Hello Azam,
Check out this web site for a good pulse width modulation using one of the timers of a 8051. http://www.iguanalabs.com/pwm.htm
I used this method to pick up a 24 VDC solenoid on a 400 day clock I modified. The objective was to make the solenoid pick up as quiet as possible.
The voltage would gradually increase until the pull in threshold was reached. It worked out very well.
Regards,
Charles Bannister
400 Day Clock
Clock was purchased at a flea market for $5.00. It was broken and I modified the clock with a 24 VDC solenoid in order for it to function. Solenoid is picked up every 15 seconds so it was important to make it operate quite as possible. The solenoid was mounted in the rear of the clock movement out of sight. The electronics was mounted underneath the clock stand.