It shouldn't be necessary for the sensor to see the steering wheel return precisely to zero. I don't need surgical precision. If it's accurate to within 5-10% that should be good enough.Here is my algorithm:
1. After the turn signals are switched on, the steering wheel must move at least 90° (or maybe 180° considering the high ratio steering) in the direction indicated by the turn signal setting. At this point a flag is set.
2. If the flag is set and the the steering wheel subsequently returns to within 10% of the start position, then the signal lights are reset.
This should prevent the circuit from being reset due normal steering wheel movement that occurs between the time that the turn signals are switched on and when the car actually turns the corner.
I was originally planning to use magnets mounted in the hub of the steering wheel and have hall sensors in the turn signal lever housing, but it should be simpler to mount the mouse sensor in the same housing and arranged so that it can pick up movement of the steering wheel hub. Remember that on these old cars, the rear of the steering wheel hub is completely exposed, unlike modern cars which have a massive housing enclosing the steering column.
When turn signals became standard equipment in North America in the early 1950's, there were a number of aftermarket turn signal kits available to upgrade older cars. The fancy versions were self cancelling, using a rubber wheel that rolled against the steering wheel hub to mechanically reset the signals. I have a couple of these units. but the rubber wheel has turned rock hard and the rest of the assembly is worn out. I suspect that these things probably weren't very reliable, even when new. I'm going to toss all of the mechanical bits from the housing except the switch lever, and replace it with electronics. I'll use an electromagnet (modified relay) to latch the switch lever.