Can anyone explain why this works? What's going on? I built this kit today and it works great, but I don't fully understand what each part is doing. Is the 555 timer being used for pulses or ossilation? How's that working? And what are the other capacitors doing? Can anyone give a simple explanation of this circuit? See attached image. Thank you!

Curcuit for Radioshack's theremin kit


It's a pretty basic 555 timer circuit, operating in astable mode. R2 (the photoresistor) is the key to changing the output sound by changing its resistance based on the amount of light. When there is less light (your hand gets closer to the sensor and blocks out more light) the resistance increases, lowering the output frequency of the 555 timer. As you take your hand away (block less light) the resistance of R2 decreases and the output frequency goes up. The frequency is dependent on the ratio of R2 and R1, which form a voltage divider. C4 is just a noise filtering capacitor. You'll see this on just about any other 555 timer astable circuit. C1 and C2 appear to be decoupling capacitors, and C3 is a DC-blocking capacitor to prevent DC from going to the speaker (which could burn it out). Other than that it's pretty straightforward.

As mentioned in the comments it is not a real theremin, which uses the user's body capacitance to change the resonant frequency of internal circuitry. It has tone AND volume control. This one just has tone control, and only does it using light. No resonance, no capacitive sensing.

  • \$\begingroup\$ Thank you very much! Great explanation! I now have the answer for my technically minded 7 yr old who was badgering me for an explanation! :) \$\endgroup\$ – Agent Zebra Nov 29 '16 at 0:35
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    \$\begingroup\$ I'm very glad I was able to help! \$\endgroup\$ – DerStrom8 Nov 29 '16 at 3:32

It's not a real theremin. This circuit creates a 'tune' based on the amount of light that hits R2. It's a simple design based on the well known 555 timer. Assumed in bright light the R2 value decreases to 1k ohm. THe output is between 700Hz and 5kHz.

A theremin is a similar principle with a RF circuit instead of light.


with: C = 100nF, R1 = 1k Ohm, R2 = 1k ohm to 10k ohm.


  • \$\begingroup\$ I was about to say the same thing! Yea NOT a real Theremin. \$\endgroup\$ – crowie Nov 28 '16 at 11:31

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