my friend was using one by cateye, wireless one
simple mechanisms
sensor on fork,
magnet on rims, every revolution made is calculated via the
circumference of wheel input, y cant it b installed on bikes
regarding doms query,
i know of one by specialized which feature lite, something like
casio illuminator,blue color, but then the bad point is, the speed
is updated every 2 second or slow, so abit laggy