I have a little ZEITEL SC03h thermal printer. It's handy for labels, to-do lists and small notes, but using it meant going through the phone app. Even when whatever I wanted to print was already on my laptop, I had to send it to my phone first.

I wanted to get rid of that extra step. That was how NuPrint started.

It wasn't something I built in one stretch. I work full time and have other things going on, so I'd pick it up when I had time, make some progress, then leave it for a while. There were a few attempts before this one that didn't get very far.

Getting it to print

There wasn't any public documentation for the printer, so I started looking through the Android app with JADX. The app already knew how to talk to it; I needed to work out what it was sending.

A lot of that was digging through commands, checksums and the way images were converted into something the printer could understand. There were also a few things that weren't obvious at all.…