The buttons
The mechanical buttons of the original were the first to be simplified away. I stil wanted input, though, so I had to think of another way to let the user input data. 9 leds, 9 buttons, all positioned over the leds... My mind went to work and eventually an idea popped up: Why not make the case from acrylic glass and detect if the light from a led scatters from the press of a finger?
To explain what I mean, perhaps a picture will help:
This is a piece of bent acrylic glass lit on one side by 3 blue LEDs. The glass
acts in the same way as glass-fibre does: the light will bounce inside the glass.
You can see my finger pressed against the glass, too; where it does, the
fingerprint lights up blue. This is because at that point, the nice reflective
glass-air-transition becomes a glass-finger-transition. The light won't reflect
anymore and bounces off the tip of my finger, to the camera's lens.
Now imagine a led in the place of the camera and a photo-sensor where the led is. Normally the light shines straight through the acrylic glass into the air, but as soon as a finger is present, the light will bounce of that and follow the acrylic glass to a photosensor which can then detect the whole event.
After some testing, I found out that the reflections generated in this way were minute, but detectible. With this in mind, I proceeded to build the actual hardware.