u could also make an alternative setup,,using the gear shift lever as input ,, just use 2 bit up/down counter and one 7 segment,,and devise some way to attach the button to the gear lever,,also this gear indicator is in built in hero bikes ,,i wonder how they did it?
something more interesting would be a rpm meter,,where u could reverse engineer the process i.e instead of using the rpms at crank u can use the wheel dia, the air pressure in tyre, then use final reduction ratio,then gears then primary then rpm at crank,,i,e go backward trough all the processes,,
btw how is rpm calculated in cg125?
and something ,ore interesting would be to use high power leds ,, they will realy brighten the road ahead of you,,