This is one of the hobby circuits that we can build by our own in our home.The function of this circuit is to indicate the temperature levels using a simple LED.Moreover this temperature controlled led circuit can be used as an indicator to denote the increase in temperature.This circuit uses two LEDs (D1 & D2), IC 7805(IC 1) which is the 5v regulator, IC LM35(IC 2) which is the temperature sensor, IC CA3130(IC 3) is the opamp,Transistors Q1&Q2 and finally resistors R1,R2,R3,R4,R5.Now lets move into the working explanation of this circuit.
WORKING OF TEMPERATURE CONTROLLED LED CIRCUIT:
Here in the above circuit 9v DC supply is used to operate the circuit.Then the 9v is given into the 5v regulator(IC 1).You can also use 5v voltage supply if you have it with you.Then the temperature sensor (IC 2) is connected to it.The output of the LM35 increases by 10mv per degree rise in temperature.Output of the LM35 is given to the non inverting terminal of the Opamp.The inverting input of the same Opamp can be given with any reference voltage using any resistor R2.And the output of the Opamp is connected to the base of the transistor Q1 and the collector of the transistor Q1 is coupled to the base of the Transistor Q2.And red colored LED is connected to the collector of Q1 and green to the collector of Q2.
If the reference voltage is 0.5v then the voltage at the non inverting input becomes 0.5v when the temperature raises to 50 degree celsius. So that both of the voltages get into the comparator as a result the output of the Opamp (IC 3) enters into the positive saturation.This trigger the transistor Q1 on and as a result the LED glows indicating the rise in temperature above 50 degree celsius.Since the base of the transistor Q2 is connected to the collector of the Q1 it remains in off state.When the temperature is below 50 degree celsius the reverse process happens making the LED 2 is light up indicating that the temperature is below 50 degree celsius.The temperature limiting range can be modified by adjusting POT R2.Now your all new homemade temperature controlled LED is ready.