can somone help me to make a windows programme that reads the arduino mega
And does what?
27 teams ride a bike for 24h on "roles"
I have no idea what "roles" means. Are the bikes stationary (that is on rollers)? Or, are they moving?
The idea is to project the kilometers
Project them where?
with a beamer
With a what?
I have one year to work this out
Good plan starting well in advance, although two weeks would be plenty.
The problems I see are that hall-effect sensors usually trigger an interrupt, since missing a pulse can be critical. That depends, of course, on how many magnets there are on the "role" that cause a pulse to be generated. The more magnets, the more pulses, and, therefore, the less critical it is to read each and every pulse.
The tradeoff, though, is that the more pulses (especially from 24 bikes), the more difficult it is to poll often enough to not miss pulses.
The Mega only has 5 external interrupt pins, so you may need to get more than 1. I'd recommend 6, monitoring 4 bikes each, using interrupts.
The challenge, then, would be connecting all 6 of them to one PC. On the other hand, one could connect 3 Megas together, so Mega 1 and Mega 2 use Serial1 to talk to Mega 3, which listens to Serial1 (for Mega1) and Serial2 (for Mega2) and uses Serial to talk to the PC. That way, only 2 Megas are talking to the PC, which should be easy to deal with.
I'd be willing to write the PC application that collects and displays, in a large simple format, the data from the clock and from the (2 or 6) Megas.