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I think it must be a short somewhere between some of the tracks. This evening I’ve built a new board from the ground up, plugged in the same Nano and stepper driver and it’s running “blink” fine. A small part of the layout is slightly different from the first one as I realised I could do it a little more neatly, but the circuit is clearly the same. I guess I just need to get the big magnifying glass out so I can check things over better.

At least I have something I can continue working with now.

Having finished the circuit board I plugged in the Nano and motor controller and connected it to a USB port (without any other power source connected). The power LED lit on the Nano, but nothing else happened and I wasn’t able to connect to upload a sketch. Unplugging the Nano from the board allows it to work exactly as expected. I can upload the “blink” sketch and it runs ok. Plug the Nano back in and the sketch doesn’t start and there’s no response when attempting to upload.

I can’t find any obvious problems with the circuit or my soldering so far, but I have to assume that either the 12V supply is required in some way that isn’t obvious to me or that there must be some issue with my soldering that I just can’t see.

As stealing the eyes of a teenager tends to be frowned upon I’m thinking my best option at this point may be to build a second board and test the Nano out at each stage to see what happens. The only other thing I can think of for the time being is to connect up the 12V supply and check if any magic smoke comes out, but I’m not so keen on that approach whilst there are still other things to try. I could perhaps attempt to build the circuit on a breadboard, but I’m not convinced there’d be room.

I now have almost all the components in place.

The large diode at the top left wasn’t in the original plan I worked from. That’s the reverse voltage protection diode. Obviously no-one, least of all me, is ever going to connect the power the wrong way around anyhow, are they? The leads on this diode were larger than the hole diameter, so I ended up buying some small diameter drills from Amazon and enlarging two of the holes to about 1.5mm so it would fit.

The top right I had to reorganise a little. Fitted around the top of the header for the Nano is the diode that protects the voltage regulator. The two resistors and the third diode form the voltage sensing circuit that means the Nano won’t try to drive the motors unless 12V power is connected. To make that all work I had to move one of the capacitors for the voltage regulator and make another cut in the track feeding the Vin pin on the Nano (which is bridged by the diode).

All that remains now is the voltage regulator itself, which will fit at the top on the very right hand edge.

As I’m not using some of the other features of the focuser controller such as the temperature sensor and the OLED display, there’s quite a bit of space free in the middle of the board. When I make the next one I might move the Nano to the left two or three columns to make a bit more space on the right hand side for the additional components at the top right.

In the end I couldn’t find a single application that would do what I wanted, so I just decided to start cutting tracks and see where I ended up 🙂

And then spent a happy hour or two soldering in all the headers,

Finally adding all the string to join the various bits of track together.

Oh, I put the two capacitors in for the voltage regulator too.

Next is a bit of testing to make sure I don’t have any bridged tracks where I don’t want them. There is actually one place where I deliberately bridged the tracks with a lump of solder. The DRV8825 “sleep” and “reset” pins are next to each other and both clamped to the 5V output of the Nano, so I just dragged a lump of solder across the tracks they use.

Once I’ve checked the soldering over there’s not a lot left to do — the resistor network for the pushbuttons, current-limiting resistor for the buzzer, two resistors and a diode for the supply voltage sensing, another couple of diodes, a capacitor and the voltage regulator and I think I’m done. I may need to break a few more tracks to fit all of those in, but that’s not a problem.

All the parts that I didn’t already arrived, so I need to get on with laying out the stripboard. I found what looks to be quite a handy tool for doing that last night (it’s probably simple enough to do by hand, but having something else check my work is always welcome) called VeroRoute. It appears to be intended for more general PCB layout, but can do stripboard as well and can check for sections that should be connected but aren’t, or vice-versa.

The first parts have arrived: a pair of NEMA17 stepper motors with 27:1 reduction gearboxes.

I’ve done a bit of reading and messing about with GIMP and this is what I think I plan to build:

I believe I understand pretty much all of the circuit now. The resistor network to the left of the Nano is just for the buzzer, manual movement buttons and LEDs. C1, C2 and the 7808 provide the 8V regulated supply for the Nano. D3 stops the Nano trying to push 5V backwards through the regulator if there’s no 12V power. R14, R16 and D2 form a power detection circuit (so the code won’t try to step the motors and get confused about its position if there is no 12V power). D1 is reverse voltage protection and C4 is apparently required across VMOT and GND on the DRV8825 for it to work.