how to convert a PWM AC dimmer circuit from normally-closed to a normally-open one?
Something like this should work. The transistor Q1 takes the place of the optoisolator in shunting the gate voltage. A second 15V supply from the second zener and 100K resistor is shorted out by the 4N35 to turn the transistor off and the MOSFET on. You will have to reverse the PWM coming from the MCU as well, or add an inverter to do it with hardware or bright will be dim and vice versa. 







simulate this circuit &ndash; Schematic created using CircuitLab