BulldogLowell:
but actually read micros() and watch what happens...
BulldogLowell, could you please explain what did oyu meant by that? Also what did Bob meant by "Note the use of micros() instead of millis() and the fact that the current time is only read once then used elsewhere in the program"
UKHeliBob, thanks, it works !! 
BulldogLowell, you program also woks as far as I could tell. Thanks both of you!