the LIGHT SENSITIVE THEREMIN page


OK, check this out. What we have here is an article about my favorite noisemaker, the "Light Sensitive" Theremin. Its probably more accurately described as a "Light Sensitive Pseudo-Theremin", as it doesn't operate via body capacitance and all that RF business. The only things that this tiny circuit has in common with real theremins is 1) it sounds kinda like a theremin, and 2) You look a bit like you're playing a theremin, as you wave your hands in the air over the thing.

As you may have gathered from the title, this "theremin" operates via a light sensitive CdS cell. The brighter the light shining on the circuit, the higher pitched sound it emits. So, if you start waving your hand over the thing, you can change the pitch, hammer out a simple tune, or just make a bunch of noise (mostly what I use it for).

Before we start, a quick sidenote re: the source of the info I'm gonna impart to you here. If you've been checking out the "experimental music instrument" scene on the web, you've probably come across a fellow who is advertising something similar for $1 and a SASE. Now, I hate to be a bigger jerk than usual, but I have to tell you: I sent for the plans, and promptly received them, no problem there. But, the plans mention that the circuit design is basically a modification of a circuit design from the long out-of-print "Electronic Music Projects, vol 1", by the great Forrest M Mims, III. Now as dumb luck would have it, not two days after I received the plans, I happened to be strolling through the local Half-Priced Books, and what should I happen upon, but a copy of the long out-of-print "Electronic Music Projects, vol 1", by the great Forrest M Mims, III. Out of curiousity, I checked out Mr. Mims' original design for the Light Sensitive Theremin. And boy-howdy, there on p.92, is the EXACT same schematic that I received in the mail a few days before [with the minor addition of a 1/4" jack, to be fair]. Hmm, somebody's takin the name Forrest M Mims III in vain, and that sort of nonsense just doesn't fly around the BlipVert ranch, i tell you what. So, in the interest of freedom of information, and also the interest in making cool noise, I present to you, the Light Sensitive Theremin. Enjoy.

Pieces Parts List:

B1 1.5 volt battery (C cell)

C1 0.1 uf capacitor (272-135)

PC1 Cadmium Sulfide photocell (276-1657)

Q1 2N3904 NPN transistor (276-1603)

Q2 2N3906 PNP transistor (2N3906-ND)

SPKR 9-ohm miniature speaker (40-250 or similar)

J1 1/4" jack (274-280 or similar)

All part numbers are Radio Shack catalog numbers, except for Q2, which is available from DigiKey. For some reason, RS no longer markets this transistor. Also, you can use the above RS CdS cell, but a cheaper alternative is to check with American Science and Surplus, they sell random assortments of CdS cells for 10 cents apiece.

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misc: you'll probably want to mount the works on either some sort of perfboard, or a little breadboard (276-175 works pretty well); a project box to put the completed circuit in; a C battery holder (270-402); and some sort of on-off switch between the battery and the photocell, so you don't have to keep taking the battery out when you're done playing with the circuit.

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I was able to slap the circuit together in about 10 minutes using a little breadboard (it hardly pays to drag out the soldering iron for this circuit). I tossed the thing into one of those black project boxes, cut 2 small holes into the top of the box, ran the terminals of the CdS cell thru these holes, and glued the CdS cell down. You may want to improvise some sort of sleeve over the CdS cell to limit the amount of light that strikes it. I used a piece of black rubber tubing about 3/4" long, and it works great.

A few other notes: The original circuit calls for a 0.01 uf capacitor, but I found that this resulted in too high a pitch to be useful. I substituted a 0.1 uf cap, and it sounds pretty good. Obviously, all this circuit is is a simple oscillator. The pitch is altered by the amount of light that falls on the CdS cell. The more light, the higher the pitch. The output volume is pretty quiet, so I'd recommend hooking this up to an external amplifier of some sort. I use a 100W bass amp, which my neighbors love. If you do this, by looking at the circuit design you might think you could leave out the little tiny 8-ohm speaker, but if you do this, the circuit won't work.

Make sure you operate this circuit in a well lit area. I've found that flourescent lights give it a strange, fuzzy kind of sound which is pretty neat. Just wave your hand over the photocell to alter the pitch. With some practice, you can hammer out some simple tunes, but I myself usually don't have the patience, I just use it to make noise. For a real kick, try running it thru a guitar delay pedal, this causes some really strange effects.

Now Hear This! A few seconds of the 3rd movement of "Metal Machine Music", as played by Billy Colbeck on the Light Sensitive Theremin w/ delay pedal.

If you have any questions or comments regarding this circuit, click here to drop me a line.

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