because the pic will be pulsing port a0 it only lets the cap charge to a certain amount anyway, i would assume that because the discharge time varies on a 0.1uf capacitor then exchanging this for a larger capacitor would produce similar or the same results.
this is quite an interesting concept that acts as an external a/d converter, the detector charges up the cap and then the mcu detects how long it takes for the cap to discharge therefore producing a pwm type signal, that needs to be modulated in software, presumably the brighter light that falls on the detector, the more charge is in the capacitor and the longer it takes to discharge through the mcu, the darker the light, not as much charge will be stored. this requires the port pin to go high for a specified amount of time (Charging period) and then go into input mode until there is no charge left.
