12v Fan Reduce speed in every minute interval using 555 timer.

[CDRIVE]

you can try to modify the pwm circuit?
like this one:
http://www.tronixworld.com/home/electronic-circuits/pwm-motor-speed-control/

Just so you don't feel ignored. The designer of that circuit didn't seem to give much thought about assuring saturation of Q1.

Chris

 

[Alec_t]

How many wires does your 'normal' DC fan have? 2,3,4,6?