I am interested to build a bike but I would also like to have free software/hardware. How can I build a cyclometer with open-source software? Would it be possible to find reflashable hardware or open schematics?
The most detailed account/details I've come across for building your own bike computer is here on PIClist.
Those pages contain detailed instructions for building a bike computer using a PIC microcontroller linked to a Hall Effect Sensor (it detects the magnet attached to a spoke as the wheel spins). In addition to the build details it also features a detailed discussion of the design process which the creator undertook whilst creating the computer.
I tried to get real time ANT sensor data from a Garmin watch to a PC. But this is difficult. BLE is easier. The following bike computer is very basic, but can be modified by the user:
Pyloton: CircuitPython Cycling Computer. Open Source cycling computer that displays heart rate, speed, cadence, and song playback info.