This is the most important part of the DONOFF electronics. D2, D3, R3, R4 and C2 function as a low power 15 Volt voltage source to drive the gate of Q1. The gate of the MOSFET is pulled to GND by R7 at rest (MOSFET switch “open” – LED lamp is Off). To close the MOSFET, the optocoupler must pull the gate to 15 Volts (MOSFET switch “closed” – LED lamp is ON). The Optocoupler (U1), R6 and R7 use about 2.2mA (15v/(150+6800)). C2 serves as an energy buffer and D2 ensures that the energy does not leak away when the rectified voltage (MAINS-DC) falls below 15 Volt. Together with the Zener Diode D3, the control voltage for the gate of Q1 remains between 12.5 – 15 Volt (Q1 is fully closed when the gate voltage Vgs > 10 Volt).

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.