uln2003

Micropython code to drive stepper motors via ULN2003

This is the ongoing work of my experiments with the BBC micro:bit and some ULN2003 stepper motors connected to 5V 28BYJ-48 Stepper Motors.

Materials

You will need:

1. A micro:bit with USB cable (and a computer to connect it to!)
2. A 5V DC power source
3. One or more 28BYJ-48 stepper motors with matching ULN2003 driver boards
4. Wires, lots of wires.
5. (Optional but handy) A breakout board to make accessing the bit's IO pins easier

Wiring / Connecting

1. Plug the stepper motor into the ULN2003
2. Connect the 4 pins from the ULN2003 into 4 separate I/O pins of the bit
3. Connect the 5v input +/- on the ULN2003 to a 5v source +/- terminals
4. Connect the bit to the computer
5. Load up the library and push to the bit (i.e. compile the program and upload to the bit)

Using the library

Code example:

# Create a stepper using the HALF_STEP command sequence 
# to a stepper which is connected:
#                     micro:bit    ULN2003
#                     pin16     -> INP1 
#                     pin15     -> INP2
#                     pin14     -> INP3
#                     pin13     -> INP4
# Set the delay between steps to 5 microseconds

s1 = Stepper(HALF_STEP, microbit.pin16, microbit.pin15, microbit.pin14, microbit.pin13, delay=5)  
s1.step(100)     # Rotate 100 steps clockwise

s1.step(100, -1) # Rotate 100 steps anti-clockwise

A delay of less than 5 microseconds may cause the motor to simply buzz and not move at all. Try and see what works for you and your motors.

Because of the gearing of this motor a full rotation isn't very exact. It is somewhere between 508 and 509 steps. The constant FULL_ROTATION has a value of int(4075.7728395061727 / 8) = ~509 steps.

Advanced Usage (Driving more than 1 stepper motor)

Being able to drive a stepper is cool but the bit has a lot more IO pins. It's possible to drive two steppers:

s1 = Stepper(HALF_STEP, microbit.pin16, microbit.pin15, microbit.pin14, microbit.pin13, delay=5)    
s2 = Stepper(HALF_STEP, microbit.pin6, microbit.pin5, microbit.pin4, microbit.pin3, delay=5)   
s1.step(FULL_ROTATION)
s2.step(FULL_ROTATION)

But you will notice that this makes stepper1 move a full circle (or a teensy bit more than a full circle) and then stepper2 will move a full circle.

If you want to move both motors at the same time then you need to interleave the commands to do that so that stepper1 moves a step, then stepper2 moves a step, then stepper1 moves a step and so on.

The library provides a way to do that:

s1 = Stepper(HALF_STEP, microbit.pin16, microbit.pin15, microbit.pin14, microbit.pin13, delay=5)    
s2 = Stepper(HALF_STEP, microbit.pin6, microbit.pin5, microbit.pin4, microbit.pin3, delay=5)   

c1 = Command(s1, FULL_ROTATION)         # Go all the way round
c2 = Command(s2, FULL_ROTATION/2, -1)   # Go halfway round, backwards

runner = Driver()
runner.run([c1, c2])

The Driver class will run 1 step from each command until there are no more commands to run which will make the two motors move (apparently) simultaneously.