Is it simply a matter of knowing what freq squarewave to detect??
If that's the case, why note use a small 8pin PIC/Picaxe chip, programmed to detect the tone one 1 pin, and trigger out on another, and obviouly easily adjustable to help calibrate it...
food for thought,
Marty.
You are correct Marty - but I always look for the method that is the easiest to maintain/repair for the future. A programmed PIC will be obsolete as fast as the chip used in the original board IMO and create servicing issues. In this case I have used a tone decoder IC that is available anywhere - it will only pass the frequency that you set with a few external parts. Set it for 40khz and away you go - simple!
I have tweaked a few of the values and I have had it running perfectly for over a day now so I am very confident all is well.
It will be interesting to have it tested in another machine to ensure all id OK.
Here is my final layout (well, final at this stage anyway...)
