Help with a simple flame detctor circuit

Mike's picture


Joined: Aug 06, 2020

Replies: 0
Help with a simple flame detctor circuit
August 6, 2020 - 6:24am

Hopefully someone can throw some ideas out about this project. I have a propane/LPG powered absorption fridge but the sight glass doesn't work for whatever reason, the only way to check the burner is lit is by removing an access panel at the back of the fridge and looking at the burner.

So I thought it should be simple enough to make a circuit that detects the light from the burner and turns on an LED at the front of the fridge.

I've been doing this for days now! Everything I have tried does not work.

First attempt was a LDR and a NPN transistor amplifier, but there isn't enough light coming from the burner to trip the circuit. In use, the burner is in total darkness. However, the flame is blue (propane).

I read on one post that the spectrum of a propane flame tends to the infrared. I got some IR photodiodes but testing them on the bench using a blue propane flame doesn't seem to work either.

I've ordered a K-thermocouple, but all the circuits use IC's to measure temperature. I don't need to know the temperature, I just want an LED to indicate that the burner is lit!

Has anyone got a simple circuit that works on 12v DC, preferably using a transistor etc, that could do this for me?

I'm at the stage now where I am planning on buying a universal thermostat with a thermocouple probe that switches on/off at 50-150C











Debashis Das's picture


Joined: Dec 02, 2019

Replies: 117

Thank you for your question, after a few google searches I have found that the propane flame indeed emits light in the UV spectrum.


Now, when that's out of the way I can suggest to you that the LDR circuit should work if you make a filter in front of it by using a UV Pen.


To test my theory just make the filter, Put it on top of the LDR, hold it in front of the flame, and measure the resistance of the LDR using a multimeter.

if the change is significant you can add an OP-Amp to your circuit and it should work just fine.




Sourav Gupta's picture


Joined: Feb 12, 2018

Replies: 696

It is difficult to answer you qustion since the construction of the oven is not described or it is impossible to check from here.

Did you used any type of Flame sensor? that does not cost much and works perfectly.