This is perhaps the simplest implementation of a temperature controlled fan. It requires only two transistors, a few resistors, thermistor and small 12V fan. While performance is not quite as good as other op amp implementations, it is a very interesting experiment.
Schematic of the temperature controlled fan circuit
The circuit consists of a four element balanced bridge, a transistor imbalance detector and a single transistor power amplifier for driving the fan. Some may recall tech school experiments that employ galvanometers as bridge imbalance detectors. In this case, transistor Q1 performs this function. Unfortunately performance is far inferior than the galvanometer due to the transistor 0.6V base to emitter junction potential.
The bridge has been modified to replace one of the resistors with a zener (D1). The advantage of doing this is to provide a low impedance point at the emitter of Q1—this is necessary to enable the transistor to output significant emitter to collector current for the purpose of driving the base of Q2. It also increases the transistor voltage gain so it can more easily detect voltage imbalance. The drawback of doing this is that it makes the temperature threshold subject to power supply voltage variations.
When the NTC thermistor (R2) is heated, its resistance drops and the voltage at the base of PNP transistor Q1 drops thus causing base current to flow. Supposing that the hFE of Q1 is 500, when its base current conducts only about 2uA, Q1 turns on sufficiently to output a 1mA collector current to turn on Q2. Supposing that the hFE of Q2 is 150, its collector current is now 150mA or full fan current.
While it is marginally sensitive enough to detect the body heat in my fingers, the soldering iron demo is a good deal more impressive. Just holding the soldering iron tip near the thermistor heats it quite rapidly, then when the fan starts, the forced air cooling cools it also very quickly.
While it functions OK, its sensitivity and temperature stability is relatively poor. However, it is a great experiment. If I was making one for a real application, I would use the LM358 op amp or LM393 comparator.