Fig. 1 Arduino with unipolar stepper driver control board.
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Arduino Power Magnetic Driver Power Board

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This consists of two parts. The diagram of the driver board is self-explanatory. I use the open-collector ULN2003A to turn on a PNP transistor Vcc switch.

I use four TIP125 power Darlington transistors. They are rated at 5 amps at 60-volts. I use this to drive a unipolar stepper motor.

Fig. 2 Electrical connections Arduino with unipolar stepper driver control board.
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Stepper motor and Arduino connections. The board has seven inputs. Four are used to drive ULN2003A inputs to drive the power transistors.

The remaining three of seven ULN2003A drivers are bought out to header pins. These are open-collector rated at 50-volts at 500mA.

Note the stepper motor connections must be connected as shown. Other motors with different colors are a matter of trial-error.

Fig. 3 TIP125 driver pair diagram for each stepper winding.
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The electrical connections to each stepper motor winding uses a pair of transistors.

A HIGH input switches the ULN2003A driver to ground. The creates an emitter-base current turning the collector-emitter current to drive the load.

The TIP125 has internal suppressor diodes from collector-emitter and I added 1N4001 diodes as shown.

Fig. 4 various parts used in unipolar stepper motor board.
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Internal parts of the driver board.

Note the motor supply voltage was passed through a constant current source set to 1 amp.

Code

The code was written originally for a 12-volt Airpax stepper motor. It consists of two subroutines forward(count, step_delay) and reverse(count, step_delay).

It consists of a sequence of 4-bit stepper out bits turning ON-OFF the various halves of each coil.

The forward or clockwise subroutine is stepped through an infinite while loop. Each step decrement a variable "count" that breaks the loop at count = 0.

Reverse or counter-clockwise is the same the sequence is reversed.

One can use direct port commands or a string of digitalWrite() commands, four per step.

See Arduino Port Registers Revisited.

The winding are switched all off before the next 4-bits are written.

The stepper motor webpage goes more into how stepper motors operate. See Using a Unipolar Stepper Motor with a Arduino.

The new code is as follows original code on the Arduino stepper motor page.

Download the Arduino code ard_stepper.txt