Thermal switch with the LM741 Circuit Idea

I recently needed a simple way to trigger a fan based on temperature and after some research and whatnot I’ve come up with this basic circuit, there is a few around the traps similar but figured it might be useful to post my version here. Using the LM741 IC, 3 resistors, a PNP Transisto, a trimpot and a 10k thermistor. An LED is added just cause :P.

So the circuit works by using the 10k thermistor as a sensor and creates a reference. The circuit can be calibrated using the 20k trimpot. The PNP transistor is capable of 1A throughput and you can use it to operate a fan, light, or even a relay to then operate a larger fan, pump or whatever you feel like. It can technically run from 10v – 36v however the 1k trimpot may need to be upgraded to prevent damage to the Transistor.

Thermal Switch Circuit using LM741

Thermal Switch Circuit using LM741

Here is a board layout I did as a single layer with standard components. Measures 25mm x 39mm. I’ve put up the “overlay” version as twice the size so it can be used as reference and the board itself is looking at the top layer same as the overlay version, so remember if you are doing an iron transfer or similar you will need to mirror it before printing.


Update! – I was playing around with this a few hours ago and wanted to prevent the fan from pulsing when close to the cut-off threshold. IE add in some hysteresis. I simply added a 220K ohm resistor between Pin 3 and 6 which gives it a few degrees of hysteresis. You can have more control over the hysteresis if you would like by putting in something like a 200-500K ohm trimpot between 3 and 6. the 220k ohm resistor gave about 0.5 degree Celsius of hysteresis. IE device triggering on at 45.8 would turn off at 45.3.


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