This design is from a magazine called "Divirta-se com a Eletrônica", which translates to "have fun with electronics", and, indeed, is very fun.
For those who don't know what a Theremin is, it's one of the first synths ever made and it's named after his inventor, Mr. Leon Theremin (a Russian guy). It's perhaps the only instrument which you don't actually touch: there's an antenna, and as the musician moves his hand near it, the pitch changes. That's the thing Page used in "Whole Lotta Love" (he used it with an echo, to get even weirder). This circuit is a very simplified version of a theremin, but is also very easy to build and really works!
How it works
It is a RF oscillator. As your hand moves near the metal plate, the capacitance "seen" at the collector of the transistor will change, and so will the frequency of oscillation. What you need now is an AM radio receptor (any) that will do the job of turning this to sound.
Details of Construction
The metal plate:
A 8 cm x 8 cm square of any metal will do the job, as long as it is thin. If you use copper, you will be able to solder, with aluminum you will have to bolt on the wire to the circuit.
1) Get a ferrite bar of 0.5 x 1 x 5.5 cm (approximate). It's that black bar you find inside AM portable receivers.
2) Get about 2 m of copper wire number 24 or 26. You can "steal" it from a burnt transformer.
3) Wire 70 turns of this wire over the ferrite bar. Solder the "center tap" at turn number 35 (you will have to scrap the varnish).
Put the Theremin VERY near the AM receiver. turn SW on and search over the receiver's dial for a whistle. Now approach your hand slowly to the metal plate - the whistle's tune will change. Try moving slow, fast, shaking your hand - with some practice it's possible to get interesting sounds. Here are some not so interesting sounds...
If the Theremin's center frequency lies over an existent AM station, try tuning it by making the turns of L1 closer or spaced. When you get a nice "tuning", you can fix it in place by dropping hot candle wax over it.
Don't mount it inside a metal box - use a plastic one.