LM358 Overheat Detector Circuit

Simple LM358 Overheat Detector Circuit with Relay is Designed to act as a Stand alone Overheat or Temperature Limit Switch. This circuit uses a NTC Thermistor as a Temperature Sensor and Operational Amplifier IC LM358 as a Comparator and a 12V SPDT Relay.

By using this Temperature Switch circuit we can control target load depends on some threshold temperature, this temperature level can be adjusted and Calibrated by using Variable Resistor RV1.

Circuit Diagram

Components Required (BOM)

2R2, R31KΩR_Axial_DIN0204_L3.6mm_D1.6mm_P7.62mm_Horizontal2
9RLY112V RelayRelay_SPDT_SANYOU_SRD_Series_Form_C1

Construction & Working

This Temperature Sensor switch circuit contains a temperature sensor (NTC Thermistor), a Comparator amplifier and a 12 Volt SPDT Relay.

What is Thermistor?

Thermal Sense Resistor or Temperature Depended Resistor is known as a Thermistor. This device Resistance changes according to the temperature. Thermistor can be a NTC or PTC which means,

NTC – Negative Temperature Coefficient.

PTC – Positive Temperature Coefficient.

Here NTC – Element Resistance Decrease when Temperature Increases. PTC – Element Resistance Increase when Temperature Increases.

In this Overheat detector circuit 10KΩ Range NTC Thermistor is connected with Inverting Input of LM358. Non Inverting input of LM358 is connected to the variable terminal of RV1. So the operational amplifier detects signal from RV1 and thermistor then compares and gives output at pin 1. This output signal is not enough to drive a 12V Relay coil hence Transistor Q1 BC549 used as a switch for Relay coil. Here LED indicates the status of ON condition of Relay.

Printed Circuit Board

LM358 Overheat Detector Circuit PCB Gerber Files.

Interactive Board Viewer


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