The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Control Circuits
disables the outputs if the chip begins to overheat. For high output currents, use a slide-on
DIP heat sink to prevent overheating.
The motor is a 4-phase, unipolar type. This type of motor has six leads that connect to two
sets of coils, with two coils in each set.
Surplus motors often don’t include complete documentation, but you can sort out the leads
with an ohmmeter and some experimenting. Begin by looking for a lead that measures an
equal resistance (typically 5 to 50 ohms) to two of the other leads. Wire this lead to +5V and
pin 2 of U5. Wire the two leads that connect to this lead to pins 1 and 3 of U5 through diodes
D1 and D2. Swaping the leads at pins 1 and 3 will reverse the direction of the motor. Identify
and wire the remaining three leads in the same way, but using pins 6, 7, and 8 of U5.
The 5V motor is powered directly by a +5V supply. This simple drive circuit is fine for many
applications, especially at lower speeds. You can find examples of other drive circuits in the
documentation from Airpax or other motor manufacturers.
The data sheet for the ’5804B recommends adding D1-D4 to prevent problems in the logic
circuits due to mutual coupling in the motor windings. Schottky diodes have a smaller
forward voltage drop (0.25V) than other silicon diodes.
Resistors R1-R4 and capacitor C1 set the frequency of U4’s output. To select a speed, you
write a number from 1 to 15 to bits 0-3 of the output port. Each bit controls one of U3’s
switches. For example, when pin 2 of U2 is high, pins 1 and 2 of U3 connect, and R1 is one
of U4’s timing components. When pin 2 of U2 is low, pins 1 and 2 of U3 are open, and R1
has no effect on U4. When more than one switch is closed, the parallel combination of
resistors forms the timing resistance. When all switches are open, U4’s output is high and
the motor stops.
In addition to the frequency of the step input, motor speed depends on the step angle of your
motor and the mode selected at U5. A typical motor has a step angle of 18 degrees, which
means that it requires 20 steps (360/18) for one full rotation. Using the resistor values shown
and a motor with an 18-degree step angle, the motor speed will vary from 1 to 15 Hz in
wave-drive or 1-phase mode.
For a different range of speeds, use the formula shown to select resistor and capacitor values.
For speeds from 10 to 150 Hz, use 0.01 microfarad for C1, or decrease the values of R1-R4
by a factor of ten. The formula assumes that in the series R1-R4, each resistor is half the
value of the preceding one. If you use a different resistor scaling, you’ll have to calculate
the values of the parallel combinations of resistors to find the resulting frequencies.
Bits 5 and 6 of the port select the operating mode. Wave-drive mode powers one phase at a
time, while two-phase drive powers two phases at once, and half-step drive alternates
The Microcontroller Idea Book 193