My first question is about what happens once the motor pulls the cable. What keeps the motor from drawing current once the latch is opened? The motor is only powered in one direction, as a spring pulls the cable back, which in turn turns the motor back to its starting position. So what in the circuit turns off the motor current, and how does it know when to turn it off?
I know that the MOSFET Q4 directly switches the motor's ground path, and that the MOSFET is controlled by U1. But how does U1 know to switch Q4 off?
My next question is about D2. Is that there just in case the batteries are put in backwards? What about D1 and D3? What are they there for?
What do Q1, Q2 & Q3 do?