DougieLawson wrote:With the speed that a bike wheel runs at it may not be a switch (I'm not sure they'd react quickly enough). It's probably using a hall effect sensor.
I got a reading of 52 miles per hour when I tripped the 40mph speed camera on the way into Brighton off Ditchling Beacon (closed roads for the BHF London to Brighton ride). With a 700C wheel that's 2136 mm (~85 inches) per revolution. 52mph is 3,294,720 inches per hour. So doing the maths that's roughly 646 revs per minute (or 10.7 pulses per second).
This is something that surprised me: with a reed switch, you can generally hear the contact closing when the magnet passes.
Even on "expensive" sensors like the Garmin GSC10, the sensor is still using a reed switch. I wonder if that has to do with the fact that many Hall Effect Sensors expect a 5V or greater supply, but the GSC10 uses a CR2032 battery. Surely there would be Hall Effect Sensors that run on 3V?