eng-bm said:
thank u mr.nigel, but could u make me a favore and send me a practical circut with a working program for it
Sorry, I don't have much time available for designing circuits and writing software for people, if I manage to find time I'll do it for you.
But everything you need is in the tutorials, I would suggest you get a PIC and have a play.
For a nice simple timer, here's a (very) simple way to write the program:
1: Read A2D to get pot position.
2: Display pot value (optional).
3: Check if start button pressed.
4: Button not pressed, goto 1:
5: Delay 'pot value' number of seconds.
6: Do what ever you need to at the end of the timed period, turn an LED on, switch a relay - anything you want.
7: Goto back to 1:
Hardware wise I would suggest something like a 16F818/9 or 16F876, an LCD display (to show what the time is set to, and you could make the display count down as well), a Start button, probably a Stop button, and some output device - an LED would do for testing.