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Climate Dependent Automatic Fan Speed Controller Circuit

Climate Dependent Automatic Fan Speed Controller Circuit

The following circuit of a temperature or climate controlled automatic fan speed regulator circuit was requested by one of the  followers of this blog Mr.Anil Kumar. Let's learn more about the proposed design.

The Design

As can be seen in the given diagram, a very simple concept has been implemented in the proposed design of a climate controlled or temperature controlled fan regulator circuit.

A1, A2, and A3 are the 3 opamps from the IC LM324 which are configured as voltage comparators and amplifier.

The diode D1 which is a common "garden diode" has a very interesting "drawback", it changes its forward voltage drop by 2mV in response to every degree rise in the ambient temperature or the temperature surrounding it.

The above drawback of the device becomes our benefit here, because the feature here is exploited for sensing the ambient temperature of the premise.

The varying voltage across D1, in response to the varying surrounding temperature is effectively amplified at the output of A3.

The above amplified response is fed over an LED/LDR opto coupler, where the LED becomes the output load of A3.

Therefore the brightness of the LED varies proportionately in response to the temperature variations, it becomes brighter with increasing temperature and vice versa.

The above illumination falls over the built in LDR of the opto, which in turn varies its resistance according to the above information from D1.

Since the LDR is fixed as the gate control resistor of the dimmer circuit consisting of R11, C5, R13, DC1 and the TR1, the voltage across TR1 starts regulating the mains AC in accordance with the fed LED/LDR response.

When the LED is bright (at higher temperatures), the LDR resistance lowers. allowing the triac to pass more current.

This increases the speed of the fan, and when the LED/LDR response decreases (at lower temperatures), the speed of the fan also decreases.

A compact power supply consisting of C3, C2, Z1 supplies the required filtered DC to the IC LM324 temperature sensor configuration for the intended operations.

Idealy P1 should be adjusted such that the LED just begins glowing at about 24 degree Celsius, initiating the rotation of the fan at the minimum level.

D1 must be kept exposed well outside the enclosure so that it is able to sense the fan breeze directly.

Circuit Diagram

climate controlled fan speed regulator circuit



About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

28 thoughts on “Climate Dependent Automatic Fan Speed Controller Circuit”

    • Gojo, this circuit will work if it's built if perfect understanding and care… and also by checking the stages step-wise before integrating together

    • first build the opamp stage, check it with a DC power supply, and make sure the LED changes its illumination when subjected to temperatures from 20 to 40 degrees C, proportionately

      after that build the dimmer stage, confirm the dimming effect using a pot instead of the LDR…and then replace the pot with the LDR concealed with the oamp LED

      all these will require a lot of optimization and understanding of the concept.

      newcomers should not try this circuit

  1. Can I use a transistor based optocopler too I got PC817 and I realised it was a transistor based isolator. I looked in the near by store and failed to find that particular one. Or maybe you could help me with the code..

    I'm in the final process of building it on PCB board

    • NO, LED/transistor opto will not work here, only an LED/LDR hand-made opto will work, because the resistance of the dimmer circuit needs to vary in response to the LED brightness levels for the required fan speed control

  2. But its possible to use a DC input circuit isolated from AC through an opto copler and Triac?
    Could you help show me the circuit.
    Well, i also needed a pin-out number of the 324 IC used. Cos im not so good in it.

    • yes you can replace the capacitive power supply with a DC adapter power supply, …..the circuit already employs an LED/LDR based opto so it is isolated from mains.

      if you are not good with the basics then I would advise you to first learn all the basics and only then try this complex circuit

  3. Well thanks for helping us upcoming ambitious sudents.
    Mine has the same principle( control of the speed fan using a DC power source with a Thermistor.
    Could you please help me with that

    • thermistors are not sensitive enough to sense minute changes in ambient temperature, so I don't think it can be used for the mentioned application….the proposed 1N4148 as the sensor in the above article is a better approach.

    • the two module were tested separately for different applications but not together, however it's recommended for experts only, newcomers will fail for sure

  4. try this:


    use BC557 and not BC547 as wrongly shown in the diagram.

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