The basic wired (or wireless) computers are essentially a mechanical odometer. There is a magnet attached to a spoke which passes by a sensor with each wheel revolution. Input the tire rollout on the display, and it will track speed, distance, and other things (i keep the time of day displayed on mine). Uses a battery that might need to be replaced every other year or so (i ride daily all year round, so most people probably get even better battery life than this). Nothing to turn on or off, as it is triggered on by the wheel magnet sensor. Only drawback for you is that the display is digital, not mechanical. However, this does help it become more versatile than the mechanical displays of the vintage odometers.