How do I prolong the reed switch output time
With reed activations for every meter traveled and 200Hz sampling rate, you're effectively limited to 10 m/s in terms of speed you can reliably measure (if you have achieved a perfect 5ms pulse time from the reed). In non-ideal conditions, you should expect reliable operation at about a half of that speed, 5m/s or 18 km/h. Assuming a bicycle application, that's not much.

My point is, you should either increase the sampling rate or reduce the number of magnets you use. The latter is the easiest option, with 2 magnets you'll be able to measure speeds up to 90 km/h, which looks reasonable.

And here's the RC circuit Steve told you about in his comment. It has a time constant of 5ms, so you'll measure between 3 and 1V in the sample following a reed activation, and less that 1V in subsequent samples.







simulate this circuit &ndash; Schematic created using CircuitLab