555 Timer Flame/High Temperature Monitor for Fridge.
The circuit below was preferred.
555 Timer IC - Fridge Gas Flame ON Indicator.
How it Works
RV1 controls the voltage to Pin 2. With the Thermistor at a high resistance when cold. this then places a high voltage on this pin. This in turn produces a zero voltage on pin 3. With no voltage the FET Transistor cannot turn on, in turn the LED is OFF
When the thermistor warms up, it's resistance falls at pin 2 rapidly, to a low value 1Kohm. At this low value the voltage, about 4 volts, will activate the trigger in the 555 Integrated Circuit, which places a voltage at pin 3. This in turn will switch the FET transistor on and allow the LED to alight.
While the above drawing is similar as the top illustration, it is the actual final working module, with one less component, i.e. the relay. I have included all details in drawing for those who want to print it out .
I have found the components are not critical in this Schmitt Trigger circuit arrangement. I have included a 10K resister in series with the variable as it reduces the voltage at pin 2. increasing the turn on speed. The 100K thermistor was changed for a 4k7 Ohm as this too speeded up the switching action.
The circuit board came from a Maplin project called 'Multi Function Car Courtesy Light Extender' The components except the Bead Thermistor came with the project, that cost about £0.49. It was used because the 555 timer was part of the project and cost only £4.99. By using only the components needed this board blends into project nicely. Maplins have a temperture monitor available which would be more suitable for others as a project.
The LED's can also be purchased from Maplin in small project packages. The MOSFET Transistor came with the project board. Again any NPN transistor could be used in this application.
The Bead Thermistor is the critical component of the project. The whole project depends on it. While it is heat detecting it wont last long placed close to a flame. It is very sensitive so fitting it around the area of the flame funnel should get it to work. So I placed it inside a thin metal tube, pinched at the flame end.
All Parts were sourced from the Electronics Dealer Maplin in the U.K. at: www.maplin.co.uk
Gas Flame Printed Circuit board
The Thermistor is enclosed in a thin walled metal tube, and placed just inside the fridge gas burner
How I constructed the fridge gas burner monitor, with tube enclosing the Thermistor.
In service when the gas was turned on, the LED turned on after about 15 seconds. When the gas was turned off, or in a failed condition, the retained heat of the burner held it on for one and a half minutes.
The LED indicator was fitted to the fridge panel alongside the 12volt indicator. Another project seen in my other pages.
This year  in the South of France I found the heat from the Heat Exchanger column would activate the device. I then adjusted RV1 until it turned off.
Regarding the construction of the enclosed Thermistor. This is rather laborious, a complete fitting can be purchased from DIY stores which supply parts for domestic boilers. It would be similar to the one used to control the original flame on sensor.