Re: What is the simplest way to control a motor's speed

Given all you want is a uni-directional motor that you can control
the speed from 0 to 100%:

Forget the steppers. They require timed and phased pulses to rotate
in any direction.

Since Servos are ususally just brushed DC motors with tach and/or
encoder feedback (there are of course brushless, AC and other
variations of servos but they require more sophisticated circuitry)
they can be considered DC motors for this project.

The simplest to control is a DC motor. Send a varying DC voltage to
the motor and the speed can be varied over about 5% to 100% range.
You can build a simple manual reversing switch if you want the motor
to change direction. Usually the problem with any of these designs
to control a motor's speed require that as the motor is loaded, the
RPM needs to stay somewhat constant. There are tricks to do that
using a loose form of feedback from the motors back EMF (remember,
all motors are generators and vise versa). If you need any level of
torque control then you will have to get more creative with the
circuitry. IF you don't mind twiddling the speed pot to adjust the
speed as load changes then read on.

The simplest way to control small DC motors is to build a cheap pulse
width generator and use that to drive a power transistor or power FET
in the ground (neg) leg of the motor. Motors take well to pulsed
DC. The venerable 555 IC chip will work and there are hundreds of
circuits available for that chip. There are pulse width specific
chips made for switching power supplies (3524 is an olde that comes
to mind)that can be used to build a PWM (pulse width modulator)
controller. It generates pulses and varies the on time of those
pulses (delivering more or less average DC to the load) by using a
reference voltage built into the chip and comparing that to a
feedback voltage. The feedback can be as simple as a pot across the
same reference voltage. Once you have the PWM that will vary with the
position of the pot, then the rest is just matching the power to the
load. Big N-Channel fets are pretty cheap, will take a lot of abuse,
and are fairly easy to drive.

Unless you are comfortable with building circuits from scratch
several of the hobby kit vendors make simple PWM motor speed control
circuits. You can't use SCR based...well, you could but that's
another story...designs. Check the Web for electronics kits, check
www.mpja.com for kits and if you can't find a cheap way to do what
you want drop me a line off list.