Motor starter circuit

// A motor starter's control circuit, at low voltage.
//
// There is no microcontroller here. Press Run and the simulator solves the
// circuit. This is a control-circuit lesson on a 12 V DC stand-in. It is not a
// motor starter for real equipment: a real starter switches a motor on mains
// or three-phase power, with a listed contactor and overload relay.
//
// THE PARTS. A DPDT relay stands in for the contactor M. Its coil is the
// contactor coil. Its first pole is the seal-in contact, its second pole
// switches the motor. The lever microswitch is STOP, wired on its normally
// closed contact (COM to NC): pressing it opens the circuit. The push button
// is START, normally open. The slide switch is the overload contact OL, which
// is normally closed: slide it to the open side to simulate an overload trip.
//
// THE WIRING. +12 V goes through STOP, then through START and the seal-in
// contact in parallel, then through OL, then through the coil to ground.
// That is why it is called 3-wire control: STOP, START and the seal-in
// contact meet at three points.
//
// THINGS TO TRY.
//  1. Press and release START. The coil pulls in, the seal-in contact closes
//     around START and the motor runs. Let go of START and it keeps running:
//     it has sealed itself in.
//  2. Press STOP. The coil drops out, the seal-in opens and the motor stops.
//     Letting go of STOP does not start it again: the seal-in is open, and
//     only START can close the circuit.
//  3. Run it, then slide OL open. The coil loses power and the motor stops.
//     With OL open, START does nothing at all. Close OL and the motor stays
//     off until you press START again. A real starter also never restarts by
//     itself after a trip.
//  4. Why STOP is normally closed: if a wire to STOP breaks, the coil loses
//     its feed and the motor stops. A broken wire fails safe.