In my projects with Raspberry PI I always has been fashinated by the infinite fields of application for this fantastic board. One of my preferred is robotics, and this requires the control of arms, wheels and gears by using motors.
Stepper motors are the most common motor type in commerce for these projects, because of their precision.
In this article I’ll show you you to control a simple stepper motor with a Raspberry Pi Zero W.
What we need
This time things are a little more complex than other previous articles, because we need to perform some cabling tasks that require attention.
For this tutorial I use the following:
- Raspberry Pi Zero W kit – including SD card, power supply and (the most important) a GPIO header
- Stepper motor – I use Elegoo 28BYJ-48
- Motor driver – I use Elegoo ULN2003
- dupont wiring (in my configuration, I will use 4x female-female + 2x female-male)
Before starting (additional HW)…
Before starting, let me expose some considerations on hardware.
Raspberry Pi Zero W kit comes with a GPIO header not connected to the board (why? I really don’t know…). So you have 2 options:
- You can use the one coming with the kit, but you will need to solder it. There are many tutorials over internet (you can just google “raspberry pi zero w gpio soldering”) and it is quite simple if you have a little of soldering manuality.
- You can buy an alternative solderless holder, which is easier to attach (you can google “raspberry pi zero w solderless gpio hammer header”). In this case you will need to add it in your shopping list
About the choice of Elegoo parts, I choose them because they are cheap and enoght for my very first experiment. I must state that these motors are a little slow for robotics projects, but they appears to be precise. They come in a kit including also some female-male dupont cables, but if you don’t have a breadboard you will need to buy one or to buy other female-female dupont cables.
Regarding the power supply, it is possible to power the motor directly from RPI GPIOs (there are some pins cabled to ground and some other cabled to 5V). The second option is to use an external power supply which can erogate 5V. The second option is mine one. Built this with an old electrical screwdriver, detaching the battery box and identifying positive and negative connectors. I used also 4x rechargable AA battery providing 1,2V each one and connected it with 2 male-female dupont cables (from male side):
The following picture shows how connections have been cabled in my lab:
The stepper motor comes with a pre-installed connector that you can directly connect to ULN2003 controller. All other connection are made with dupont cables.
To identify GPIO in Raspberry Pi Zero, refer to the following picture:
This numbers both GPIO IDs and phisical positions. Using RPI.GPIO python libary requires the use of GPIO ID instead of phisical position. The picture above shows also pins usable to supply power (5V and 3V3) and ground.
- GPIO 17 (connected to ULN2003 IN1)
- GPIO 27 (connected to ULN2003 IN2)
- GPIO 22 (connected to ULN2003 IN3)
- GPIO 23 (connected to ULN2003 IN4)
so having the following:
If you want to power the motor with RPI power PINs, you will connect one of Ground pins to ULN2003 negative pin and one of 5V pins to ULN2003 positive pin.
In the following picture, the ULN2003 wiring:
…and the overall picture:
Preparing the test
We’ll start from a clean raspbian stretch lite installation (see here for steps).
Once connected via ssh to your RPI, update first of all:
sudo apt-get update
sudo apt-get install rpi.gpio
Now we need a Python script to control the stepper motor with our Raspberry Pi. A good hand comes from Matt Hawkins (http://www.raspberrypi-spy.co.uk/), who prepared a simple script ready to be used. We can just download and align the script to our wiring configuration.
So, first download:
Identify the row where StepPins are defined (row 33). The original one appears as the following (including comments):
# Physical pins 11,15,16,18 # GPIO17,GPIO22,GPIO23,GPIO24 StepPins = [17,22,23,24]
Comments are useful to remember phisical wiring and corresponding GPIO IDs that must be declared in StepPins array. If you cabled like me (GPIO 17, 27, 22 and 23), edit these 3 rows having a final result like the following:
# Physical pins 11,13,15,16 # GPIO17,GPIO27,GPIO22,GPIO23 StepPins = [17,27,22,23]
Now you can easily test your lab by launching a simple command:
sudo python stepper.py
and you should see the motor rotating.