Automatic temperature controlled fan
Here is a circuit through which 
	the speed of a fan can be lin-
	early controlled automatically, depending on the room temperature. The circuit is highly efficient as it uses thyristors for power control. Alternatively, the same circuit can be used for automatic temperature controlled AC power control.
In this circuit, the temperature sensor used is an NTC thermistor, i.e. one having a negative temperature coefficient. The value of thermistor resistance at 25oC is about 1 kilo-ohm.
Op-amp A1 essentially works as I to V (current-to-voltage) converter and converts temperature variations into voltage variations. To amplify the change in voltage due to change in temperature, instrumentation amplifier formed by op-amps A2, A3 and A4 is used . Resistor R2 and diode D2 combination is used for generating reference voltage as we want to amplify only  change in voltage due to the change in temperature.
Op-amp A741 (IC2) works as a comparator. One input to the comparator is the output from the instrumentation amplifier while the other input
is the stepped down, rectified and
suitably attenuated sample of AC voltage. This is a negative going pulsat-
ing DC voltage. It will be observed that with increase in temperature, pin 2
of IC2 goes more and more negative and hence the width of the positive
going output pulses (at pin 6) increases linearly with the temperature. Thus
IC2 functions as a pulse width mo-
dulator in this circuit. The output
from the comparator is coupled to
an optocoupler, which in turn contr-
ols the AC power delivered to fan  (load).
The circuit has a high sensitivity and the output RMS voltage (across load) can be varied from 120V to 230V (for a temp. range of 22 to 36oC), and hence wide variations in speed are available. Also note that speed varies linearly and not in steps. Besides, since an optocoupler is used, the control
circuit is fully isolated from power circuit, thus providing added safety. Note that for any given temperature the speed of fan (i.e. voltage across load) can be adjusted to a desired value by adjusting potmeters VR1 and VR2 appropriately.
Potmeter VR1 should be initially kept in its mid position to realise a gain of approximately 40 from the instrumentation amplifier. It may be subsequently trimmed slightly to obtain linear variation of the fan speed