Stepper Motor Driver

The stepper motor interface board was built by Bret Victor for his ditch day stack. If a stepper motor is ever needed for the Coke machine, well, we've got one. The reason a PAL is used instead of a 374 is that I had a ton of 20V8's lying around, so it was cheaper than buying a new chip. There are two boards. The board on the right goes near the controller board, and is attached via a very long ribbon cable to the board on the left, which goes near the actual motor. The board near the motor requires an external power supply (9-12VDC or so, standard barrel connector with center ground).

Chip List

U1: PAL20V8 - latch to store the data bus on write cycles: latch.pds
U2: LM324 - op-amp to buffer the signals from the PAL.

There are four TIP112 darlington transistors to drive the motor coils, and of course, protection diodes.


Write a byte to any address to set the stepper motor pattern. Only the low four bits matter.


The motor itself has a connector with six wires in the following order: yellow, brown, grey, green, orange, red. Brown and orange are on either end of one coil, and green is the center-tap. Yellow and red are on either end of the other coil, and grey is the center-tap. The motor driver board is wired with the center-taps attached through resistors to positive voltage, and the ends of the coils attached to the collectors of the TIP112 power transistors. There is a protection diode across each half-coil, from coil end to center-tap.