 ### Author Topic: Some explanations on Temperature controlled LEDs  (Read 1721 times)

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#### Banart • Beginner
• • Posts: 4
• Helpful? 0 ##### Some explanations on Temperature controlled LEDs
« on: September 16, 2016, 12:59:15 AM »
Hey! 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.
Description.
The circuit is nothing but two LEDs (D1 and D2), whose status are controlled by the temperature of the surroundings. The famous IC LM35 is used as the temperature sensor here. Output of LM35 increases by 10mV per degree rise in temperature. Output of LM35 is connected to the non inverting input of the opamp CA3130.The inverting input of the same opamp can be given with the required reference voltage using POT R2. If the reference voltage is 0.8V, then the voltage at the non inverting input (output of LM35) becomes 0.8V when the temperature is 80 degree Celsius. At this point the output of IC3 goes to positive saturation. This makes the transistor Q1 On and LED D1 glows. Since the base of Q2 is connected to the collector of Q1, Q2 will be switched OFF and LED D2 remains OFF. When the temperature is below 80 degree Celsius the reverse happens.IC1 produces a stable 5V DC working voltage from the available9V DC supply. If you already have a 5V DC supply then you can use it directly.

Notes.

The circuit can be assembled on a Vero board.
IC3 must be mounted on a holder.
The temperature trip point can be set by adjusting POTR2.
Type no of Q1 and Q2 are not very critical. Any general purpose NPN transistors will do it.

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.

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