Marc Dougherty

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Thermal Receipt printer experiments

30 June 2017

I did not really need a thermal receipt printer, but i bought one anyway. Then i tried to make it useful.

Code on github

The Basics

First, i followed the excellent tutorial from Adafruit(where i bought the printer). It covers all the initial setup stuff very well, and got me to a working printer that outputs simple text, with some basic text styling features (and other stuff like barcodes i did not expect to use).

Printer Drivers

A printer is fun, but I don’t typically have a monitor connected to my raspberry pi, so printing things was infrequent, as I had to ssh into the pi, and run a script.

I found another excellent adafruit tutorial that described the setup process for a network-connected thermal printer.

This let me sent more complex content to the printer, and print from other computers. This setup is great for printing out a grocery list before heading off to the grocery store. It uses the thermal printer just like any other printer.

I ran into some interesting paper sizing troubles here, where pages were printed in the aspect ratio of a 8.5x11in page, but only 2in wide.

Being able to print the grocery list, or my todo list, is good, but still involves using a regular computer to print out the list. The next idea was to set up physical buttons that would cause the pi itself to fetch the list, and print it.

I keep a few different lists on todoist, which has a decent api, and a python client library. Python is perfect, since the Pi also has a python-based library for reading the GPIO pins (where we can attach the buttons). With some small refactoring of the gpio_listener.py script from the above example, it will listen for multiple buttons, and launch the todoist script to print any number of pre-configured lists.

Be aware that the search strings “supported” by the todoist api are not the same as the searches done through the search box. Many search features are for paying customers only. There is a todoist help article on Filters that explains available filters, but only some of them appear to work correctly for me (not a paying customer). Notably, the no date, today, and overdue filters dont appear to work. :frowning:

I was still able to build a reasonable set priority searches that work well enough for me.

Aside: packaging

My process so far has been to develop on my laptop, and copy files over to the Pi for testing. This workflow gets a little tedious, and means i’m constantly overwriting files with new copies, from a machine which cannot run most of this code (the pi’s GPIO libraries are not available for my laptop).

To ease this tedium, and allow for easier rollbacks, I decided to put the todo-printer files into a quick-and-dirty debian package, so i’d only need to copy a single file over to the pi, and could install/uninstall as needed. My process for doing that will be covered in a separate post about quick-and-dirty debian packaging.

Enclosure

The last step of the project is to build a suitably attractive case for the pi and printer. I settled on a cube ~4.5” in all dimensions. This size allows plenty of room for the printer, and a set of 4 small pushbuttons on the top surface.