Russell McMahon wrote:

>I have an application requiring very short range low speed telemetry.
>Low cost is an overwhelmingly significant factor.
>The application is to transfer an occasional on/off signal over a distance
>of around 1 metre.

I assume:

  1) It is a person who want to originate the signal

  2) That person can tell wether the current state is on or off, so he only need to be able to assert a "toggle" signal.

Like... Clap his hands! (once...)

(Assuming there is only one machine in the room, i.e. home usage )

The detection should pe simple, as a clap has
High frequency (lot of frequency above hearable)
High pressure level
Steep attack/decay

So a simple mic, amp stage with high pass filter, rectifier, and peak detect to toggle a flipflop... sorry, I mean give signal to a PIC.

Using a PIC we can use double claps, or clap-pause-clapclap, and we can control the whole home, just we can memorize it all.

Once I had an idea to control all lights etc in the home etc clapping, but realised it would get pretty annoying...

Cost of transmitter: Zero
Battery lifetime: lifetime
Cost of reciever: also low


Regards
/Morgan