This is the most important part of the DONOFF electronics. D2, D3, R3, R4, and C2 function as a low-power 15 Volt power supply to control the gate of Q1. The MOSFET's gate is normally pulled to GND by R7 (MOSFET switch "open" - LED light is Off). To close the MOSFET, the optocoupler needs to pull the gate to 15 Volts (MOSFET switch "closed" - LED light is On). The Optocoupler (U1), R6, and R7 use about 2.2mA (15v/(150+6800)). C2 acts as an energy buffer and D2 ensures energy doesn't leak if the rectified voltage (MAINS-DC) drops below 15 Volts. Together with the Zener Diode D3, the control voltage for the gate of Q1 stays between 12.5 - 15 Volts (Q1 is fully closed when the gate voltage Vgs is > 10 Volts).

The voltage is normally 15 Volts but drops to 13 Volts at 100% PWM (the MOSFET is then constantly closed)

The current flowing through R4 is also about 2.2mA, which means that R4 has to convert about 0.7 Watt (I2 * R = 4.8 * 150) into heat. The temperature of R4 becomes a maximum of 15°C higher than the ambient temperature. It is therefore necessary that a 400 Volt 2 Watt resistor is used for R4 (a bit over dimensioned).

Resistor R6 limits the current through the optocoupler and the gate of Q1.
C1 filters the “MAINS” DC voltage.