Wiring circuit

This schematic diagram show a single chip Theremin circuit.  Theremin is an electronic music instrument which sense hand movement to control the tones/frequency.  This Theremin circuit uses two separate Colpitts LC oscillators to produce a beat frequency. 

The frequencies of  two Colpitts LC oscillators are mixed and then rectified. This rectification  demodulate the mixed signal to get the beat frequency which is in audible range. This beat frequency or difference is the real  Theremin’s output. The oscillator is operated at high frequency (inaudible) to get wide audible frequency range of beat frequency when two oscillator output is mixed.  This circuit uses a 4011 quad gate to construct the high frequency oscillator operating at 250kHz. Here is the schematic diagram of the circuit.

The metal probe that is used to sense your  hand produces only small frequency shift in term of percentage of original frequency, that’s why we need to derive the beat frequency to get wide audible frequency range as the result of  high frequency shifting. The IC2, an LM741 is used to amplify the mixed signal before rectification. 

The D1 will  rectify the mixed signal to detect the audio (the beat frequency).  This audio signal is then filtered by an adjustable bandpass filter IC3. The further audio amplification before power amplifier IC5  is done by IC4. The metal toilet-tank float  is used for the hand probe since is has better sensitivity than a simple wire antenna, but any conductive material will work.

Theremin circuit shown in this schematic diagram uses digital component, so we can call it a digital Theremin. This circuit employs  logic inverter 74C04 or CD4069 hex inverter and CD4046 phase-locked-loop (PLL) IC. 

The CD4069 logic inverter is operated as a fixed-frequency oscillator with  frequency around 100kHz. The CD4046 is operated as a variable frequency oscillator which is adjustable around 100kHz. The exact center frequency of the on-chip oscillator is determined by R4, C2 and R3. Here is the schematic diagram of the circuit. 

The frequency of variable oscillator frequency circuit can be shifted several kilohertz by moving your hand approaching the antenna since the C2 and the antenna form an equivalent parallel capacitance. The frequency of the variable oscillator should be set to the same frequency of fixed oscillator when there is no hand or human body close to the antenna. 

This  calibration is done by adjusting the  zero control R4 pot with this simple rule: If  both oscillators (the fixed and the variable) are set to the same frequency then the Theremin will produce no output (silent). This Theremin circuit will start  producing audible tone if you move your hand approaching the antenna since it will shift the frequency of the variable oscillator. You can play this Theremin circuit by moving your right hand around the antenna and at the same time turning the volume knob R5 with your left hand.

This simple but complete Theremin with Inverter Gates is constructed using only two inverter chip plus one regulator IC. This Theremin circuit consist of five functional blocks: power supply regulator, hand controlled oscillator, null oscillator, mixer, and filter. Here is the complete schematic diagram.

The power supply regulator consist of LP2950 regulator IC, which stabilize the voltage from battery to 5V. You can use more popular 7805 IC for this, but since the power consumption of this Theremin circuit is very small, then you can use 78L05 which is smaller.  CR1 diode is used to protect from inappropriate battery polarity,  shorting the battery voltage together with R8 100 Ohm resistor which prevent the large current when the battery is installed in wrong direction. Although the inverter chip will work well for 9V battery, there is a benefit of using voltage regulation to regulate the battery voltage at lower voltage level, that the voltage will remain constant for until the end of battery life. This will avoid frequency drift of the Theremin’s null oscillator  which should be carefully adjusted to zero the output frequency, which can be affected by the supply  voltage.

The hand proximity sensor is an oscillator which has antenna extension which shift the capacity coupling in the loop. This capacitance shift occur when we move our hand approaching the antenna. Since this change is very small in percentage, we need this oscillator to be high enough to produce notable frequency difference.  This oscillator block is built around U1A, U1B, and U1C. This oscillator give oscillation at around 73kHz. This frequency is not directly audible, we have to process this signal further to produce audible signal.

Null oscillator is employed to produce a constant frequency oscillation that will be used to produce differential frequency which is audible.  This oscillator block is built around U2A, U2B, and U2C. This null oscillator should be adjustable to set the null point where the Theremin should produce no oscillation at certain hand position.  At this point, the null oscillator should be adjusted to have  same frequency with the hand controlled oscillator since the audible Theremin output is the product of the difference between hand controlled and null oscillator frequencies.

The mixer is used to mix the signal from two oscillators, the hand controlled and the null oscillators. This mixer produce an output which contain many frequency components, not only the difference but also the original and the sum, since the amplifier U1F is basically a digital inverter which has non-linear transfer function. Fortunately, all the frequency components, except the difference, will be much higher than the needed signal and inaudible. This make it easy to remove by simple low pass filter to obtain only the differential frequency component.

As stated before, we need to obtain only the audible frequency component by low-pass filtering. Although the high frequencies is inaudible, we still have to remove it since it can cause damage in audio power amplifier is the level is too high. It can distort the audible signal, and it can eat the electric power like ghost, frying your amplifier or at least make it overheated. This  Theremin circuit use simple low pass filter consist of C4, R5, and R7 for the passive stage, and C2 inside the inverter amp loop.