Odp: [sdiy] 3D phototheremin [was: What makes you drool?]

[Roman Sowa]

> When you get this one figured out, Roman, let me know... I'll like to
> breadboard it.  ; )

sure thing!
I have now only strong feeling that it is doable. All I've done so far
is 1D theremin, works pretty well (yeah, perfect for me, as I haven't
had a chance to compare it to any other).

Here's my idea:
4 LEDs, driven in circle fashion, one at a time, and one photo transistor
in the middle. Let it be HC138 controlled by 2-bit counter. Same
address is used for a MUX to distribute phototransistor's output
(after filtering, conditioning etc.) to 4 detectors, or as we'd say here,
envelope followers. Then 2 differential amplifiers generate X and Y
control voltage depending on difference of light level reflected to
phototransistor from 2 LEDs in one axis. I hope it's clear so far.
Z output would be simply mean value of all detectors.
To make it more usable, Z could also control VCAs of X and Y
outputs to compensate small difference of received signal
when hand-in-da-beam is far from the 'eye'
Now, the trick is to get LEDs with proper beam angle, and
put them in right positions.

Roman