A simple pulse width modulator speed control for a brushless DC "pancake" type fan motor utilizes a type 555 timer for driving a transistor switch, connected in series between the DC voltage supply and the fan motor, at about a 10 Hz rate. The duty cycle of the pulsed output of the timer is controllable...http://www.google.com/patents/US4856078?utm_source=gb-gplus-sharePatent US4856078 - DC fan speed control
