[Bearleener]

I'd thought about an automatic stop/start too, but decided for manual because it's easier and it allows me to switch off the engine while rolling. I have separate start and stop buttons above the shift lever.

The easiest way to switch off the engine is to interrupt the signal from the Hall sensor on the distributor, which tells the engine computer (ECU) how fast the engine is turning and when cylinder #1 is at top dead center. It's usually a yellow/green wire. Interrupt this signal and the ECU thinks the engine is stopped and stops firing and injecting. You have to keep the "interrupt" button pressed long enough until the engine really stops turning (about 1 second) or else it might fire back up using its remaining momentum.

To tun on the engine I tapped the wire going from the ignition switch to the starter solenoid and switch it to the ignition-switched positive line with a relay. But if I'm still rolling above about 15 km/h I restart by bump starting.

To automate this at a stoplight you would need some control electronics and monitor the following signals (and suggested algorithm rules):
> Vehicle speed (< 8 km/h in the French system, elsewhere presumably 0)
> Engine speed (wait until < 100 rpm when stopping; wait until > 600 rpm when starting)
> Gear is in neutral (my car doesn't have this signal, but would be easy to put a magnetic proximity switch on the gear shift linkage)
> Clutch pedal pressed (my car doesn't have this signal, but curiously does have a pre-drilled mounting hole for such a switch on the clutch pedal mounting [VW once offered a Golf III with auto start/stop in the 1990's; the later version used a flywheel to restart the engine])
> (optional) Engine temperature (don't stop if engine too cold)

Algorithm: if engine warm enough and vehicle is (almost) stopped and gear in neutral and clutch pedal released, then interrupt engine speed signal until engine stopped. If engine stopped and clutch pedal pressed then run starter until engine is running.