Understanding a heat sensor circuit?

AI Thread Summary
The discussion centers on the temperature dependence of the β parameter in npn transistors and its effect on collector current, which activates an LED. Participants express confusion over the resistor arrangement and the necessity of a diode that maintains forward bias on the Base-Emitter junction. There is a debate about whether the diode's temperature-dependent forward voltage is counterproductive to the circuit's intended function. Clarification is sought on how the potentiometer maintains sensitivity in the circuit. The circuit is suggested to function effectively as a differential thermometer by measuring temperature differences between the diode and transistor.
Raihan amin
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I am builiding a heat sensor circuit using this circuit model. But first I want to understand how this circuit is working.
Circuit link: https://circuitdigest.com/electronic-circuits/heat-sensor
What I understand is that the β parameter of the npn transistor is temperature dependent. Increasing the temperature increases the collector current and hence the LED turns ON. But I don't understand the resistor arrangement.Also I can see that the diode is keeping the Base-Emitter junction in forward bias. But why this is necessary? I believe the pot is supposed to maintain the sensitivity but how is it doing this?

Looking for some thoughtful explanation.
 
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The use of the diode seems counter-productive to me, since its ## V_f ## is temperature dependent as well, and would seem to counter the temperature effect that they are trying to use the transistor for...
 
Raihan amin said:
What I understand is that the β parameter of the npn transistor is temperature dependent.
It is not beta that is temperature sensitive, it is the base-emitter voltage.
I agree with @berkeman The reference is also a PN junction, namely the 1N4148, so it gets confusing as to what temperature is being measured, relative to what. Was it a failed attempt to build a bandgap reference?
 
That circuit would work fine if you consider it a differential thermometer.
It reacts to the temperature difference between the diode and transistor.

Cheers,
Tom
 
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