Elektor "304 Circuits" #119 page 157
'On' indicator for gas-operated fridges
119This indicator is intended primarily for refrigerators used by campers or caravan owners. Whether the gas flame is on may be indicated with the aid of a thermocouple, a device that reacts to variations in temperature.
It normally consists of two dissimilar metals - here iron and constantan soldered or welded together at one point. When the temperature at the weld of the thermocouple is about 150°C, the e.m.f. across B is also 0.7 V. This causes the LED to light, indicating that the gas flame is on.
Circuit in ABSE