Proximity sensor?

[Andrew Schrock]

On Thu, 30 Sep 1999, harry bissell wrote:
> I'd use a light sensor (beam break) you can have the electronics up by the
> exhibit, send a beam to a retroreflector (one of those corner prism units they
> sell for marking driveways etc.) The retroreflector sends light back exactly
> from the direction it came, so alignment is not real critical. This could give
> an output that is maintained as long as the person breaks the beam..
> 
> IR units are sensitive only to CHANGES in heat signature... if you stand still
> they will go off for sure...

Yeah, that's what I meant when I said I haven't had terrific luck with IR
sensors being reliable. 

> Ultrasonics will do the same, but they are sensitive to motions of about 3.3
> inches
> (assume 40khz) so you have to stand "really still". But they are also
> sensitive to air movement etc...  They are "change only" devices also...
> 
> I worked in the motion sensor field for a couple of years...
> 
> How about a "Tapeswitch" mat hidden under a carpet....  Buy from a local
> burgular alarm shop ???

This sounds like the most reasonable idea. I'm sure I could rig up
something cheap and easy to work well. The CHEAPEST way I can think of
involving a floor sensor is a few tact switches beneath a piece of
plywood, but I think people might notice that, it would be a bit unweildy. 

I'll have to check out burglar alarm shops in my area.. or steal one from
an alarmed house! I bet they didn't think that would ever happen ;) 

Thanks
Andrew

-| Andrew Schrock | aschrock at cs.brandeis.edu |-