I worked on this on an off for the past couple of weeks. Using the A4988 product page I had this functional on the breadboard pretty quickly.
I was running things off two separated power rails here and forgot about it after getting excited and plowing through to a premature prototype phase.
For power I have 29V DC so I need something that can step that down. I found two buck modules that I found interesting. One based on the LM2596 and one on MP1584. I went with the MP1584 even though the listed maximum input voltage is 28V. I looked at the datasheet for the MP1584 and found that it lists an absolute maximum input voltage of 30 and I am not going to need anywhere near 3A to power a promini and the logic rails of the A4988.
I did some testing on the bench with this setup and it seems to perform pretty well, driving the stepper motor for far longer periods of time than I will require for this project. The capacitor recommended by the A4988 product page is pretty neccesary.
During this project I wanted to try out some new techniques. I haven’t done a circuit layout on paper in quite a while so I went for it. I had to learn some old lessons like not to use a ball point pen. On a trip to get a new pencil I learned about dot grid. Apparently though, dot grid index cards are complicated to get your hands on (shoutout to this incredibly detailed blog post) so I made my own.
Figuring out how to control the power state of the A4988 wasn’t as straight forward as it could have been. The information in the datasheet is a bit spread out. I suggest setting Reset
HIGH and leaving Enable floating. Then control the Sleep pin using a
10k pulldown resistor which is what I’ve done here.
I had a little trouble because this MP1584 wasn’t quite 0.1" pitch. :(
I normally build my circuit prototypes by bridging traces on a proto-pcb. For this one I wanted to just try making perfectly sized wire segments.
Illuminated Automotive Switch
As a bonus, I took apart the switch I am considering using. I found that if you swap the output/power terminals on your standard automotive illuminated switch the led will be on regardless of the switch position, which is actually what I was looking for with this project.