I need to be able to turn 30 identical stepper motors on and off individually.  I can only afford to have one unique wire going to each stepper unit because I only have ~40 digital outputs to work with.  I can afford to have a few common outputs that are jumped from board to board.  basically I need to turn four inputs that go in a pattern into one input.

The four lines on the motor driver (H-Bridge) basically take turns going high when I want the motor to turn.  When I want it to stand still, only one of the lines should be high.  This is called “wave driving” a stepper motor.  Here is what happens when a bipolar stepper motor is wave driven.

t=0:  A=1  B=0  C=0  D=0

t=1:  A=0  B=1  C=0  D=0

t=2:  A=0  B=0  C=1  D=0

t=3:  A=0  B=0  C=0  D=1

t=4=0

The Solution

A Serial in Parallel out (SIPO) Shift Register does basically exactly what I’m looking for.  If I have one common clock (a line that goes high every 1/4 step) and connect the 4th output to the data input, then the four parallel outputs will shift through my 4 states like a champ.  The only catch is that I need to seed the circuit with the initial “1″ so that the “1″ can move along the shift register.

Luckily, because a shift register is just 4 flip-flops lined up in a row, I could build my own shift register out of flip flops, and access the set/reset abilities of the individual flip-flops.  So in the final setup, I had a single clock coming from the Arduino (pulsing at 100ms intervals) which controlled the speed of the motor, and a “stop” pin coming from the Arduino to control whether or not the motor was turning.

The “stop” pin was tied to the reset pin on the first flip-flop and the set pins on the other 3.  This means that when the “stop” pin was driven low, it would force the shift register into the “1-0-0-0″ state, and when it was released the “1″ would shift sequentially at the speed of the clock to drive the motor.  Here is a view of the protoboard layout (the center IC is the motor driver, and the other two each contain two flip-flops):

Pros

This is a huge improvement over controlling all 120 lines individually.  An Arduino mega can easily output a single clock and 30 control lines.  The cost of each circuit is about $4 in parts (three Integrated Circuits, or ICs), more if you PCB it.  It works, and it lets you do a powered brake as well.

Cons

The two IC’s with flip-flops are about $2.50 0f the total parts cost.  For this price ($2.50×30=$75) it would technically be cheaper to buy some other board that can take serial from the Arduino and control the 120 outputs.  Also, the wiring is a bit complex and uninsulated because each flip-flop’s output feeds into the next one’s input.

More Photos