102 Minimum Theremin Schematic and Circuit Description

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102 Minimum Theremin Revision 1 Schematic

102 Minimum Theremin Circuit Description

Two identical integrated circuits, U1 and U2, known as "QUAD 2-INPUT NOR GATES" perform the theremin's primary functions. They are CMOS (Complimentary Symmetry Metal Oxide Semiconductor) devices. Each IC contains four identical sections; thus the term "quad." They are typically used in computer circuits to perform the "inverted-or" Boolean logic function, and may also be used in the linear mode, as in the 102 Minimum Theremin circuit, to provide other functions such as oscillation.

U1 sections A and B, in conjunction with R1, R2 and C2, comprise a variable-frequency oscillator that produces waveforms with frequencies ranging between about 163.5KHz and 165KHz. The ANTENNA forms one-half of a variable capacitor that is part of this oscillator's frequency-determining network, and the player's hand forms the other half. As the distance between the hand and the antenna varies, so does the hand-to-antenna capacitance, and therefore the oscillator's frequency. The variable oscillator's frequency becomes lower as the hand is moved closer to the antenna.

Upon initial consideration, the U1A-U1B circuit section would, in itself, seem enough to make a simple theremin, since it satisfies the objective of frequency dependency on hand position. However, the range of frequency produced by the U1A-U1B oscillator is inaudibly high, far-exceeding the human hearing limit of about 20KHz. While the oscillator's frequency may be reduced to the audible range by increasing the value of C2, the effect of the hand-to-antenna capacitance would, in such a case, only cause a very slight change in frequency, resulting in a theremin with a pitch range of less than a single tone interval.

To convert the variable-frequency oscillator's high-frequency range to an audible range, its output is applied to one input of NOR gate U1C (pin 9) which performs the function of a mixer. The mixer "heterodynes" the variable-frequency oscillator's waveform with the output from a second oscillator called the "local oscillator," connected to U1C's other input (pin 8). The heterodyne process produces a waveform containing the sum and difference values of the two oscillators' frequencies. The difference frequency is also called the "beat frequency."

The theremin's local oscillator is comprised of U2 sections A and B, in conjunction with R7, R8, RV2, and C12. U2 sections C and D are not used, so their inputs are grounded. The local oscillator's frequency is coarsely adjusted with the board-mounted RV2 ZERO CAL (zero calibration) trimmer potentiometer, and finely adjusted with the front panel PITCH potentiometer. The potentiometers are adjusted so that the local oscillator's frequency is equal to the the variable oscillator's frequency with the hand furthest away from the antenna (about 165KHz for a hand distance of 18 inches). Due to the capacitive coupling from their physical proximity, the two oscillators' waveform frequencies synchronize. Under these conditions, no beat frequency waveform is present at the U1C mixer output.

As the hand is moved closer to the antenna, the variable-frequency oscillator shifts lower in frequency, breaking out of synchronization with the local oscillator. Under this condition, the mixer output includes frequency content that is the difference between the two oscillators' frequencies. For example, when the hand-to-antenna distance is 6 inches, the variable-frequency oscillator produces a waveform with a frequency of 164.7KHz. The difference between this frequency and the local oscillator's frequency of 165KHz is 300Hz. 300Hz is an audible frequency, closely corresponding to the musical note D4, that will be part of the waveform at the U1C mixer output.

The PITCH potentiometer, configured as a voltage divider in conjunction with R10, is not directly connected to the sensitive time-constant section of the oscillator (U2 pins 1, 3, and 4). Instead, it controls frequency indirectly via U2's power supply pin 14, which is decoupled with capacitor C13. This method of frequency adjustment is advantageous, since the impedance at pin 14 is very low, and therefore immune to interference. The length of wires between the PITCH potentiometer and the circuit board is therefore unconstrained, allowing the potentiometer to be located at any convenient position.

In use, the PITCH potentiometer is adjusted by the player so that the theremin is silent with the hand furthest away from the antenna, and produces the lowest tone when the hand is at the maximum playing distance of about 18 inches.

