frosted halfhemisphere hollow plastic ball (cut from ball salvaged from rollon deodorant) (optional) siliconchip.com.au 23108141 IC2 TL072 (SMD) IC1 MC1495 (SMD) 1 1 Fig.5: here’s how to mount the alternative SMD ICs on the back of the PCB. Our prototype used SMDs for IC1 & IC3, as shown in the inset photos. 1 1 IC3 TDA7052 (SMD) 1 cedure. Once all the pins have been soldered, you can remove any excess solder using solder wick. If you’re not using SOIC ICs, or once you’ve finished fitting them, install the resistors. Be sure to push them all the way down so that they sit flush against the PCB before soldering their leads. Table 1 shows the resistor colour codes but you should also check each one using a DMM before soldering it in position. Next, fit any DIP ICs, either by soldering them directly to the PCB or using IC sockets. That done, fit PC stakes to the three GND positions (ie, TP GND and the GND pads adjacent VR1 and T1), then TP1, TPS (adjacent IC2) and for the two wiring points for coil L1. The two 3-way pin headers for LK1 and LK2 can then go in. Diodes D1 and D2 are next on the list, taking care to ensure that they are correctly orientated. Bridge rectifier BR1 can also be installed at this stage, with its ‘+’ pin positioned as shown. Follow with JFETs Q1-Q4 and trimpots VR3 and VR4. Note that VR4 is orientated with its adjustment screw adjacent to LED5. The capacitors can then all go in but be sure to orientate the electrolytic types correctly. Table 2 shows the codes used on the lowvalue capacitors. LED3 is installed next and must be pushed all the way down onto the PCB before being soldered. Its anode (A) lead is orientated as shown. siliconchip.com.au 1 Once it’s in, the two adjacent M205 fuse clips (used to connect the antenna) can go in. These must have their end-stop tabs broken off before installation, by bending them back and forth using small pliers. These fuse clips are both mounted slightly proud of the PCB and their pins soldered on both sides of the board, to make a secure mounting receptacle for the antenna. Do not push the fuse clips all the way down onto the PCB as they could short to LED3’s pads. The two oscillator coils, T1 & T2, can now be installed. These are both white-cored IF transformers and only go in one way, since they have three pins on one side and two on the other. Push them all the way down onto the PCB before soldering their pins and don’t forget to solder the mounting pins on either side of the metal cans. Once these parts are in, install switch S1, power socket CON1, 3-way screw terminal block CON2 and 3.5mm jack socket CON3. Note that the wire entry side for CON2 must go towards the adjacent edge of the PCB. 9V regulator Regulator REG1 mounts horizontally, with its leads bent by 90° so that they go through the PCB holes. Secure its tab to the PCB using an M3 x 6mm screw and M3 nut before soldering the leads. Don’t solder the leads first; if you do, the PCB tracks could crack as the screw is tightened. Next, cut the shafts of VR1 & VR2 to suit the knobs that will be used and clean up the ends with a file. That done, snap off the small lug next to the threaded shaft bushing on each pot and install the two pots on the PCB. The metal body of each potentiometer must be earthed to the PCB via a GND PC stake. For VR1, the GND stake is immediately adjacent and the pot’s metal body is connected to it using a short length of tinned copper wire. Note that it will be necessary to scrape or file away a small section of the passivation layer on the pot’s body to allow the solder to adhere. By contrast, VR2’s GND stake is some distance away, to the left of coil L1. It should be connected using medium-duty hookup wire. This earth position was necessary to remove background hiss from the Opto-Theremin’s audio outputs. Front-panel LEDs The remaining LEDs (LED1, LED2, LED4 & LED5) must all be mounted on 35mm lead lengths, so that they later protrude through the lid of the box. This means that you will have to extend their leads using short lengths of tinned copper wire. Keep the anode leads slightly longer than the cathode leads, to make it easy to check the polarity when the LEDs are installed. It will be necessary to sleeve at least one lead of each LED September 2014  29 Table 1: Resistor Colour Codes o o o o o o o o o o o o o o o o No.   9   1   1   1   2   1   6   1   2   2   1   1   2   1   1 M3 x 10mm SCREW 2 