Understanding a heat sensor circuit?

[Raihan amin]

TL;DR

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.

 

[berkeman]

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...

 

[Baluncore]

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?

 

[Tom.G]

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