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.

Circuit diagram.



  • 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.