AC Phase Angle Control for Light Dimmers and Motor Speed Control using 555 Timer and PWM Signal

This circuit regulates alternating current (AC) power delivered to loads like incandescent lights or motors using a phase angle control technique. At the heart of the system is an Arduino Nano, which generates a Pulse Width Modulation (PWM) signal based on control inputs. This low-voltage microcontroller logic is safely isolated from the high-voltage AC mains by components like the PC817 optocoupler, which transmits signals across an optical barrier, and an LM7805 voltage regulator that steps down the supply to provide a stable 5V rail for logic components. The timing and pulse generation circuitry—featuring an NE7555 timer IC along with various supporting resistors, diodes, and capacitors—processes the control signals to determine the precise moment during each AC half-cycle to trigger the load. When the timing sequence matures, the MOC3021 opto-triac driver fires, isolating the control side while safely triggering the power semiconductor stage (which includes the IRF9540 MOSFET and other passive components) to switch the main AC current. By delaying the firing angle relative to the zero-crossing of the AC sine wave, the circuit effectively chops the AC waveform, reducing the root-mean-square (RMS) voltage supplied to the output connected via the screw terminal connectors. This variable RMS voltage achieves smooth dimming or speed control.