level under 120 degree Celsius. A 10K variable resistor is used to adjust the circuit to activate on the desired temperature. An LED is also use to provide the visual indication when the temperature is reached on the preset level and the relay switch is activated.
The circuit is simple and very easy to build. It is using two different ICs. First is a LM35 IC which is a transistor shape temperature sensor. It is a readymade digital output temperature sensor and accurately calibrated in centigrade by the manufacturer therefore doesn't require any calibration. The current consumption of this IC is only 60uA due to which it is ideal for battery operation.
Another IC used in the circuit is a CA3140 operational amplifier IC, that is used in comparator mode here. A relay switch and an LED are connected to the output pin 6 of the IC. A buzzer can also be connected between the pin 6 and ground if desired. The operating voltage of the circuit is 5V to 12V DC.
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Here is the project of a temperature controlled switch using LM35 and CA3140 ICs. The circuit can be used for variety of temperature sensing purposes. A relay switch is used at the output of the circuit that can control or switch ON and OFF any AC and DC appliance on the desired temperature
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