On Sun, Aug 16, 2009 at 9:44 AM, Robert Bullock wrote: > I would not rule out the use of a Hall Effect sensor quite so quickly. > ... > > Researching more, I see that Infineon has one that looks perfect for your > application. > It is very low power, (typically 4 uA averaged supply current ) and is > designed for > Battery operation from 2.4 to 5.5 volts. > > Infineon TLE4913 > > Digikey TLE4913INCT-ND =A0$0.92 qty 1 > As I mentioned in my message, "low-power" sensors are actually duty cycled. This one is on for 50 us, then off for 200 ms. It is not suitable for measuring the period of a bicycle wheel: the spoke magnet could easily pass by in the 200 ms that the sensor is off. Even if you find one with a fast enough sampling frequency, it will still quantize the period to that frequency. So short-term speed resolution will suck compared to a reed switch. These hall sensors are made to notice your cell phone or laptop being open, not to actually measure a period. The humble reed switch is best for this application. I'm not sure why everyone is looking for something else. Regards, Mark markrages@gmail -- = Mark Rages, Engineer Midwest Telecine LLC markrages@midwesttelecine.com -- = http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist