5. How to Control Motor Speed with a PWM Circuit  (article
continued from previous page)  Pulse-width modulation is an
effective method for adjusting the amount of power delivered to
an electrical load. A simple circuit containing an inverter
chip, diodes, trimpot, and capacitor creates the variable duty-
cycle PWM. A resistor and transistor switch heavier loads than
the 74AC14 chip can drive by itself.  Let’s say you have a nice
DC gearmotor for your robot. The problem is, this gearmotor is
marginally too fast for debugging the robot, or perhaps even too
fast for final usage. For debugging, you’d like the motor to run
as slowly as 1/3 or 1/4 of the full speed. But, replacing the
gearmotor would be expensive and you’ve already installed it
with home-made mounting brackets.  You measure the no-load motor
current at 5V to be 88 mA. You figure that’s an apparent
resistance of 56 ohms (5V / 0.088 A). Can the speed of the motor
be lowered to 1/3 if you insert a 112 ohm resistor to take up
2/3 of the power?  To expe