x 3mm ID WASHERS PCB Value 100kΩ 47kΩ 10kΩ 6.8kΩ 2.2kΩ 1.2kΩ 1kΩ 820Ω 680Ω 560Ω 220Ω 150Ω 100Ω 82Ω 39Ω RANGE SENSOR SHARP GP2Y0A41SK0F M3 NUTS with heatshrink tubing after attaching the wire extensions, to prevent them from shorting. Once the extensions are in place, mount the LEDs on the PCB (red for LED5, blue for the others), taking care to ensure that they are orientated correctly. It’s a good idea to slide a 35mmwide strip of cardboard between the leads of each LED when mounting it in position. It’s then just a matter of pushing it down onto this spacer before soldering its leads. Equalising coil Equalising coil L1 consists of a bobbin and two ferrite core halves. The first step is to jumble-wind (ie, randomly wind) 260 turns of 0.25mm enamelled copper wire onto the bobbin. When the winding is complete, lightly twist the two free ends together for about 2mm to prevent the winding from unravelling, then cut the leads to 20mm and scrape away the insulation from each end. Next, cover the winding with a layer of insulation tape. Alternatively, shrink some 20mm-diameter heatshrink tubing around the bobbin. The coil can now be assembled onto the PCB, as follows: (1) Position one ferrite core section on the PCB and fit the bobbin in place. 30  Silicon Chip 4-Band Code (1%) brown black yellow brown yellow violet orange brown brown black orange brown blue grey red brown red red red brown brown red red brown brown black red brown grey red brown brown blue grey brown brown green blue brown brown red red brown brown brown green brown brown brown black brown brown grey red black brown orange white black brown M3 x 10mm SCREW 2 x 3mm ID WASHERS Fig.6: this diagram shows the mounting details for the Sharp optical distance sensor. Note the 3mm stacked washers used as spacers. (2) Slide two 4mm-ID Nylon or polycarbonate washers inside the bobbin, so that they rest on top of the inner part of the bottom core (these are needed to provide a 2.5mm spacing between the two cores). (3) Place the top core in position and secure the entire assembly to the PCB using an M4 x 25mm Nylon or polycarbonate screw and an M4 nut. Be sure to orientate the coil as shown on the parts layout diagram (Fig.4). (4) Solder the coil wires to the adjacent PC stakes. Volume control PCB That completes the main PCB assembly – now for the volume control board. Start by installing the two 470Ω resistors and diode D3, then fit regulator REG2. As before, be sure to secure the regulator’s tab to the PCB using an M3 x 6mm screw and M3 nut before soldering the leads Next, fit the 10µF and 1000µF electrolytic capacitors. As shown, the latter must be installed with its body lying horizontally and its leads bent down through 90° to go through its solder pads. The 10µF capacitor will also need to be bent over slightly so that it later clears the case lid. The two blue LEDs can go in next. 5-Band Code (1%) brown black black orange brown yellow violet black red brown brown black black red brown blue grey 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 green blue black black brown red red black black brown brown green black black brown brown black black black brown grey red black gold brown orange white black gold brown Table 2: Capacitor Codes Value 470nF 100nF 22nF 1nF 330pF 100pF 68pF µF Value 0.47µF 0.1µF 0.022µF 0.001µF NA NA NA IEC Code 470n 100n 22n 1n 330p 100p 68p EIA Code 474 104 223 102 331 101 68 These are mounted with their bodies close to the PCB and are bent slightly towards the 470Ω resistor, so they do not later directly shine into the player’s eyes. If you are not using a translucent case, then the LEDs will need to be mounted about 10mm proud of the PCB, so they later protrude through the case. The distance sensor is installed by first soldering a 3-way pin header to the pins of the right-angle 3-way connector on the underside of its PCB. This is clearly shown in one of the accompanying photos. The sensor is then mounted on the volume control PCB and secured using M3 x 10mm screws and nuts, with two stacked M3 washers serving as spacers on each side – see Fig.6. Tighten the screws down firmly before soldering the 3-way pin header to the PCB. That’s all we have space for this month. Next month, we’ll describe how the two boards are assembled into their boxes, give the final mechanical assembly details and detail the simple SC test and adjustment procedure. siliconchip.com.au