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.


  1. Yana Shamni

    Can i get the full circuit of this project? Component used in this project?

  2. sir can u exactly explain how temperature plays role on the ckt …. more over does the ckt built for particular temperature????
    plz sir reply utmost important …. !!!

  3. hi sir
    if you please tell the full working of this circuit…… i want to understand the working principal.
    thank you!!

  4. gilmar albuera

    hi sir
    im electrical engineering student a want this circuit for my research study
    I very need the Temperature Indication and Temperature control circuit diagrams, Pls send as soon as possible. Thank you.

  5. Hi, I am currently using Orcad Capture Pspice lite, and am unable to insert the lm35. What is the alternative and how would I simulate it? I also have to clamp my circuit from 0-3.3v only, any ideas on how I would achieve this using the same circuit you have?

    Thanks Scott

  6. what are the applications of this project sir…?????

  7. Mahendran

    Dear Sir/Madam,

    I want the Temperature Indication and Temperature control circuit diagrams, Pls send as soon as possible. Thank you.

  8. IC CA3130 is not available in market. so which model i can use instead of IC CA3130?? plss reply quickly.. i need it urgent.. plsss.. thank u. 🙂

  9. Can I use and LM741 op-amp instead of CA3130? And Can I use 2N3904 IC instead of BC107. Thank you

  10. muralidhar

    hi..can this circuit be used to connect a fan and a heater instead of that i can swich on either fan or heater depending on my set point..please tell me..tomorrow i need to submit if the circuit suits my need..

  11. If i am going to use this circuit to switch something, how can it be done? hank you.

    • Seetharaman

      Hi Jack connect a 5volt @ <100mA relay in parallel with R4 D1 combination, do not for get the free-wheeling diode (1N4000 series in parallel with the relay in reverse biased condition.

  12. I want the detailed explantn abt this and i liked it and i need it urgently!!!!!plz do it as soon as possible

  13. Seetharaman

    Hi Bhavin IC3 is cmos by using a leaky soldring iron you may kill it. hence solder the socket and carefully insert the IC. Adjust the pot to the required trip value between 0 and 88deg C. calibrate the potentiometer scale linearly from 0 to 88deg C.

  14. why IC3 must be mounted on a holder ? and what is temperature trip point ?
    and hoe i can adjusting POTR2 ?

  15. seetharaman

    Hi Gerald you can use any board. IC2 is the temperature sensor, to be located at the place where the temperature to be sensed, thrugh 3 core extension cable from the board.

  16. sir is it ok to use breadbroad instead of vero board? 🙂

  17. zunair shahid

    please can some one describe whole of this circuit .em quite new to these things so i cant really get that which bit is doing what…can some one help me out please.
    zunair shahid

  18. Seetharaman

    Hi Max the ic capability is from +2deg to +150deg C. you can use within this range. But with the pot R2 range it is restricted in this circuit by R1 to 90deg C.
    The IC output is 2 to 150deg C is 20 to 1500mV. Set 400mV for 40 deg C at IC3 Pin No 2.

  19. What should i do if i can to measure 40 degree celsius?