by Hays Turner
Project Overview
This project involves designing and building a compact, two-oscillator theremin that generates sound based on hand proximity. The instrument uses a pair of high-frequency RC oscillators whose slight frequency difference creates an audible beat frequency through the process of heterodyning. The design aims for smooth pitch variation and recommends several strategies to reduce unwanted oscillator synchronization. The final goal is a playable, breadboard-friendly theremin that demonstrates capacitive sensing to generate an analog audio signal.
Tools and Materials
- - 74HC132 NAND Gates with Schmitt-Trigger Inputs (2)
- - LF411 FET-Input Op-Amps (2)
- - 100 pF capacitors (2)
- - 1 nF capacitor (1)
- - 0.1 μF capacitors (3)
- - 4.7 μF capacitor (1)
- - 5 kΩ resistors (3)
- - 10 kΩ resistors (8)
- - 100kΩ single-turn potentiometer (1)
- - 10kΩ multi-turn potentiometer (1)
- - tinned copper wire, 16 AWG (~ 15cm) for pitch antenna
- - PB-505 prototype board or breadboard (1)
- - DC power supply (or built-in power supply on prototype board)
- - Dual-output function generator
- - Oscilloscope
- - Digital multimeter
- - Banana cable or 1' metal rod (antenna) (1)
Project Files
Step-by-Step Instructions
To start, build two nearly identical high-frequency oscillators on a prototype board. The lower oscillator will be the "reference" oscillator set to a fixed frequency by a multi-turn 10kΩ potentiometer. The upper oscillator (with a banana cable acting as the antenna) will have a varying frequency that depends on capacitance between the user's hand and the cable. Each tank circuit should use a separate NAND IC. Construct these oscillators on the breadboard and physically separate them by at least a few inches. This will reduce any coupling effects between the two circuits that inhibits the proper functioning of the theremin. As another preventative measure, place 0.1 μF capacitors between +5V and GND power rails to reduce high-frequency oscillations along the rails. Probe each output using the oscilloscope. Expect to see a 5Vpp square wave in 500-1000 kHz range. Confirm that the oscillators run a few kilohertz apart without pulling into synchrony, ensuring proper heterodyning later on.
Next, assemble the first op-amp in a differential-amplifier configuration using four 10 kΩ resistors. The two inputs (V1 and V2) will be the two high-frequency oscillators. To test this configuration, use the function generator to send in two 5Vpp square waves at slightly different frequencies (say 800kHz and 805kHz). When functioning, this setup should heterodyne the two high-frequencies to create a beat frequency at the rate of the difference (5kHz in this case).
After heterodyning the two HF oscillators, you will get an audio signal that still contains unwanted high frequencies. Start by passing it through a simple RC low-pass filter (5 kΩ + 10 nF) to smooth the waveform. Depending on your desired signal, you can calculate the cutoff frequency using the equation f = 1/(2pi*RC). Next, feed the filtered audio into the 100 kΩ potentiometer wired as a voltage divider: input on one outer terminal, GND on the other outer terminal, and the center wiper as the adjustable output. This pot acts as your volume control, letting you modulate the signal amplitude. Verify on the scope that turning the pot cleanly scales the audio amplitude without distortion. From the center input of the potentiometer, place a 4.7 μF capacitor and 10 kΩ resistor to remove the DC offset of the signal (high-pass filter).
Lastly, take your volume-controlled audio signal and send it into a non-inverting op-amp amplifier with a gain set by the negative feedback and grounded resistors (Vout = Vin*(1 + Rf/Rg)). This will bring the signal up to a suitable for the built-in speakers in your prototype board. You may now plug in the output of your op-amp into one of the speaker inputs. Verify on the oscilloscope that the output is of the right shape and at a sufficient amplitude (at least 2Vpp).
Important things to keep in mind when building:
- You must physically separate the two HF oscillators (use different 74HC132 chips if possible and keep their capacitors on separate ground paths) to prevent unwanted coupling or injection-locking when the frequencies get close to each other.
- Add 0.1 µF capacitors across VCC and GND rails to reduce noise within the board.
- Test your differential op-amp using similar (but not the same) HF inputs to ensure that you can achieve a beat frequency from square wave inputs that corresponds to their frequency difference.
- Antenna positioning can have a significant impact on the frequency of the antenna oscillator.
