Whether you're looking for a cool science fair project or you just want some soldering practice, building an optical theremin can be a fun and educational experience.
Unlike true theremins, optical theremins use light sensing "photoresistors" instead of capacitance to control the pitch of the instrument. Most optical theremins are pitch-only, meaning you cannot change the volume. Nonetheless, they can be a fun way to make some crazy sounds and learn a little about basic electronics. Connect it to some guitar effects pedals, and you'll be all set for Halloween!
Here's a round-up of 3 introductory optical theremin kits for under $50. If you're looking for a school science fair project or just want to get in some soldering practice on a fun noisemaker, these kits can't be beat.
Synthrotek Optical Theremin Kit
Best known for making Eurorack modular synthesizer gear, Portland, OR based Synthrotek has a great little optical theremin kit for just around $22. Available at Amazon.com or directly from Synthrotek. If you want to source your own parts, you can also buy the empty circuit board only.
3pdt Optical Theremin Kit
We found this one on Amazon.com for just around $40. All necessary parts are included (except a battery). The seller lists this as an easy build, and from the looks of it, we'd agree.
XLPC Optical Theremin
Here's another optical theremin on Amazon.com that actually requires no soldering! This unit is pre-assembled and ready to be mounted in the housing of your choice (perhaps a plastic pumpkin for Halloween?). This is the most expensive of the 3 kits we found at around $50, but if you aren't comfortable with soldering or don't have the equipment, this might be a good choice for you.