in case you're using a LDR (or high impedance analog source), i see some advantage of RCTime.. unless, i really don't. A simple transistor+few resistor + PIC adc, would be my solution if i need accuracy.

Level detector? ---> PIC Comparator

OR, PIC16F785 i've ordered a thousand of them yesterday, sOOOO much nice stuff on-board... built-in OP-AMP, 2 phase PWM + asynchronous feedback as a teaser...

NEW TOY, NEW TOY