Thermocouple wire going to a comparator. Simple, cheap, and dead

reliable.

This is more or less the fire loop used in commercial aircraft engines.

Mike Harrington

Seems there was an article in kitplanes many moons ago describing fire
detectors that used a low melting point metal. the metal melts, circuit
opens, the big red light comes on. This has the advantage of letting you
know if the circuit goes bad.

Monty

 
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