AC Fan Speed Control using Arduino and TRIAC

This project regulates the speed of a 230 VAC axial AC fan using phase-angle control driven by an Arduino Mega 2560 or Arduino UNO. The system relies on a step-down transformer connected to a 4N25 optocoupler to function as a zero-crossing detector, which signals the microcontroller every time the AC mains voltage crosses zero. By tracking this zero-crossing point, the Arduino can precisely time a delay before sending a trigger pulse. This pulse is sent to a MOC3021 optocoupler, which safely isolates the low-voltage digital circuitry from the high-voltage mains while firing the BT136 TRIAC. The TRIAC acts as an AC switch, turning on midway through each half-cycle to chop the sinusoidal AC waveform supplied to the fan. Adjusting the 10k potentiometer changes the analog input value read by the Arduino, which in turn alters the phase delay of the TRIAC firing angle. Shortening the delay delivers more power to the fan, increasing its speed, while lengthening the delay reduces delivered power and slows the fan down.