[Cool toys] Tablet PC (1)-- Dissecting the tablet pen

I've been a tablet PC user (Toshiba Protege M200) for over a year now. The tablet is an amazing piece of engineering; it allows you to write directly on the laptop screen, as if it were a piece of paper. The digitizer pen writes like a smooth felt-tip. It even senses your writing pressure at 8 bit resolution, so in some graphics programs, if you press harder, you get a thicker line.

How does it work?

The digitizer pen has no power source, and is just an RLC circuit that resonates at a certain frequency-- this resonant frequency is modified by the pen pressure and by button presses.

To read the location of the pen, the tablet first sends out a pulsed radio wave through an antenna array which is embedded in the screen. This is recieved by the pen's circuitry, which resonates at a given frequency. Next, the tablet switches to antenna mode, and senses where on its antenna array the resonating pen is located. It apparently does this 50 or so times a second.

One of the few complaints that I have with the Toshiba unit is the ridiculously cheap digitizer pen. It's way too light, and feels like a cheap plastic ballpoint--which is not what you'd expect from an expensive unit.

This cheap digitizer pen broke on me recently, and I guess the least I can do is to post a photo for those who might be interested but don't want to destroy their pen for curiosity's sake. (The photo shows circuitry from the top part of the pen, which acts as an eraser. I imagine that another circuit exactly like it is embedded in the tip as well.)

Links

• How does a Graphics Tablet work?
• Wacom Graphire graphics tablet