How about a Walmart Smoke and Flame ionization
detector for 10 bucks. Or get one of the photoelectrics for smoke and use the ionization for flame. Get the models that slave or simply put a resistor and optoisolator across the piezo beeper. You could probably connect a low voltage
bidirectional PLC input directly. They've got all kinds of UL CIL,yada, yada, certifications and are highly reliable. Or you could get one from the commodity home alarm vendors. Or you can get a special automation model for $1000.00 that
detects "industrial" smoke and flame.
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