What is the role of the diode in this heat sensor circuit?

Question

^{simulate this circuit – Schematic created using CircuitLab}

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.

Answer 1

The diode forms a crude reference. It is assumed to be at constant (room) temperature. If you heat the diode up you'll get an opposite effect to heating the transistor.

The effect is primarily due to the Vbe of the transistor decreasing, thus tending to turn it on as the temperature rises, not hFE.

Here is a simulation with the transistor heated from 20°C to 200°C (pot set at 50%):

The plot indicates LED current. Depending on the LED it may be pretty visible at a fraction of 1mA.

If you want a more stable sensing for a few pennies more you can connect the transistor as a diode, add a reference (eg. TL431) and use a comparator or an op-amp to do the comparison.

Answer 2

BE on the Transistor is a diode as well. With the diode D1 you're basically making a low reference voltage that can be set more precisely with the potentiometer.

Answer 3

You set the brightness to room temp and the use the diode as a heat sensor isolated from the transistor at room temp.

The PN junction has a NTC value approx -2mV/+1'C with the initial somewhat balanced current shared between diode and Vbe and the LED is ON. As the diode heats up Vbe drops due to the diode voltage dropping as it draws more current and the base draws less current and the LED gets dimmer.

The relationship of Vbe to Ic is a quadratic one and even if the hFE was constant the base current is not a linear one even with a fixed low base resistance in the pot as the transistor Vbe also changes with base current. In the end it is exponential so if a 50'C rise was considered full scale minimum LED current, the 25'C rise won't exactly be the mid-point of LED current. But it will indicate an inverse brightness with diode temp rise.