For testing you only need to drive one PUL pin. It simplifies things immensely.Thanks for your detailed response. I have done most of the tests you suggest, so I'm currently at the stage of having verified that the Pi, the DM556T stepper drivers and the NEMA17 motors are working correctly. And the wiring between all components is fine.- start with a simple test .. etc
I have checked the current being drawn by the drivers from the Pi, which is 6.38mA - good because it's very slightly less than I was expecting - and because I am only driving four pins (two PUL and two DIR pins) it is unlikely to exceed the limits of the Pi (which I understand is around 50mA in total, and 16mA per pin). The test program called for a PUL signal of 1s duration to give the multimeter time to catch up, so I'm confident this is a true value.
Connecting a Sparkfun opto-isolator in between the Pi and the stepper driver, with a 5v supply attached to the HV side, and checking the current in the same way I get 0.36mA and no stepper movement, and a voltage of 1.36v. And this is more or less the same for every type of logic level shifter I've tried. I will try the Mosfetti board again, although it has not worked before and nothing has changed that would make me think it will do any differently. Even the tech support guy at MonkMakes (the manufacturers of the Mosfetti board) says that he sees no reason why the circuit I have shown here does not work.
This is why I am asking if anyone has ever wired up a logic level shifter that actually works - there is plenty of advice on the internet along the lines of "just use a mosfet" or "try a Darlington pair" but not one example of "here's what I did and it definitely works".
Post a picture of your wiring, and a schematic of what you have built.
Statistics: Posted by ame — Mon Mar 31, 2025 7:59 pm