This simple circuit will produce an alarm whenever the temperature falls below zero degree. A thermistor is used here to sense temperature. The op-amp LM7215 is used to compare the reference voltage and voltage from the thermistor network. Reference voltage is given to the non inverting input (pin3) of the IC and voltage from thermistor network is given to the inverting input (pin4).When temperature becomes less than zero degree the voltage at the non inverting input becomes larger than the voltage at the inverting input and the output of the op-amp becomes high. This makes the transistor Q1 ON and drives the piezo buzzer to make the alarm. In the power supply section, IC 7805 is used to derive 5V from the 9V battery.

Circuit diagram with Parts list.


Power supply for this circuit.



  • Assemble the circuit on a good quality PCB.
  • The thermistor used here is a glass bead thermistor, type No: KEYSTONE RL0503-5536K-122-MS (361K @ 0 degree Celsius and 100K @ 25 degree Celsius).
  • The IC1 must be mounted on a holder.
  • The battery B1 can be a 9V PP3 battery.


  1. water level measure

    Awesome issues here. I’m very happy to see your post.
    Thanks a lot and I’m taking a look forward to touch
    you. Will you kindly drop me a mail?

  2. Hello..i choose this circuit as mini project…do you have picture of this circuit? coz i need to see it..picture as in the real picture when this zero degree alarm bcome working

  3. mohamed arfeen

    what if i want to make it a 100 degree celesius alarm ? am i going to change the thermistor only or what i have to do ? thanks