Wawa:
Where to put the opto transistor. As in the diagram for Q3 or for Q4.
Not conducting transistor is almost ideal open circuit, but conducting has a saturation voltage.
Fet Vgs 'off' voltage range is small, but Vgs for 'on' can be anything between 6volt and 12volt.
I think it's therefore better to have the resistors between gate and source, and the transistor between gate and ground/12volt.
Leo..
Ao you're saying i should rather do it like I drew it on O3 rather than how I drew it on Q4?