deadman / watchdog circuit

[digiman]

For a project, I need to design a circuit that will make sure that a relay (connected to a load) will not get stuck "on" if the computer that controls it freezes with the input to the circuit being either high or low. I think I recall this being called a dead man switch.

COMPUTER ----> DEAD MAN CIRCUIT W RELAY ------> LOAD

The best thing I could come up with is a pulse having to be generated by the driving computer that must be present for the deadman circuit to remain active and the relay closed. If the driving computer freezes with the output high or the output lo, the circuit opens the relay and shuts down.

The responsiveness could be very slow -- Ideally a pulse of once per second would be sufficient and the response time for the circuit could even be as long as 30 seconds.

Attached is a rough "guess" for what is needed... Anyone have any ideas? Any better way of doing this? Do the values look kind of correct in the attached circuit?

ANY help would be greatly apperciated!

Ted

[pebe]

Can you reduce the size of your circuit file?
It's too large to read.