Having played with LEDs, switches and buzzers I felt the natural next step was playing with a stepper motor or two. This might form part of an idea I had to create an automated stop motion animation “turn table” for rotating and photographing objects.
There is a huge selection of motors to buy but I decided to experiment with a 28BJY-48 with ULN2003 control board.
The reasons I chose this device where :
- It is cheap
- Widely available from both overseas and UK sellers
- Easy to obtain with a controller board
- Small but relatively powerful
- Runs on 5V
- Easy to interface
I ordered two from “4tronix_uk” on eBay and they arrived the next day. There are additional details in the Stepper Motor 28BJY-48 Datasheet
Interfacing With The Pi
- 5V (P1-02)
- GND (P1-06)
- Inp1 (P1-18)
- Inp2 (P1-22)
- Inp3 (P1-24)
- Inp4 (P1-26)
The P1-XX references above represent the Pi header pins I used. These are defined in the Python example below in the StepPins list so if you use different pins be sure to update the Python list as well. You can use other GPIO pins if required just remember to update your Python script.
To rotate the stepper motor you provide a sequence of “high” and “low” levels to each of the 4 inputs in sequence. By setting the correct sequence of high and low levels the motor spindle will rotate. The direction can be reversed by reversing the sequence.
Here is a copy of the script I used to rotate the stepper motor. It uses version 0.2.0 of the RPi.GPIO library and defines a 4-step and 8-step sequence.
#----------------------------------- # Name: Stepper Motor # # Author: matt.hawkins # # Created: 11/07/2012 # Copyright: (c) matt.hawkins 2012 #----------------------------------- #!/usr/bin/env python # Import required libraries import time import RPi.GPIO as GPIO # Use BCM GPIO references # instead of physical pin numbers GPIO.setmode(GPIO.BCM) # Define GPIO signals to use # Pins 18,22,24,26 # GPIO24,GPIO25,GPIO8,GPIO7 StepPins = [24,25,8,7] # Set all pins as output for pin in StepPins: print "Setup pins" GPIO.setup(pin,GPIO.OUT) GPIO.output(pin, False) # Define some settings StepCounter = 0 WaitTime = 0.5 # Define simple sequence StepCount1 = 4 Seq1 =  Seq1 = range(0, StepCount1) Seq1 = [1,0,0,0] Seq1 = [0,1,0,0] Seq1 = [0,0,1,0] Seq1 = [0,0,0,1] # Define advanced sequence # as shown in manufacturers datasheet StepCount2 = 8 Seq2 =  Seq2 = range(0, StepCount2) Seq2 = [1,0,0,0] Seq2 = [1,1,0,0] Seq2 = [0,1,0,0] Seq2 = [0,1,1,0] Seq2 = [0,0,1,0] Seq2 = [0,0,1,1] Seq2 = [0,0,0,1] Seq2 = [1,0,0,1] # Choose a sequence to use Seq = Seq1 StepCount = StepCount1 # Start main loop while 1==1: for pin in range(0, 4): xpin = StepPins[pin] if Seq[StepCounter][pin]!=0: print " Step %i Enable %i" %(StepCounter,xpin) GPIO.output(xpin, True) else: GPIO.output(xpin, False) StepCounter += 1 # If we reach the end of the sequence # start again if (StepCounter==StepCount): StepCounter = 0 if (StepCounter<0): StepCounter = StepCount # Wait before moving on time.sleep(WaitTime)
In this example the wait time is set to 0.5 seconds. To make the motor turn faster you can reduce this value. I found I could reduce it to 0.004 before the motor stopped working. If the script runs too fast the motor controller can’t keep up. This performance may vary depending on your motor and its controller.
The 4 step sequence is faster but the torque is lower. It’s easy to stop the rotation by holding the motor spindle. The 8 step sequence is slower but the torque is much higher. For my turntable application I prefer the torque over speed so I will be using the 8 step sequence.
As with all Python scripts that use the GPIO library it needs to be run using “sudo” :
sudo python stepper.py
Press Ctrl-C to quit.
You can now control a stepper motor using a Raspberry Pi and some Python script. Add another motor and you’ve got the beginnings of a small robot!