I have been trying to understand the operation of the above circuit. To simplify, I am ignoring K1, Q1, R4 and D2. I also assume there is a jumper across T1 and T2. Since I assume there is no K1, I assume that R3 is connected to 12V rail and the LEDB junction is connected to the drain of Q3.
As I understand, this is a circuit which is used to power up a device from mains (through a 12V relay) using a momentary switch. When the circuit powers up C3 will charge up to 12V. Q3's drain is also at 12V. Q3's gate, because the capacitor charges up to 12V, should be at almost 0V and therefore it should be off.
What I don't understand is what happens when the switch is pressed: I assumed that Q3's drain is at 12V and the cap is also at 12V. But if that is the case, nothing should happen when the switch is pressed because both points are at 12V.
So I must be wrong with some of my assumptions.