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
As usual, I suggest adding from now to your favorite e-commerce shopping cart all needed hardware, so that at the end you will be able to evaluate overall costs and decide if continuing with the project or removing them from the shopping cart. So, hardware will be only:
- Raspberry PI Zero W (including proper power supply or using a smartphone micro usb charger with at least 3A). Also newer Raspberry PI boards will work with same procedure
- micro SD card (at least 16 GB, at least class 10)
- Stepper motor – I use Elegoo 28BYJ-48 – with motor driver – I use Elegoo ULN2003
- dupont wiring (in my configuration, I will use 4x female-female + 2x female-male)
Check hardware prices with following links:
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 Raspberry PI OS Lite installation. Once connected via ssh to your RPI, update first of all:
sudo apt update
sudo apt install rpi.gpio
Now we need a Python script to control the stepper motor with our Raspberry Pi. For this purpose, I prepared a simple python script. You can just download it and align the script to yuor wiring configuration:
Logic Behing Python Script
This script is based on producer step sequence schema:
Each sequence step (from 1 to 8) is a GPIOs status to set from RPI to ULN2003.
You can image this table as a composition of 8 arrays (each one with 4 digits). This can be also seen as an ordered composition of 2 kind of arrays (0,0,0,1 and 0,0,1,1) alternated and rotating by one digit. In python, array rotation can be implemented with the simple line:
rotated_array = array[n:]+arr[:n]
where “n” is the number of rotation steps. To rotate by 1 step, this string becomes :
rotated_array = array[1:]+arr[:1]
Because arrays are composed by 4 digit, opposite rotation can be achieved with a 3 step rotation:
rotated_array = array[3:]+arr[:3]
So, first download the script in your RPI:
If your cabling is different from mine, identify the row where chan_list is defined. The original one appears as the following (including comments):
chan_list = [17,27,22,23] # GPIO ports to use
You can also edit delay var from current “.001” value to higher one. The higher the value the lower the rotation.
delay=.01 # delay between each sequence step
Now you can easily test your lab by launching a simple command:
and you should see the motor rotating.
For opposite rotation, as previously described edit:
arrOUT = arr1[1:]+arr1[:1]
arrOUT = arr1[3:]+arr1[:3]
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