–Pairs of
back-to-back diodes can be used to slow DC motors
•Each pair drops
the voltage by about 0.7 volts
•This circuit can
be adjusted to drop from 0 to 4.9 volts
http://www.trainelectronics.com/artcles/AutomaticBlockSwitchController/Part_3-diodes_speed_control/part_III.htm