“BrachioGraph - arm-writer - is an easy-to-build pen-plotter, driven by a library of simple Python applications.
Components and materials
You’ll need: a Raspberry Pi, three servo motors, two arms and other small items mentioned below (such as a clothes-peg) depending on exactly how you build the machine. You’ll also need some strong adhesive.
If you are using SG90 motors as suggested, arms of about 8cm are suitable for drawing an area approximately 16cm wide by 11cm high. This fits well onto a sheet of A5 paper. To help understand the relationship betweem arm geometry and the drawing area, this can be visualised using the script provided.”