I am working on a circuit for a science competition, and one of the tasks is cooling a thing of water to a certain temperature (we are doing about 25C to 18C) and then using that to trigger a circuit. To do this, I am using a PTC thermistor, I have a TTC-103 one. The challenge I am facing is how to set up the circuit, so that when the resistance drops to a specific amount with this thermistor, to trigger another circuit, here a UV light.

One way I thought was just wiring the thermistor in parallel and once the resistance drops, the light can turn on. But with such high resistance still, the light is weak and the circuit is not as cleanly cut off as I would like. Someone suggested a voltage matching circuit, where the resistance drops enough that the voltages are equal. I, however, do not fully understand how I would implement this with a thermistor, so if you could provide me with an example of how to set one up, that would be amazing.

Thank you!


1 Answer 1


I can give you the below example for starters. Just get rid of the pnp part for avoiding negative current.

*Addition, transistor part is representation of the Darlington pair.

TEC cooling


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