The schematic of an automatic cooler fan for audio amplifiers is given here. The circuit automatically switch ON the cooler fan whenever the temperature of the heat sink exceeds a preset level. This circuit will save a lot of energy because the cooler fan will be OFF when the amplifier is running on low volume. At low volume less heat will be dissipated and it will not trigger the cooler fan ON. The temperature is sensed using an NTC (negative temperature coefficient) thermistor R2. Junction of thermistor r2 and resistor R1 is connected to the inverting input (pin3) of IC1 which is wired as a comparator. The non-inverting input (pin2) is given with a reference voltage using the preset R3.

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As temperature increases the resistance of NTC thermistor will drop and so do the voltage across it. When the voltage at the inverting input becomes less than that of the reference voltage (set for a particular threshold temperature) the output of the comparator goes high and switches the transistor Q1 ON. This will activate the relay and the cooler fan will be switched ON. When the temperature decreases the reverse happens. LED D2 will glow when the fan is ON. Diode D1 is a freewheeling diode.

Components Required
1. Temperature Sensor LM35
2. Op-Amp IC (LM324/741)
3. Motor Driver IC L293D
4. Potentiometer-10k
5. DC Toy motor
6. Propeller
7. 7805 Voltage Regulator
8. 100uF Capacitor
9.Multi-meter (Digital one is Preferred)
10.A Breadboard
Description
Basically the circuit switches ON a toy motor fan connected to the motor driver whenever heat is applied to the temperature sensor unit using say a burning matchstick. The wind from the motor fan would blow off the matchstick and switches OFF itself automatically.

Here the Temperature is the output to be controlled. A feedback is generated using the sensor which is fed into comparator which acts as a controller here. And the power is the input. The Temperature sensor LM35 is very accurate whose output is directly proportional to its surrounding temperature. For every 1°C of rise/fall in temperature, the output voltage of the sensor varies by 10mV.

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