Hello Everyone, in this post I will share with you guys a simple circuit without a microcontroller, which can be used to control or switch an external load based on the temperature information.
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| Temperature Controlled Switch |
The temperature-controlled switch triggers automatically when the ambient temperature goes beyond the set temperature (in the above-mentioned circuit it is set to 50 degree Celsius but can be changed easily to any other value with the help of a potentiometer).
The circuit has three main parts.
- The Temperature Sensor LM35
- The Main Control Unit (Simple Circuit based on OP-AMP)
- Relay (used to control the external load, here it is used to control the Lamp)
Circuit and Working
The circuit comprises a step-down transformer, a bridge rectifier and a voltage regulator 7805 integrated circuit chip, a temperature sensor integrated circuit chip LM35, an Operation Amplifier LM35, and a changeover 5V relay, apart from this we have used some simple components like resistor, capacitor, diodes and LEDs.
The IC LM35, the heart of the circuit, is a 3-pin device like a transistor with pins for VCC, GND, and
OUTPUT. It gives variable voltage at the output based on temperature up to 150 degrees Celsius. This popular and inexpensive temperature sensor is generally used in digital thermometers.For every one-degree centigrade rise in temperature, there is a 10mV increase in voltage at the output pin. Let us say, at 0°C the output of the sensor is 0V. When the temperature rises to 10°C the output will become 100mV; at 25°C it will become 250mV.
Temperature sensor LM35 senses the temperature and gives the signal to the controller built around ICLM358. The data received by the controller is then given to the relay driver circuit built around the driver transistor and changeover relay. On receiving the signal from the controller, the relay is turned on/off. This process repeats forever.
Calibration of the LM35 sensor should be done as we have used opamp LM358 to compare the output voltage of LM35 with the reference voltage. Since the circuit has been set for the threshold voltage of 50°C,
to trigger the op-amp we need to set the reference voltage to 0.5V, as at 50°C the output of LM35 will be 0.5V. The reference voltage is at pin 2 of LM358.
The working of the circuit is simple. When power is switched on, the LED glows and the circuit is ready to use. As soon as some heat is provided near the LM35 sensor, the voltage at pin 3 of the Operational Amplifier goes higher than the reference voltage at pin 2, so the output of the op-amp at pin 1 goes high and relay energies. As a result, the light bulb gets connected to the 230V AC supply and switches on.to trigger the op-amp we need to set the reference voltage to 0.5V, as at 50°C the output of LM35 will be 0.5V. The reference voltage is at pin 2 of LM358.
When the temperature at pin 3 of the op-amp goes lower than the reference voltage at pin 2, the output of the op-amp at its pin 1 goes low and the relay de-energises. As a result, the light bulb gets disconnected from the 230V AC and switches off.
The bulb can be replaced with any other appliance, such as a fan or an air cooler, which will switch
on automatically when the room temperature goes beyond a limit. The circuit can also be used as a fire
alarm circuit by setting the reference temperature very high, like 100°C, and connecting an electric bell or siren as the load. After using a higher current rating relay, the circuit can be designed to automatically switch on even an air-conditioner when the room temperature becomes high.
on automatically when the room temperature goes beyond a limit. The circuit can also be used as a fire
alarm circuit by setting the reference temperature very high, like 100°C, and connecting an electric bell or siren as the load. After using a higher current rating relay, the circuit can be designed to automatically switch on even an air-conditioner when the room temperature becomes high.
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