electronic heater controller circuit schematic

Smart Electric Heater Control Circuit

Minuscule circuit of the electric AC heater controller presented here is built around the renowned 3-Pin Integrated Temperature Sensor LM35 (IC1) from NSC. Besides, a popular BiMos Op-amp CA3140 (IC2) is used to sense the status of the temperature sensor IC1, which also controls a solid-state switch formed by a high power Triac BT136 (T1). Resistive type electric heater at the output of T1 turns to ON and to OFF states as instructed by the control circuit. This gadget can be used as an efficient and safe heater in living rooms, incubators, heavy electric/electronic instrument etc.

Normally, when the temperature is below a set value (Decided by multi-turn preset pot P1), voltage at the inverting input (pin2) of IC1 is lower than the level at the non-inverting terminal (pin3). So, the comparator output (at pin 6) of IC1 goes high and T1 is triggered to supply mains power to the desired heater element.

When the temperature increases above the set value, say 50-60 degree centigrade, the inverting pin of IC1 also goes above the non-inverting pin and hence the comparator output falls. This stops triggering of T1 preventing the mains supply from reaching the heater element. Forunately, the threshold value is user-controllable and can be set anywhere between 0 to 100 Degree centigrade.

The heater control circuit works off stable 9 Volt dc supply, which may be derived from the mains supply using a standard ac mains adaptor (100mA at 9V) or using a traditional capacitive voltage divider assembly. You can find such power circuits elsewhere in this website.

Note:CA3140 (IC2) is highly sensitive to electrostatic discharge (ESD). Please follow proper IC Handling Procedures.

Electric Heater Controller Circuit Schematic

electric heater controller circuit schematic


Join the conversation!

Error! Please fill all fields.
  • amir

    i design this with proteus 8.3
    this is file

    can anyone check and correct this please?


    I build this project it works very well but I notice that the tempreture is stable but as time go by the tempreture drops,how can I do so that a given tempreture remain constant throght out because I am using it to produce chinken ie egg incubator.

  • David Dubins

    I would like to thank you for providing this circuit diagram and for your explanations and answers. I successfully implemented this strategy for a water bath heater. I attached the LM35 to XLR cable, and encased the exposed leads in an epoxy putty to protect from water. I then used a heat shrink tubing for good measure (leaving the IC body exposed).
    One improvement I made to your circuit was to move the LED to pin 6 of IC2, and this way rather than a power light, it lights up when the circuit is heating. This helped me immensely to calibrate the pot. I used it on 115V (our power in Canada). One more minor detail for future users: make sure the 10K pot is linear, not log. The electronics store I go to was out of linear, and I am only using 1/6 of the range of the log pot with tiny increments between 5 degree jumps. I will replace when they get more in stock.

    One question I have, if you swap pins 2 and 3 inputs on IC2, would this work as a cooling circuit? I suspect the answer is yes but I thought I would throw that out there.


  • Harsha

    What is the use of the 470nF capacitor??

  • Sum Guy

    Possibly this triac would be better:

    Q6012LH1LED (12 amps on-current rating)

    Maximum output current of CA3140EZ is 10 ma. Many versions of BT-136 require more than 10 ma. Gate trigger current of Q6012LH1LED is 10 ma and hold current is 8 ma.

    I suggest that this circuit be improved to include transformerless (capacitive half-wave rectified) low voltage 9vdc supply for control circuit.

    • zainahmedsharif2705gmail-com

      hey man. I wanted to ask if there is an alternative to this BT136 which can be found in proteus ? I want to build a schematic in proteus. I will be really obliged if you can help me out here

  • Robin

    i have been messing with this circuit i get the low voltage to function but the triac ( bt-136 ) doesn’t work i noticed that their is a lot of different bt-136s do i have the Wrong one ??????????

    Thanks Robin

  • rodel

    Sir, can I replace the LM35 to an ntc

  • Namle

    Sir, can you please post a circuit as electric AC “coller” controller that I can use to open a 220V AC fan in automatic when the temperature ambiance increase more than a fix setting temperature. Thanks in advance.

  • Kuberkoos

    Oops! I forgot… your power supply to the control circuit must also be suitable for 110V input and the specified output

  • Jeremy

    What would need to be changed to convert this to 110 instead of 230?

    • Pedro Quadros

      You can also look for BT138 or BT139 series. The last one, in my country, have a lot more max current allowed and is still cheaper! Oversize a component, sometimes, can save money (beyond the headache)

    • Kuberkoos

      Connect it to a 110V supply, and utilize a heater rated for that specific voltage. No other modifications necessary.

Looking for the latest from TI?