I've had good luck with SunDings, bought on eBay for $3-$5, a few dollars more for wireless (meaning no cable running from the sensor to the 'computer'). They do nothing but keep time and count revolutions, and what they can compute from that data (speed, distance, distance since last reset...) They've only failed because I broke them (say by turning the bicycle upside down without removing them). At that price, I was careless. Bizarrely, though most vendors sell them cheaply, 1 listed them at $108 once.