-----Original Message----- From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of Mark Rages Sent: Sunday, August 16, 2009 12:09 PM To: Microcontroller discussion list - Public. Subject: Re: [PIC] Bicycle Speedometer Project 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 Ok, I should have read further regarding the duty period, however, I still would not rule out using a hall effect sensor. The continuous duty ones typically draw 2.5 to 5mA and that in itself is not too bad for a battery powered application that your making yourself. However, it isn=92t great either, but there is a fairly simple solution to that too. After you have sensed a couple of rotations, you could start to shut off the hall effect sensor until, say, 80% of the previous periods time frame is = expired. You could also extrapolate what the predicted period will be for this by factoring in the current rate of change as well. You might miss a measurement during periods of extreme acceleration however it would recover quickly since you would turn off the power down feature until the acceleration became stable again. Regards, Bob Bullock -- = http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist