Selectric Typewriter Goes From Trash Can To Linux Terminal

July 25, 2023 by Dan Maloney 19 Comments

If there’s only lesson to be learned from [alnwlsn]’s conversion of an IBM Selectric typewriter into a serial terminal for Linux, it’s that we’ve been hanging around the wrong garbage cans. Because that’s where he found the donor machine for this project, and it wasn’t even the first one he’s come across in the trash. The best we’ve ever done is a nasty old microwave.

For being a dumpster find, the Selectric II was actually in pretty decent shape. The first couple of minutes of the video after the break show not only the minimal repairs needed to get the typewriter back on its feet, but also a whirlwind tour of the remarkably complex mechanisms that turn keypresses into characters on the page. As it turns out, knowing how the mechanical linkages work is the secret behind converting the Selectric into a teletype, entirely within the original enclosure and with as few modifications to the existing mechanism as possible.

Keypresses are mimicked with a mere thirteen solenoids — six for the “latch interposers” that interface with the famous whiffletree mechanism that converts binary input to a specific character on the typeball, and six more that control thinks like the cycle bail and control keys. The thirteenth solenoid controls an added bell, because every good teletype needs a bell. For sensing the keypresses — this is to be a duplex terminal, after all — [alnwlsn] pulled a page from the Soviet Cold War fieldcraft manual and used opto-interrupters to monitor the positions of the latch interposers as keys are pressed, plus more for the control keys.

The electronics are pretty straightforward — a bunch of MOSFETs to drive the solenoids, plus an AVR microcontroller. The terminal speaks RS-232, as one would expect, and within the limitations of keyboard and character set differences over the 50-odd years since the Selectric was introduced, it works fantastic as a Linux terminal. The back half of the video is loaded with demos, some of which aptly demonstrate why a lot of Unix commands look the way they do, but also some neat hybrid stuff, like a ChatGPT client.

Hats off to [alnwlsn] for tackling a difficult project while maintaining the integrity of the original hardware.

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Hackaday Prize Semifinalist: OpenBionics Affordable Prosthetic Hands

September 20, 2015 by Dan Maloney 16 Comments

The human hand is an amazing machine, and duplicating even a fraction of its abilities in a prosthetic is a daunting task. Flexible anthropomorphic prosthetics can reach tens of thousands of dollars and are beyond the means of many of the people who need them. So imagine the impact a $200USD prosthetic hand could have.

For such a low, low price you might expect a simple hook or pincer grip hand, but the OpenBionics initiative designed their hand from the outset to mimic the human hand as much as possible. The fingers are Plexiglas with silicone knuckles that are flexed by tendon cables running in sheaths and extended by energy stored in elastomeric material running along their dorsal aspects. Each finger can be selectively locked in place using a differential based on the whiffletree mechanism, resulting in 16 combinations of finger positions with only a single motor. Combined with 9 unique thumb positions, 144 unique grasp are possible with the open source hand built from hardware store and 3D printed parts. Stay tuned for a video of the hand in action after the break.

3D printing is beginning to prove it’s the next big thing in prosthetics. Hackers are coming up with all kinds of static artificial hands, from the elegant to super-hero themed. Maybe the mechanism that OpenBionics has come up with will find its way into these hands – after all, it is an open source project.

The 2015 Hackaday Prize is sponsored by:

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Wiffletree: A Mechanical Digital To Analog Converter

November 28, 2010 by Mike Szczys 15 Comments

This isn’t a hack. But it is a decidedly interesting piece of mechanical technology. The Whiffletree shown above is a way to turn binary data into a mechanical analog value. [Bill Hammack] explains how this assembly is used in a typewriter and how a whiffletree can convert binary data to a set of analog outputs.

These linkages are what makes an IBM Selectrix Typewriter work. You know, the one with the globe stylus instead of individual hammers for each key? [Bill] uses the typewriter as the example in his illustrations that show how each bit of data positions the output in a predictably different location. We’re familiar with other mechanical representations of binary data but converting to an analog value mechanically is a new concept for us. Lukily, the videos that [Bill] put together are fantastic at explaining the concepts. Not surprising, since he is a professor at the University of Illinois at Urbana-Champaign . See them both after the break.

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