In the seventies, in the middle east, we had an automatic fire extinguisher installed (for insurance requirement) which was based on compressed CO2. The flame sensors over the generators were microswitches held open by treated cotton cord, described as "fulminating cotton", which would instantly ignite and release the switch when touched by an open flame. Searching this on line, I cant find any reference to it, except that "fulminating cotton" is evidently gun-cotton, which isn't something you'd want around the house! There do appear to be many flame sensors available, however.
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