By this circuit, we can control electrical load or appliances automatically depending on temperature, and handle it with extreme care and protection because it is involved in the operation of High voltage or AC. Now when the output is higher than 2.5V from the operational amplifier Q1 transistor turns ON and connects the relay coil to the Ground supply hence the coil gets energized and makes the N/O (normally open) contact with a closed one. Every inductor coil produces equal and opposite EMF when switched OFF suddenly, this may cause permanent damage to components, so a diode must be used to prevent reverse current. And diode is used to prevent reverse current flow when the transistor is switched OFF. The resistor is used to provide biasing to the transistor. Transistor is used to amplify the current so that full current can flow through the coil to fully energize it.

Here relay is configured by using a small driver circuit which consists of a Q1 transistor, diode, and a resistor.

The relay coil is connected between +5V and Q1 transistor collector terminal. The output of LM358 is connected to the base of NPN transistor Q1, so Q1 also becomes ON which triggers the Relay, and the bulb gets turned ON. And as we have to set the reference voltage low at Pin 2 of LM358, so now the voltage at pin 3 (non-inverting input) becomes higher than the voltage at pin 2 (inverting input) and the output of op-amp LM358 (PIN 1) becomes HIGH. Temperature sensor LM35 gives output voltage linearly proportional to the centigrade temperature and LM358 operational amplifier helps us to choose the temperature level through VR1 variable resistor, the output of this operational amplifier drives the Q1 transistor. To eliminate AC ripples from rectified DC it goes through a filter capacitor, here C1 reacts as a filter then linear regulator IC 7805 regulates DC voltage and gives a constant 5V DC supply. And then rectified into DC through a bridge rectifier. As we can see this circuit starts with a rectifier and regulator unit, in which with the help of a step-down transformer high voltage AC (110 – 220V) supply is reduced to 9V AC.

Lm358 pinout download free