Flame detection

[Homer J Simpson]

Electronics isn't really my field so this is an appeal for help for
something which is probably embarrassingly easy if you know what you are
doing.

I want to detect the presence or absence of a small gas flame (caravan
fridge) and light an led if the flame is there and turn it off if the
flame isn't there. A reasonably quick response time is needed.

Presumably a thermocouple (I think I have read the term "K type"
somewhere), a transistor an led and a resistor or two will be involved! It
would be nice if I could run it off 12v DC and it would be nice if the
current drain could be zero or at least tiny if the led is off.

Elektor "304 Circuits" #119 page 157

'On' indicator for gas-operated fridges

119

This 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