Automatic Temperature Controlled Switch

We can make control temperature using this circuit; it will make control temperature automatically. LM35 is the temperature sensor used in this circuit for detection of temperature and also it helps to turn ON/OFF the output devices or appliances.
Once we tune the LM35’s sensitivity level of temperature, the circuit becomes control as an automatic switch. Easily available components can be used for developing this circuit prototype with small PCB boards like line/dot PCBs.




Construction & Working

Regulator unit and rectifier are the first stage of this circuit. 110V to 220V AC Supply is the input voltage and it is converted into 9V AC by using step-down transformer. After that, it is being rectified into DC voltage using bridge rectifier. Capacitor C1 reacts as filter to remove AC ripples then using 7805 regulator IC regulates provides constant 5V DC Voltage Supply.
LM35 is the temperature sensor used in this circuit and it gives an output voltage linearly proportional to the centigrade temperature. And an operational amplifier LM358 used here to help us to choose the temperature level through the variable resistor, VR1 and output of this Op-Amp is drives the transistor, Q1. In between +5V DC and collector terminal of the transistor Q1, the relay coil has been connected. When output voltage is higher than 2.5V from Op-Amp transistor Q1 turns ON and it connect the relay coil to neutral/ground. Hence, the coil gets power and makes the Normally-Open contact to Normally-Closed one. So, we can control electrical loads or an appliance automatically depends on temperature.


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