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Capacitive Sensing And Old IBM Keyboards


The pen is mightier than the sword, but the IBM Model M keyboard, properly applied, can knock teeth in. There are a few more IBM keyboards even better suited to blunt force trauma – the extremely vintage beam spring keyboards made for terminals and desktop publishers. Being so very old, there’s no easy way to connect these keyboards to a modern system, so when [xwhatsit] wanted to make his work, he needed to build his own controller.

The beam spring keyboards use capacitive switches, and with 122 keys, the usual method of reading capacitance – putting a capacitor in an oscillator – would be far too slow to be of any use in a keyboard. There is another method of reading capacitance: measuring the current going through the capacitive switch. This can easily be accomplished with an LM339 comparator.

[xwhatsit]‘s keyboard controller uses this capacitive sensing circuit to read the four rows of keys, with a few shift registers taking care of the columns. An ATMega32u2 is the brains of the outfit, running LUFA to translate the key presses to USB.

If you’re lucky enough to have one of these ancient keyboards, [xwhatsit] is selling a few over on the usual mechanical keyboard forums.¬†There’s also a controller for the Model F keyboard using the same basic circuit. If you need one just drop him a line or grab the gerbers and roll your own.



Custom Mechanical Keyboards


[Wyager] was shopping around for a mechanical keyboard, and after noticing custom PCB manufacturing had come down in price so much, he decided to build his own. The end result is a keyboard that’s so elegant in its design, that it could, with a little work, become a very interesting Kickstarter project.

The design had three requirements: cheap, mechanical switches, and extremely customizable. The cheap requirement was solved by splitting the keyboard into two parts with a master/slave arrangement. The boards are connected by a 1/8″ TRRS jack conveying an I2C bus. Since both boards are identical except for the code running on the Teensy dev boards, [Wyager] saved a bit of cash by using two of the three PCBs that came with his OSHPark order.

The mechanical switches – Cherry MX Blues – are rather expensive parts for a failed project. For fear of failure, [Wyager] first ordered a PCB containing the footprint of only one key. With the footprint correct, he graduated to a 2×2 matrix. Once that was verified, the 6×5 matrix was ordered. Everything worked perfectly the first time, something we can’t say about many of our projects.

The code, board files, and schematics are available over on the github

USB adapter for an old VT100 keyboard


Ah, the VT100, the first dumb terminal that was controlled with a microprocessor. This ancient beast from the late 70s is quite unlike the terminals you’d find from even five years after its vintage – the keyboard connects via a TRS quarter-inch jack – the electronic and code design of this terminal is a bit weird. [Seth] was up to the challenge of making this mechanical keyboard work as a standard USB device, so he created his own USB adapter.

On the little quarter-inch to USB adapter, [Seth] included an HD 6402 UART to talk to the keyboard, along with a Teensy dev board and a few bits of circuits stolen from DEC engineers. The protocol between the keyboard and terminal is a little weird – first the terminal sets a bit in a status word, then the keyboard scans all the key rows and columns in sequence before telling the terminal it’s done. Yes, this gives the VT100 full n-key rollover, but it’s just¬†weird compared to even an IBM Model M keyboard that’s just a few years younger.

[Seth] finally completed his circuit and wired it up on a perfboard. Everything works just as it should, although a little key remapping was done to keep this keyboard adapter useful for Mac and Windows computers. It’s a wonderful bit of kit, and any insight we can get into the old DEC engineers is a wonderful read in any event.

Vidias below.

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