Proximity sensor?

[Fraser, Colin J]

> -----Original Message-----
> From: Andrew Schrock [mailto:aschrock at cs.brandeis.edu]
> Sent: 30 September 1999 02:53
> To: SynthFolk
> Subject: Proximity sensor? 
> 
> my question today is: what do you 
> think is the
> best way to implement a proximity sensor? I was thinking some 
> sort of IR
> switch, but I've always had a bit of trouble with those being 
> reliable. I
> need it to turn the sound on when somebody walks in front of 
> it and go off
> when they walk away. Seems fairly simple, but it has to be 
> rock-solid. Oh,
> did I mention cheap? :) 

Sounds very like a device invented in the 20s by a Russian living in America
call Leon Theremin.
He built a device that used a pair of high frequency oscillators and an
antenna, that would generate a sound when someone walked past it.
The idea was it could be mounted in a doorway as an automatic 'doorbell'.
It turned out to have musical applications too.

You can build a cheap one with cmos logic oscillators, and use a rectifier
to detect the audio output / presence of a body.


Colin f