Today’s column makes me wonder if new engineers know the origin of “carriage return“. What does a carriage have to do with a computer, and why (and where) does it need to be returned? A clue to the answer comes from the USB typewriter, invented by experimenter Jack Zylkin.
Jack has taken a number of old manual typewriters (put your hand up if you learned to type on one of these) and converted them into working USB keyboards. Plug the typewriter into your computer, and start banging away. You can even type simultaneously onto paper. The concept is brilliant, and his implementation is terrific. There are two main components to his kit, a controller and a sensor board, and installation doesn’t require any permanent modifications to the typewriter.
Every manual typewriter has a part called a crossbar. When you type, part of the linkage for the keys presses against the crossbar and moves it. The motion of the crossbar is how the ink ribbon and carriage are advanced with each keystroke. Since all (or most) of the keys strike the crossbar this is where the kit detects which key has been pressed. The crossbar is first wrapped in insulating tape, and then thin flexible metal strips are wrapped around it, one for each key, and carefully aligned with each key. This way, when you press a key it now makes an electrical contact with one of the thin metal strips as it is moving the crossbar.
The sensor board has a number of shift registers on it, and the shift register outputs are connected to the thin key sensing strips wrapped around the crossbar. The controller steps a one down the length of the shift register. When a key is pressed, a circuit is completed between the shift register output, the key linkage and chassis of the typewriter, and an input pin on the microcontroller. All of the sensor strips are connected to the same microcontroller input pin, this works because the microcontroller knows which shift register output is activated, and therefore which key is completing the circuit back to the input pin.
Many important keys, like shift and space, do not use the crossbar. Zylkin works around this by using a few reed switches, along with magnets glued in appropriate places, so that when these special keys are used the magnet activates the reed switch, alerting the controller that the key was used.
Zylkin has a promotional video on his WWW page, take a minute to watch it. I especially like the one that shows a tablet computer sitting on the typewriter carriage, so that it moves as he types. The project is open source. If you don’t feel like designing your own you can buy a kit from Zylkin, or indeed a fully converted typewriter, although you’ll have to be prepared to spend some money on the ready-to-go model.
Steve Ravet
Design News Gadgeteer
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