What Next? 285Experiment 33: Moving in Steps- ULN2001A or ULN2003A Darlington arrays by STMicroelectronics.
Quantity: 2. - CMOS octal or decade counter. Quantity: 2.
- Various resistors and capacitors.
Exploring Your Motor
I’ve specified a unipolar, four-phase, 12-volt motor because this is a very com-
mon type. A typical sample is shown in Figure 5-105. If you can’t easily find
the one that I’ve listed, you should feel safe in buying any other that has the
same generic description. “Unipolar” means that you don’t have to switch the
power supply from positive to negative and back to positive again, to run the
motor. Four-phase means that the pulses that run the motor must be applied
in sequence to four separate wires. Because you will be running your motor
directly from 555 timers, the lower its power consumption, the better.
First, though, we can apply voltage to the motor without using any other com-
ponents at all. Most likely it will have five wires already attached, with the ends
stripped and tinned, so that you can easily insert them into holes in a bread-
board, as shown in Figure 5-106. Check the data sheet for your motor; you
should find that four of the wires are used to energize the motor and turn it in
steps, while the fifth is the common connection. In many cases, the common
connection should be hooked to the positive side of your power supply, while
you apply negative voltage to the other four wires in sequence, one step at a
time.
The data sheet will tell you in what sequence to apply power to the wires. You
can figure this out by trial and error if necessary. One thing to bear in mind: a
stepper motor is very tolerant. As long as you apply the correct voltage to it,
you can’t burn it out.
To see exactly what the motor is doing, stick a piece of duct tape to the end of
the shaft. Then apply voltage to wires, one at a time, by moving your negative
power connection from one to the next. You should see the shaft turning in
little steps.
Inside the motor are coils and magnets, but they function differently from
those in a DC motor. You can begin by imagining the configuration as being
like the diagram in Figure 5-107. Each time you apply voltage to a different
coil, the black quadrant of the shaft turns to face that coil. In reality, of course,
the motor turns less than 90° from one coil to the next, but this simplified
model is a good way to get a rough idea of what’s happening. For a more pre-
cise explanation, see the upcoming section “Theory: Inside a stepper motor.”
Bear in mind that as long as any of the wires of the motor are connected, it
is constantly drawing power, even while sitting and doing nothing. Unlike a
regular DC motor, a stepper motor is designed to do nothing for much of the
time. When you apply voltage to a different wire, it steps to that position and
then resumes doing nothing.
Figure 5-105. A typical stepper motor. The
shaft rotates in steps when negative pulses
are applied to four of the wires in sequence,
the fifth wire being common-positive.12V DCFigure 5-106. The simplest test of a step-
per motor is to apply voltage manually
to each of its four control wires, while a
piece of duct tape, attached to the output
shaft, makes it easy to see how the motor
responds.