For proper functioning of a circuit, its ambient conditions are to be maintained in an optimized manner. There shouldn’t be much of a mess with the temperature, neither humid condition are acceptable. Tackling out the heat factor of a circuit is this heat sensing circuit which alerts the user whenever the circuit tends to get excessively heated in its due course of operation.
This simple heat sensor circuit acts as a warning device whenever the temperature exceeds the preset level in its vicinity. It is an ideal circuit to monitor the heat generating devices like PC, Inverter etc. The circuit is compact and can be incorporated in the device itself.
The heat sensing element is the NTC Thermister and its resistance drops when the temperature increases. The image below shows how a NTC Thermister looks like:
Working of Circuit
Resistance of Thermister is set by VR and if the temperature is below the preset level, transistor T1 will be out of conduction and keeps the buzzer and LED off. When the temperature near the Thermister increases, T1 conducts and buzzer beeps.This Heat Sensor Circuit can be powered from the device itself or use two 3 volt lithium cells. If it is used in PC, 5 volt DC can be tapped from the USB port. Use a small DC buzzer that works off 3-12 volt DC.
TH 4.7 K Thermister NTC
R1,R3 470 Ohms, R2 1K,
T1 BC 547
D1 Red LED, BZ 3-12 Volt Piezo buzzer, S1 Toggle switch, 3V x 2 Lithium cells