Building a Transistor Theremin

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by Mia David

Project Overview

This project includes instructions for the construction of a transistor based theremin, a simple electronic musical instrument. The theremin takes a constant DC signal as input and returns a clean sine wave (amplified and sent through a speaker to create a tone) as output; the frequency of the output audio signal is adjusted via a variable capacitor consisting of a control antenna and the performer’s hand. This particular design uses the Colpitts oscillator configuration to generate the oscillating output signal, which makes use of simple components including a bipolar junction transistor, a few capacitors, and an off-the-shelf inductor.

The basic functioning blocks of the theremin include a reference oscillator (which outputs a constant frequency), a pitch oscillator (the frequency of this output signal is controlled by the variable capacitor formed by the player’s hand and the wire), a mixer to combine the two signals, filtering to eliminate unwanted frequencies, amplification of the signal, and finally the loudspeaker.

Tools and Materials

  • - 4.7 nF capacitors (5)
  • - 0.01 μF capacitors (2)
  • - 680 pF capacitors (2)
  • - 220 pF capacitor (1)
  • - 260 pF capacitor (1)
  • - 15 pF capacitors (2) — these replace the trimmer capacitors C15 and C25 in the schematic
  • - 39 pF capacitors (2)
  • - 100 pF capacitors (2)
  • - 8.2 pF capacitor (1)
  • - 0.22 μF capacitor (1)
  • - 1000 μF capacitor (1)
  • - 100 Ω resistors (2)
  • - 9.2 kΩ resistors (2)
  • - 3.3 kΩ resistors (2)
  • - 390 Ω resistors (2)
  • - 4.7 kΩ resistors (2)
  • - 20 kΩ resistors (6)
  • - 470 kΩ resistors (2)
  • - 2.2 MΩ resistor (1)
  • - 10 kΩ resistor (1)
  • - 2N2222 npn transistors (2)
  • - 1N5711 schottky diodes (1)
  • - LM741 op amp (1)
  • - LM386 low voltage audio power amplifier (1)
  • - 8 Ω speaker
  • - 530 μH inductors (2)
  • - tinned copper wire, 16 AWG (~ 15cm) for pitch antenna
  • - prototype board or breadboard (1)
  • - DC power supply (or built-in power supply if using prototype board)
  • - Oscilloscope
  • - Multimeter for testing

Step-by-Step Instructions

Begin by constructing the circuit for the reference oscillator. Trimmer capacitor C15 can be replaced by a standard capacitor in the specified range (15-60 pF). Once the circuit is constructed and connected to power, use the multimeter to check the voltage at the base of the transistor; this should be at around 1.9 V, which puts the 2N2222 transistor into saturation mode (any voltages between 1.2V-2.0V will work, if the resistors in the voltage divider need to be adjusted). Check the output using the oscilloscope probe at the point specified on the schematic; frequency of the oscillating signal can be calculated using the included formulas, which only require the values of the components in the “tank circuit” (highlighted on the schematic). The values for the components in the tank circuit in the schematic should yield an output signal with a frequency around 536 kHz.

Build the pitch oscillator. For initial testing purposes, I would recommend adding a regular capacitor in place of the pitch antenna to make sure that the circuit behaves as expected; after the rest of the circuit is finished, replace that capacitor with the tinned copper wire which acts as the pitch antenna. The construction of the pitch oscillator should be very similar to the reference oscillator other than the additional capacitor; depending on the capacitor you choose, the output frequency will differ. As with the reference oscillator, I would check the voltage at the base of the transistor to make sure it is in saturation mode.

Build the mixer. This consists of the schottky diode, a very large resistor, and a capacitor. Note that between the oscillators and the mixer are a couple of resistors and capacitors, which are intended to decouple the oscillators from the mixer. Measure output before C2 using the oscilloscope to confirm you are getting a new frequency.

Construct the remainder of the circuit. Before adding the speaker, measure the signal at the output of the audio amplifier to confirm it is a clean sine wave. Add the speaker. Depending on the capacitance of the capacitor substituted for the pitch antenna, the frequency of the tone will be different. Assuming that an audio signal is being produced, now replace the substitute capacitor for the wire used for the antenna. Move your hand closer and further from the antenna to observe how the pitch changes. At this point, you can try switching out the capacitors or inductor in the tank circuit for both oscillators to see how the audio output signal changes.