@torben
i've tried to build a distance meter using microprocessor. the algorithm is to measure the time delay between transmitted and received signal. "It doesn't work" because it can only measure for a short distance (less than 10cm). I need to measure up to 1m (with minimum distance --> +/- 10cm). Since it was an assignment, i can't buy sensor like ping (parallax) nor the range finder (devantech)..
thanks for all your help, and i'm really sorry if my english is so bad..