Mains box heat monitor

Here is a simple circuit mains box heat monitor for sensing high temperature due to overheating. This circuit helps to prevent disasters caused by only sparking in the mains box due to any fault like short circuit over loaded. This circuit is so designed that it also automatically switches on a bright while LED (LED3) in case of power fails. This light is sufficient to check the mains box wring or fuses in darkness.

Circuit Description Mains Box Heat Monitor

The circuit is build around timer IC, a sensor and two transistors. The input AC mains is stepped down by using a 0-9v 500mA transformer and rectified by using bridge rectifier build using four diodes D1 through D4. The rectified voltage is further filtered by using 1000 µF of capacitor (C1) to smoothen the voltage and bypass the ripple. The output is given to LED1 through resistor R1 to indicate the power-on condition. Resistor R1 is used here as correct limiter.

circuit diagram of mains box heat monitor

Diode D5 is a germanium diode which is here act as temperature sensor and is used in reverse bias mode. The resistance of diode D5 is high at normal temperature areal as a result transistor T1 conducts to hold pin 4 of IC1 in low state. Potentiometer VR1 is used hear for adjust sensing capability of circuit.

Timer IC (IC1) is configured here as an astable multivibrator and generate frequency when the temperature around diode D5 rise due to overheating of the fuse. The resistance of diode D5 is decrease when temperature rise and transistor T5 stop conducting and this enable IC1. The output of IC3 is given to LED2 through resistor R6 for visual indication. A piezo buzzer (PZ) is connected parallel to LED1 as shown in circuit diagram for audio indication.

The emergency light circuit is build around PNP transistor (T2). It is powered by a 9V rechargeable battery. The battery B1 is also charged constantly via forward bias diode D6 in mains on condition. The forward bias of diode D6 results in reverse biasing of transistor T2 and thus white LED (LED3) is off. Similarly in absence of mains transistor T2 is forward bias and light up the LED (LED3)

The circuit beeps once when power fails and again when power resumes.

BEP NOTE: Diode D5 should be placed close to the fuse and the LED3 should be directed towards the fuse such the maximum lights falls on the fuse. In BEP LAB we test the circuit by putting the hot tip of soldering iron near diode D3. The piezo buzzer PZ1 will sound to indicate the high temperature.


Resistor (all ¼-watt, ± 5% Carbon)

R1 = 330 KΩ

R2 = 100 K KΩ

R3 = 1 K KΩ

R4 = 56 K KΩ

R5 = 4.7 KΩ

R6 = 220 Ω

R7 = 47 Ω/0.5W

R8 = 470 KΩ

R9 = 150 KΩ


C1 = 1000 µF/25V

C2 = 0.01 µF


IC1 = NE555

T1 = BC548

T2 = BC558

D1 – D4 = 1N4007

D5 = 1N34

D6 = 1N4001


X1 = 230V AC primary to 0-9V 500mA, Secondary Transformer

PZ1 = Piezo Buzzer