Overheat/Overcooling circuit breaker using temp sensor (LM35) & comparator (LM339)

This simple project with easy to build electronic circuit is very useful to fight with problems like overheating or overcooling of electronic devices which in turn leads to their damage. This circuit automatically switches off the circuit in event of excess heating or excess cooling. With small modification it can work as both overheat circuit breaker and overcooling circuit breaker.

This simple project has a circuit is based around the temperature sensor LM35, which senses the surrounding temperature and gives the corresponding voltage output at its output pin, its output voltage when multiplied by 100 gives the temperature sensed in degrees. By using this fact the reference can be set accordingly.

For overheat circuit breaker, reference is set to a higher value. A NPN transistor is used to drive relay as the output current of the comparator is not sufficient before to drive the relay. When temperature is below the reference temperature set, the output of comparator is low and the transistor is in the cut off state and consequently relay does not work. When temperature goes beyond the set reference value than output of comparator becomes high, thus biasing the transistor T1 and activating the relay and breaking the external circuit.

For overcooling circuit breaker reference is set below the surrounding temperature and PNP transistor is used. In default state output of comparator is high but PNP transistor works as not gate as it does not gets biased at high voltage and consequently relay does not work. When temperature becomes lower than the set value, than output of comparator becomes low thereby biasing PNP transistor and thus activating relay and breaking external circuit.