I want the cheapest, most basic method of turning off the motor while an electronically assisted vehicle is moving at over a certain fixed speed.
I don't need to measure the speed other than it being 'slower or faster' than a fixed rate. As the motor runs via gears to the wheels, I need something linked to RPM of the wheel.
For the sake of wiring I am thinking about a magnet on the wheel spokes and a sensor on the frame - and a hall switch linked to resetting some kind of countdown timer - while the timer is at zero, the switch stays open for a fixed period (enough time for an other revolution and the timer to hit zero again), if the timer doesn't get to zero in a fixed period then the wheel must be doing a revolution in less than the timer value, so the sensor switches off the power until the time does start getting back to zero (the wheel is now moving more slowly).
Is there a better way? If not, what sort of circuit should I be looking at for my timers?