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 request and the proposed design.

The Request

Hi Swagatam,

am doing project on automatic speed control of ceiling fan depending on temperature. am an engineering student. can help me abt circuit and all. please...

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.

WARNING - THE CIRCUIT IS NOT ISOLATED FROM MAINS AC...... BE VERY MUCH CAUTIONED WHILE BUILDING AND TESTING THIS CIRCUIT.

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