A Light-Controlled 'Theremin'

The working principle of this electronic music toy is different from that of a traditional theremin, but it also allows the performer to change both the pitch and volume of the sound - without touching anything with the performer’s hands. This control is being achieved thanks to solar cells, that's why I called the device 'light-controlled'. Just hover your hands over two black boxes, and you'll get some sounds or could play a simple melody.

I offer my apologies to the audience for low volume of the sound in the video, the reason is I used only the device's embedded preamplifier; perhaps, you might need to increase the volume while watching the video.

Electronic components used in the circuit:

IC LM331

Q1 2N3904

Q2 2N5551

R1, R7 R9 10 KOhm

R2, R12 1 KOhm

R3, R4 100 KOhm

R5 47 Ohm

R6 12 KOhm

R8 6.8 KOhm

R10 5.6 KOhm

R11 27 KOhm

R12 1 KOhm

R13 200 Ohm

C1 1.0 x 10 V (electrolytic)

C2 0.1 microfarad

C3 0.01 microfarad

C4, C5 4.7 x 10 V (electrolytic)

C6 100.0 x 10 V (electrolytic)

LS1 a small loudspeaker 4...8 Ohm, 100 mW

The circuit is assembled on a laminate board sized 90 x 50 mm. The power is supplied by 4 x 1.5 V batteries (6 V in total).

The circuit is based on an IC LM331 that converts voltage into frequency; thus, if the voltage applied to pin 3 changes, the IC’s output frequency also changes. A solar cell battery PV1 produces this voltage; the performer covers or uncovers the battery with his/her hand thus changing the amount of light falling to the battery. Therefore, the battery’s voltage and, by consequence, the sound’s frequency changes.

Resistors R1 and R2 form a bias that doesn’t allow the frequency to decrease significantly when the battery is completely covered.

The signal from pin 7 enters a preamplifier built around Q2; a small loudspeaker LS1 is connected to the preamplifier’s output just to show the device’s work in the video. A power amplifier would be needed to get a louder sound.

The volume is controlled by changing the voltage produced by the solar cell battery PV2. This voltage is applied to the base of Q1 to open it progressively; the more Q1 opens, the more signal from pin 7 goes to the ground instead of entering the preamplifier; therefore, the sound’s volume decreases. The volume is maximum when the battery is completely covered with the performer’s hand.

Solar battery PV1 is a battery from an electromechanical toy, its voltage changes between 0 and 1.5V. The battery is placed into a cardboard box sized 70 x 70 x 70 mm; thus, the light can fall to the battery only from above; otherwise, the light reflected from the table’s surface would have influenced the battery’s output.

Solar battery is from a solar powered garden lantern, it develops voltage between 0 and 0.8 V. In fact, some elements of the battery were not functional, so I had to use only those that worked. The battery is placed into a box sized 85 x 85 x 70 mm; I also had to make a cover with a 60 x 60 mm opening to be able to completely cover the battery with my hand.