Using Electromagnetism to Create a DIY Drawing Tablet

Most people don’t know and don’t care how the technology they use actually works. But you’re reading this, so I know you’re different. I’m sure you’ve thought about how your smartphone’s touchscreen knows where you’re tapping, for example. If I’m right, then you’ll enjoy this project that relies on electromagnetism to make a DIY drawing tablet work.

This is kind of like a homemade touchscreen, except it doesn’t sense your fingertip and it doesn’t work by measuring capacitance or resistance. Instead, it works by using the “puck” (a kind of flat active stylus) to detect magnetic fields in the “tablet.” It is kind of like a magnetic encoder in two dimensions and the similarities aren’t coincidental.

The magnetic fields are created by two lengths of wire that cover the bottom of the tablet in serpentine patterns. One length of wire is turned 90 degrees relative to the other, so they overlap to form a grid. Current flowing through the wires creates magnetic fields and the overlap results in areas with alternating rotation.

The puck contains a MLX90393 3D magnetic sensor module monitored by an Arduino UNO R3. Starting it one corner of the tablet, it counts the field transitions it crosses to determine its position, like a magnetic encoder does. But that is a very rough position. It can also determine its position within a field by looking at the phase of the overlapping sources. Thus, it can report its position on the 2D plane and that changing position over time can be used to draw (or trace) lines.

The positioning isn’t very accurate in this demonstration, because everything was wired by hand and the wires aren’t perfectly flat. Small variations in the wire path distort the readings. But with a simple two-layer PCB and clean power, this could be made to be very accurate.