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

[PRCamann at aol.com]

In a message dated 5/20/2003 12:48:38 PM Eastern Standard Time, modular at go2.pl writes:

> 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.

Sounds similar to the arrangement used in the Alesis airFX and airSynth.  I bought a used airFX and opened it up; now I'm trying to get some sort of usable analog signal from its detectors....

Paul Camann