Anyone who’s shivered in the dark at a scary movie or laughed at the unintentional cheese-ball of a bad sci-fi (paging Ed Wood) knows the eerie sounds of the theremin. In the classic theremin design, two antennas control pitch and volume, and you play the instrument by moving your hands near the antennas without touching them.

This simpler design uses interrupted photons (light) instead of radio waves, and can be built with a handful of common components, including the versatile 555 timer chip. When we’re done, we’ll have a decent sounding mini-theremin. You can experiment with its sound by changing the type of light sensor used and the capacitance of the circuit.

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## Project Steps

Gather together the breadboard, capacitors, speaker, resistors and photoresistor (or photodiodes).

TIP: I nearly always buy the large multi-packs of resistors; it saves time and money in the long run.

The schematic I based this project on called for a 0.47μF capacitor. I didn’t have any, so I used two 0.22μF capacitors in parallel. The values add, giving 0.44μF — and that’s close enough!

We are going to be building a 555 timer-based “astable oscillator circuit.” It sounds complicated, but really, it’s not that hard.

### Install the 555 Chip and Power Lines

Our first task is to place the 555 timer IC on the breadboard. Note the location of the small dot indentation (which I painted white to make it more visible). That dot always marks Pin 1 on a chip.

I also added the basic power lines — red is +6v, and black is 0v (Gnd).

The two red wires carry the power lines between the top and bottom horizontal power “rails” on the breadboard.

### Install the Resistors

Breadboard the 10KΩ (Brown, Black, Orange, Gold) resistor at the top, and the 1MΩ (Brown, Black, Green, Gold) resistor at the bottom, as shown.

Disregard the blue color of the 10KΩ here; your resistor will likely be beige in color.

Add both 0.22μF capacitors in parallel.

Be careful that the legs of the capacitors do not touch!

Remember: I’m using two .22μF caps in place of the .47μF called for in the schematic. If you have a .47μF cap, you can use that.

Also add support wires as shown (the two brown wires, and one white one).

### Install the Remainder of the Parts

Note: Electrolytic capacitors are polarity-sensitive. They can only safely go in one way. Note the orientation of the black band; it marks the negative lead.

Add the speaker. Note the orientation of the red (+) and black (-) wires; it also needs to be connected with the correct polarity.

Install the two photodiodes (second image).

You should now be able to power up the device and hear a buzzing tone coming from the speaker. Move your fingers towards the photodiodes, and the pitch should go down.

That’s it! Move your fingers around the photodiodes to create different notes and sound effects.

### Experiment: Photodiodes vs. Photoresistors

Photodiodes work in this circuit, but you can get a broader range of tones by swapping in photoresistors, which RadioShack sells in a 5-pack.

Try different types of photoresisor, and also try removing one of the 0.22μF capacitors — this will alter the range of pitches you can produce.

See and hear the Light Theremin in action here and here.

## Conclusion

The venerable 555 timer integrated circuit used in this project is the most popular IC of all time. You can learn more about it here and read about a chance encounter with the designer of the chip, Hans Carmenzind, on MAKE.