Hi, As you see in schematic, the circuit is a normally-closed AC dimmer. Therefore when the connection between microcontroller - which produce pwm - and mosfet get halted, the lamp will be always on without any dimming. I want to make this circuit in a way which eliminating microcontroller would turn the lamp off. Is it possible?
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.
Convert your inverting optocoupler circuit to a non-inverting one. Done!
You'd achieve that by putting the optocoupler in series with D1 on the high side.