Resistor R4 and capacitor C6 comprise a low-pass filter that reduces the inaudibly-high waveform content from the mixer's output, leaving the audible beat-frequency content. This waveform is coupled to U1D's input by C3. U1D converts the waveform to a square shape compatible with the theremin's loudspeaker. The R4-C6 filter rolls-off the waveform amplitude as the audible frequency increases, so the board-mounted RV1 BIAS ADJ trimmer potentiometer serves to bias U1D's input precisely at its threshold voltage to ensure adequate waveform conversion at high audio frequencies. Resistor R3 and capacitor C4 comprise a second low-pass filter to remove residual, inaudible, high-frequency content from the audible square waveform.

The waveform is applied to the VOLUME control potentiometer via DC blocking capacitor C9, and then through C10 and R9 to audio amplifier U3. Capacitor C10 blocks the DC voltage present at U3's pin 4 from the potentiometer, and the ratio of R11 / R9 sets the amplifier's gain. Capacitor C15 provides further low-pass filtering to reduce harsh audible harmonics. U3 is a "bridge" type amplifier that provides an inverted voltage signal to one side of the SPEAKER and a complimentary, non-inverted signal to the other side. This permits the speaker to be direct-coupled to the amplifier without requiring a large DC blocking capacitor, and also provides the advantage of power gain for ample volume.

To ensure immunity from changing battery voltage, low-dropout voltage regulator IC VR1, with C7 and C8, furnishes the U1 and U2 circuits with a steady 5 volts. The theremin will operate with a battery voltage as low as 6 volts. The circuit typically uses about 4.3 milliamperes with the volume control set at minimum (no speaker output), 14 milliamperes with the volume control at mid-point, and 70 milliamperes at full speaker volume. If the theremin is used only to drive an external amplifier, a wire jumper may be connected between the two points labeled "INH." This will place the audio amplifier IC U3 in a shut-down mode, lowering the current drain from the battery by approximately 3 milliamperes. Rectifier CR1 protects the circuit from accidental battery reversal.

The 102 Minimum Theremin may be connected to an external amplifier via the OUTPUT jack which provides an audible waveform directly from the R4-C6 low-pass filter via DC-blocking capacitor C5 and attenuation resistors R5 and R6. The OUTPUT jack amplitude is about 700 millivolts, peak-to-peak, rolling-off to 500 millivolts peak-to-peak at 1000 Hz. The OUTPUT jack amplitude is not affected by the VOLUME control.

For a theremin to operate properly, sufficient electrical coupling must exist between the player and the instrument. A player is coupled to earth's ground due to the relatively large capacitance between the body and earth (about 100 to 200 picofarads). When the theremin is connected to an external amplifier, it too is coupled to earth's ground via the audio output connection between the theremin amplifier, which in turn, is connected to earth's ground by its power cord. The player's earth coupling, along with the amplifier's connection to ground, provide the desired common connection between the player's body and the instrument.

When used only with its internal loudspeaker, and not connected to an external amplifier, there may not be sufficient electrical coupling between the theremin and player. This may be easily remedied by connecting the circuit's GROUND jack to earth or a nearby metal object. If this is not practical, attaching a wire to the GROUND jack and placing the wire in near-proximity to the player will provide sufficient coupling. In all cases, sufficient coupling between the theremin and player will ensure the best pitch range and sensing distance.

The 102's simple design employs the use of resistance-capacitance-controlled "relaxation" oscillators that are inexpensive, easy to build, and easy to calibrate. Oscillators of this kind, in comparison with inductance-capacitance-controlled "resonant" designs, are more susceptible to interfering sources such as radio stations, appliances, and power lines. Adverse effects may be reduced considerably by locating the theremin away from interference sources.

The jumper points (FRAME GROUND), shown in the schematic to the right of CR1, are provided to directly bond the circuit's ground to the metal front panel through the printed circuit board's lower-right mounting standoff. The jumper is generally inserted, as it improves grounding for the theremin circuit. When constructed in the recommended manner, this grounding will also occur by default via the metallic bushings of the front-panel-mounted OUTPUT and GROUND jacks.


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