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Like to set the 50 degree Celsius temperature as triggering value, you can set the potentiometer roughly at 8k:2k like:
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By using the above formulae you can set the reference voltage accordingly and can change the Threshold Temperature. Now to set the reference voltage, we have created a Voltage Divider circuit by using resistor R1 and Variable resistor RV1 of 10k. Reference voltage is the voltage at Pin no 2 of LM358. As mentioned we have set the circuit for threshold voltage 50 Degree, so to trigger the op-amp at 50 Degree, we need to set the reference voltage to 0.5 volt, as at 50 degree temperature LM35 output voltage will be 0.5 volt or 500mV. Here we have used Op-amp LM358 to compare the output voltage of LM35 with the reference voltage. Setting up Reference Voltage for Op-amp LM358: So if the temperature is 0◦ centigrade the output of sensor will be 0V, if the temperature is 10◦ centigrade the output of sensor will be +100mV, if the temperature is 25◦ centigrade the output of sensor will be +250mV. LM35 is very popular and inexpensive temperature sensor generally used as digital thermometer or to measure temperature.įor every +1 centigrade rise in temperature there will be +10mV higher voltage at the output pin. LM35 provides output in degree Celsius and can sense up to 150 degree Celsius temperature. This sensor provides variable voltage at the output based on temperature. LM35 is three pin transistor like device. It can also work as fire alarm if you set the threshold temperature very high like 100 Degree Celsius and connect a alarm in place of bulb or you can configure it to automatically switch on the Air Conditioner beyond a particular temperature by using a Relay of proper rating. Here you can replace the bulb by any AC home appliance, like if you replace it by fan then it will serve as Temperature Controlled Fan Circuit. We have used a simple LED bulb in this Temperature Controlled Switch Circuit for demonstration, means if temperature goes above than 50 Degree Celsius then bulb will switch on automatically and if temperature goes below 50 degree, bulb will be switched off automatically. This threshold temperature of 50 degree value can be changed by adjusting the Variable resistor in the circuit, according to requirement. We are using LM35 as temperature sensor here. This circuit will serve as automatic light switch which will trigger if temperature goes beyond a particular level (50 Degree in this circuit). Now we are enhancing that circuit by using a Relay, and now we are going to Control Home AC appliances according to Temperature. Previously we have built Temperature controlled LEDs Circuit, in which two LEDs were glowing according to the temperature.