heat sensor with transistor

Heat Sensor Circuit

Here are 2 heat sensor circuits, first one is build with the 4060 IC, the second one is simpler than the one before and has fewer components. It is an ideal add on circuit to monitor heat in CD players, Amplifiers etc. It senses heat from electronic devices and gives a warning alarm.

Heat Sensor Schematic with the 4060 IC

heat sensor with 4060 IC

Heat Sensor Schematic with the transistor

heat sensor with transistor

The heat sensing element is the NTC Thermister and its resistance drops when the temperature increases. Resistance of Thermister is set by VR and if the temperature is below the preset level, transistor T1 will be out of conduction and keeps the buzzer and LED off. When the temperature near the Thermister increases, T1 conducts and buzzer beeps.


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  • PhilB

    How could this circuit be reconfigured to detect a voltage (about 6v)from an analog automotive temp gage and when it reaches an over-temperature state it would sound an alarm?

  • Colin Mitchell

    The 100R is reducing the effectiveness of the 4k7 by a factor of 4700/100 = 47 !!!!

  • V.Sambath kumar


    Thank you for the feedback.

  • Colin Mitchell

    Let me explain two features that make the circuit a poor design.

    The 100R pot feeding the NTC thermistor is far too low.
    It reduces the NTC output enormously.
    The PNP Darlington is not needed. A normal transistor will do the job.
    The emitter-collector drop will be about 1v and Rx will not be needed for a 6v supply and 6v relay.
    The 1N4007 is not needed because the 100u dampens the spike.
    The push-switch is drawn incorrectly.
    The BC547 can drive the relay AND the buzzer.
    How many more things do you want me to mention?

  • Colin Mitchell

    Sambathkumar I have never seen such a bad circuit as you have produced. IT WILL NOT WORK. Don’t put such rubbish on the web without testing.
    You are just as bad as Professor D Mohankumar who doesn’t know a thing about designing a circuit.

    • V.Sambath kumar

      Colin Mitchell,

      It is a suggestion that i had put forward for Mohan’s circuit.

      I believe this is a forum where we can develop/clarify our technical knowledge/doubts and not one to ridicule someone else.

      If everyone knew everything there would be no need of this forum.

      So if you could advise on how to make the circuit work instead of saying how unskilled I am, it would be really helpful.


  • Sambathkumar

    Hi Mohan kumar,
    Pin no 12 should have a voltage all most near to the supply voltage of the ic for the purpose of reset, once reset is done pin no 5 of the ic goes high.I have done a small modification ie vr1 value changed fr 10k to 100 ohms, pin 7 not used, output of pin 5 is used to drive an npn transistor for activating the buzzer and also driving a rly driver tip127 transistor for cutting the supply to the cd player an additional reset manual switch push to open is provided for further safety if this not required you can avoid this ,during load cut off led connected to the rly will switch on to show that the cd power is switched off due to over heat the rly once switched on will remain on until the sw1 is pushed because rly coil will get positive supply through the no contact of the rly. I am attaching a diagram of the same for your ref pl check it and tell me whether it works or not. Waiting for your reply.

    Thank you

    Regards V.Sambathkumar

  • Karthik Hebbar

    yes!! I have seen piezo buzzers operating at even 5mA as long as the voltage is sufficient.
    I have the working circuit with me now!!

    • Tatenda Ronnie Ngwaru

      what are using the circuit for
      im looking for a project for my second tear second semester electronic engineering so myt want some ideas

  • Colin Mitchell

    Don’t forget, the chip can only deliver about 10mA

  • Karthik Hebbar

    The specs of HXD piezo buzzer I bought,

    Rated Voltage: 1.5~24VDC
    Operation Voltage: 1.3~28VDC
    Rated Current: ≤ 42mA
    Sound Output: ≥ 85DB
    Resonant Freq: 2300± 300HZ
    Operating Temp: -20º C~+45º C
    Storage Temp: -20º C~+60º C
    Weight: 2g

  • Colin Mitchell

    Corrections to above reply

    The thermistor will work with the CD4060.
    Show me the part number of the piezo buzzer