Originally Posted by
Iowegian
I am (obviously) not an analog guy so bear with me on this....It seems to me that whether the switch is NO or NC it has to be connected to some source of power to generate a transition to whatever is counting the rev's. I'd imagine it's a normally open switch that momentarily closes to complete some circuit and this generates a clock pulse to a counter or something along those lines. But wouldn't that quickly drain the battery if the switch was left in a closed state?
That's all true, but a microcontroller awaiting a state change in deep sleep mode can pull as little as a few nanoamps. I've done a little work with AVR controllers, and in deep sleep mode waiting for a state change, battery drain is such that a standard CR2032 button cell will last on the order of 3 or 4 years. It's true that if the switch happens to be closed, the current draw will go up a bit, but the pullup resistors can be quite high resistance and the drain would probably only kill the battery in a year or so. Since the odds of the wheel sitting where the switch is closed is probably about 1 in 100, in general you're still looking at multiple years, unless you happen to park it with the switched closed and leave it alone for 6 months.