Capacitive touchscreens and touch sensors ubiquitous on the computers around us now, and they’re starting to be common on DIY projects as well. When you place your finger on a phone touchscreen, the conductivity of your fleshy bits causes a small change to the capacitive coupling between thin transparent electrodes printed under the glass. This is great for quick, coarse interactions with our pocket computers, but it isn’t designed for accuracy, and the range is typically limited to direct contact.
For creative work, it’s useful to have a precise input device that you can hold like a pencil. This isn’t just a plastic stick, like you’d see on the resistive touchscreens from the Palm Pilot era, it’s a thicker stylus that contains some active circuitry which forms a specific position reference that the tablet device can “see” from a distance. The pen typically also provides buttons and pressure sensors.
These graphic tablets or pen tablets have been popular with digital artists since the 1970s, but in the last decade or so prices have dropped enough to finally make them appealing for hardware experiments.
Wacom is the biggest name in tablet devices, and they have a unique (patented) method for powering the pen circuitry from the tablet, so you don’t need to recharge or replace the pen batteries ever. The latest from Wacom will set you back hundreds of dollars, but the going rate for a used CTE-450 Bamboo Fun tablet, with an active area of 5.82″×3.64″, is only $15.
We’ll also look at an alternative made by Huion, popular with artists on a budget. The H610 is almost enough for a full page, with a 10″×6.25″ active area. This device takes a simpler approach, with a battery-powered pen that transmits a continuous-wave signal back to the tablet. They’re between $50 and $80 new.
HOW PEN TABLETS WORK
Pen tablets work by mutual inductance, as if the pen and tablet are each half of a transformer. This may sound familiar if you’ve read about RFID cards, but where a keycard reader only needs one coil, a tablet needs a whole array of coils to sense the pen’s position.
Just behind the pen nib you’ll find a wire coil made from about 30 turns of thin copper. The tablet’s counterpart is far simpler: just a few traces on one side of a circuit board.
This is the key engineering that made both tablets simple enough to build for a reasonable price. The coils are in fact implemented as two perpendicular low-resolution arrays, with horizontal coils on one side of the board and vertical on the other. The tablet’s firmware can scan each axis separately to locate the strongest signal source, using the pattern of signal strengths among adjacent coils to estimate the true two-dimensional position of the pen.
With the constrained size of each coil, the tablet sees only a weak coupling to the pen’s signal. The tablet uses a single chain of amplifiers to filter and boost this signal, shared among all the coils using a bank of analog multiplexer chips. There wouldn’t be enough time to scan every coil and keep an interactive frame rate. Instead, the firmware needs to switch between a slower search pattern and a more targeted fine tracking pattern whenever the pen is located.
At this point the Wacom and Huion designs diverge. Huion’s pen is a single-transistor oscillator. Pressure on the nib changes the frequency from 255 to 266kHz by tuning the inductor, and the two buttons switch to 235 or 245kHz with additional capacitors.
The simplest Wacom pen would be a resonant LC circuit tuned to 750kHz. To transmit button and pressure status, an additional digital circuit modulates the resonant damping to send out individual bits of sensor data on each carrier burst.
For more details, see episodes 12 and 13 of my video blog.
Also see the journal PoC||GTFO, Volume 13:4 for some extreme reverse engineering and repurposing of the CTE-450 tablet … to read RFID tags!
WHAT YOU CAN DO WITH THEM
There’s plenty you can do with a graphic tablet without modifying it at all. You can emulate a Huion pen using a Raspberry Pi or Arduino with PWM. Emulate a Wacom pen by choosing a capacitor and inductor that resonate at 750kHz.
All modern tablets are USB input devices, but due to protocol complexities beyond our scope, it’s helpful to have drivers that understand the compatibility modes used by individual tablet models. The easiest way to use a tablet in your project is to attach it to a Raspberry Pi, where these Wacom and Huion and many other tablets will be supported by the evdev interface in Linux.
You can start to think of a tablet as an absolute position sensor, like a two-dimensional pair of calipers. If you build a pen emulation into the build plate of your 3D printer, you could drive the x- and y-axes with DC motors and use the tablet for servo feedback. Or put a pen in the hand of an inaccurate robot arm, and use the tablet as fine position feedback to help it draw. Even a vibrating bristle bot can draw, with a little bit of position feedback from a pen tablet!
MICAH ELIZABETH SCOTT (scanlime) is a maker, reverse engineer, video producer, and live-streamer who likes to take things apart on camera and build complicated robots for her cat.