I am building a heat sensor circuit using this circuit model.
What I understand is that the \$\beta\$ parameter of the NPN transistor is temperature dependent. Increasing the temperature, i.e, lighting something in front of the transistor, increases the collector current and hence the LED turns ON.
I don't understand the resistor arrangement. I can see that the diode is keeping the base-emitter junction in forward bias ,but why is this necessary? I believe the potentiometer is supposed to maintain the sensitivity, but how is it doing it?
I want to understand how this circuit works.
The circuit model can be found here.