LM35 Temperature Sensor Switch with IC LM358

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The temperature sensor switch is built across the two ICs LM35 and LM358. The relay in the circuit activates any electronic circuit connected to it when LM35 senses the desired temperature level. Furthermore, the circuit is very simple involving just the two ICs, a transistor, and a relay connecting other electronic devices/circuits.

Hardware Required

S.no	Component	Value	Qty
1.	Temp. Sensor	LM35	1
2.	Operational Amplifier IC	LM358	1
3.	Variable Resistor	10KΩ	1
4.	DC Battery	5V-12V	1
5.	Diode	1N4007	1
6.	PNP Transistor	2N3906	1
7.	Relay	5-12V	1

IC LM35

VCC	Positive supply of battery
OUT	Analog output voltage proportional to temperature
GND	Negative supply of battery

Features

Celsius calibration
Range -55°C to 150°C
Low Cost
Operating voltage 4V to 30V

IC LM358

PIN No	PIN Name	PIN Description
1	Output 1	Output of operational amplifier 1 of the IC
2	-input 1	Inverting input of operational amplifier 1 of the IC
3	+input 1	Non-Inverting input of operational amplifier 1 of the IC
4	Ground	Negative battery supply
5	+input 2	Non-Inverting input of operational amplifier 2 of the IC
6	-input 2	Inverting input of operational amplifier 2 of the IC
7	Output 2	Output of operational amplifier 2 of the IC
8	Vcc	Positive battery supply

Features

Large voltage gain: 100dB
Very low current drain
Unity gain through internal frequency gain
Wide range of supply voltage: 3V to 32V

Circuit Diagram

temperature-sensing-switch-using-lm35-&-LM358.gif — **LM35 Temperature Sensor Switch Circuit**

Circuit Explanation

The IC LM35 is a temperature-sensing IC that detects temperatures up to 120°C. The 10KΩ variable resistor is set according to the desired temperature sensing level. Once the value of the variable resistor is set, LM35 when senses the desired level of temperature, thus, producing a signal sent directly to LM358. LM358 is a dual-operational amplifier IC. Moreover, LM358 works as a comparator of the input voltages.

LM358 receives the input signal and amplifies it. Furthermore, to operate the relay, transistor 2N3908 amplifies the signal. Moreover, the diode connected to the relay prevents voltage reversal. Also, the relay is selected so that its rating be same as the battery voltage. At last, the relay is activated as soon as it receives the signal and, thus, activates the electronic device or circuit connected with it.