This device, as the title says, is a piston based letter display. What it does, is that you press a button, and then in a 10x8 gold blocks display, it shows the letter. There's 26 buttons, each one corresponds to a letter.
So basically, as you can see in the images, there's a hole in the wall, where you can see the display.
The buttons are located in a room made of iron, diamond and gold blocks, with a piston door made of glowstone, covered by pine. This door will close when you activate on lever from each side (left and right) and it also works with one from the outside and one (the opposite one) from the inside. It also has a lightswitch on the inside, with an enormous circuit on the roof and walls.
Some letters don't look good (M, W, X, Y), but that's because the size of the display (each pixel is 2x2, so it's 5x4 pixels) isn't really appropiate, but i just noticed that when i finished it, so i would have to remodel it completely to add another line of pixels.
NOTE: Remember to press the button you pressed again so it resets. If you press one button, and without reseting it, you press another, it will get messed up. In some cases that can be fixed by pressing both buttons, but in other cases, you're goinna have to go the piston memory thing (behind the display, the nearest thing, with pistons) and turn on once, with anything you want (button, redstone torch) every one that has the output on. So yeah, try to remember to don't press 2 buttons.
Anyway, enjoy it, and have fun!