Thermistor Temperature Sensing Alarm

This circuit is a temperature sensing as well as alarm circuit. The circuit raises an alarm whenever the temperature crosses a certain limit. Temperature monitoring is a very important and frequently used application in industries and in many other places where the temperature should be kept below a maximum allowable level. This circuit comes to our rescue when a situation of that sort arises.

The circuit is made to be low cost and reliable so that you can make it with less resources and without compromising on the performance. Although it is not an industry level calibrated circuit, it is quite sufficient where it is not a mission critical application. Another advantage of this circuit is that the circuit has a flexibility to adjust the temperature level to which it should be set. This means that you can set the temperature above which the circuit should activate the alarm.

Do you know how a High and Low Voltage Cutoff with Delay Alarm Circuit Works?

Circuit Diagram of Thermistor Temperature Sensing Alarm:

Temerature Sensor Circuit Diagram

The element in the circuit that senses the temperature of the environment is a thermistor. The name itself has its meaning. Thermistor means thermal+resistor. It means that the resistance of the thermistor varies with change in temperature. The relationship between the resistance of the thermistor and temperature is inversely related. This means that if the temperature in the atmosphere increases, the resistance offered by the thermistor decreases and if the temperature outside decreases, the resistance of the thermistor increases. This property of the thermistor helps us to make use of it to sense the temperature of the surroundings.

The circuit makes use of two BC547 NPN transistors to switch the alarm when the temperature above desired value is detected. The IC 4011 which is used in the circuit is a quad NAND gate integrated circuit. It has four NAND gates assembled in the single IC itself. This reduces the space and complexity of the circuit.

The combinational circuit which is built using the NAND gates is an oscillator circuit. As we know, any combinational circuit has an inherent time delay between the input and the output. This time delay is usually considered as undesirable but in this case we are making use of it to make it work like an oscillator. The circuit turns on and off repeatedly with a time delay operating as a square wave oscillator. The output of the oscillator is given to a buzzer which thereby operates at the audio frequency. The capacitors used in the circuit acts as filters to remove unwanted components of the signals and hence ensuring stability and proper operation.

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24 Responses

  1. Thank you for the circuit diagram..
    Could someone please explain the use of 4 NAND gates and the working of this sensor??

    1. I’m still working through the circuit, but essentially when the input 1 to U1:A changes the NAND gates begin to flip on and off because of the way the outputs feedback to the inputs. The delay in that propagating through 4 NAND gates means that it is not instantaneous and it makes a square wave at the output 11. The frequency of the square wave is in the audible range making the buzzer make a noise. It’s quite clever.
      I was looking for something just like this to make a warning sensor for a freezer. Assuming the buzzer is loud enough I can put this circuit in an altoids tin and store it in a garage chest freezer.

      1. instead of using 470 nano capacitor we can use some other ??
        or not
        please reply we r having so less time we have choosen thi s project
        we have to submit it as quick as

  2. Thanks for the Circuit.
    When i remove the component for the 4 Nand gate my buzzer is going on straight non-stop.
    But if the Nand gate is there it works fine.
    Can i know which part i’m wrong or i need to disconnect the Q3

    1. components,
      – 1 x 10K thermistor
      – 1 x 100Ω POT
      – 1 x 120Ω
      – 3 x 4.7K
      – 2 x 1k
      – 3 x BC547
      – 2 x 470K
      – 1 x 56K
      – 1 x 4.7M
      – 1 x 100k
      – 1 x 1M
      – 1 x 470nf
      – 1 x 1000pf
      – 1 x buzzer
      – 1 x CD4011

  3. How much resistance should be set in variable resistor for 10k thermister and what is the exact sensing range of 10k thermistor

  4. i know it’s kinda awkward question to ask but i have to;

    i actually didn’t understand how we set the temperature level that activates the alarm, i mean there is no microcontroller. so how do we manage to set the temperature level?

    sorry,if this is a dummy question. i am new to electronics.


    1. Could you please be more specific ???
      Or else if you have doubt on calculating frequency for IC 4011 then the formula is,
      Frequency output for U1:A & U1:C = 2.2 R8xC1
      Frequency output for U1:B & U1:D = 2.2 R10xC2

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