The stepper motor spins only if i touch the STEP pin on the driver A4988

I am using an Arduino mega 2560 rev3 board together with an Arduino CNC shield and stepper motor driver A4988 to run (for now) only one stepper motor (Nema 17 39.5 mm-1.5A). I want to use them to first get some experience and later use them to build up a 3D printer machine. But, when I connect everything, the stepper motor runs only when I touch the STEP pin of the driver. I have spent a lot of time figuring out what the issue is and how to solve it, but I still couldn’t succeed. Would appreciate it a lot if someone could help me solve this issue.

More about the issue. What I find quite strange is that the stepper motor spins even when no code is running. So, just connect the Arduino mega board to the laptop, connect the power supply of (19 V) to the CNC shield, insert the A4988 driver and touch the STEP pin → stepper motor spins.

Some more data in case needed. I want to use Python programming to control the stepper motor.

Thanks in advance

In the first image, the pin that I’m pointing to is the STEP pin.


Unfortunately, the boards in your picture are not genuine Pololu parts, so we cannot offer support for them. Also, please note that our documentation probably isn’t valid for them either. From your description, it sounds like maybe the STEP pin is floating or there’s a lot of noise on the signal. You might try contacting the manufacturer to see if they have any suggestions.

For future reference, if you want to be sure you’re ordering genuine Pololu products you can order through our website, or you can get them from our authorized distributors. Additionally, since you’re using 1.5A stepper motors, you might consider using a more powerful driver such as the DRV8825 or MP6500.