Hand Wired, Arduino coded Televideo 925 keyboard

[OldIsNew]

I've been wanting to try hand wiring and writing firmware for a keyboard, and after trying out mepler's TI-99 keyboard conversion tutorial (http://www.instructables.com/id/RaspTI- ... 994A-into/), I set about attempting to hand wire a Televideo 925 keyboard and then modify/write the Arduino firmware code for it. The Futaba switches on this board looked good for this kind of project and I saw that others have done it before with success . I used the Teensy++2.0 since it has plenty of pins and I just found its size a little easier to work with for this project than the Teensy2.0.

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First desoldered the PCB, then wired the rows and columns:

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Next decided which pins to use and then using mepler's code as a template, made the code modifications and additions for my keyboard matrix. A bit tedious coding for each key but I really just wanted the experience of doing it. One code modification I made was to use an Active Low configuration using the Teensy internal pull-up resistors, rather than the Active High with external pull-down resistors configuration mepler used in his TI-99 code.


Then wired the very messy looking initial breadboard, uploaded my hex file to the Teensy, did a bit of debugging, and it worked! Once I was sure everything was working I soldered directly to the Teensy and tucked everything back in the case. I'm sure my code is not the most elegant solution but I do feel like I learned quite a bit by doing it (again much thanks and credit to mepler for his TI-99 matrix template and tutorial).

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[Daniel Beardsmore]

So ... Enter is a pair of lightweight switches, and those have a cream or pale yellow base (seemingly a more saturated shade than the switches).

But left (but not right) shift, and print and funct use a switch that appears to have a darker yellow base. What's different about the feel of those keys compared to the others? Why would right shift not use this switch?

[OldIsNew]

So ... Enter is a pair of lightweight switches, and those have a cream or pale yellow base (seemingly a more saturated shade than the switches).

But left (but not right) shift, and print and funct use a switch that appears to have a darker yellow base. What's different about the feel of those keys compared to the others? Why would right shift not use this switch?

I can`t tell too much difference between the cream colored switches on the Return key and the others. Maybe when they were new it was more noticeable.

It`s hard to see in the first pictures I posted, but the bases of the other three differently colored switches are discolored and pitted - almost like they were exposed to a solvent or something. Other portions of these switches are the same orange as all the others. You can see some of the same effect on the center bar of the Back Tab switch ( rightmost top row in the pic below). Fortunately they work and feel fine so I didn`t look to replace them.

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[Daniel Beardsmore]

I have a bunch of the black ones and lots of those have orange bases that are very noticeably faded-looking, so it may just be that you've got the faded-looking ones together in a cluster.

The return key switches should feel substantially lighter: instead of a stabiliser wire, some keyboards have two lightweight switches side by side for one or more heavier keys, which appears to be the case here. However, the yellow base is also used in other keyboards for normal switches, suggesting that either the colour usage was inconsistent, or the switches are not anywhere near as light as they would be on an SMK or Micro Switch keyboard.

[OldIsNew]

The keyboard also has an internal speaker that I'm guessing was used for keyboard clicks. I went ahead and added a piezo speaker on an empty Teensy pin and a simple subroutine to provide a keystroke click. I may later try wiring the original speaker but this was a quick fix for now.

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I actually like it! I'm primarily a BS keyboard fan and the Futaba switches were just a little too quite for me.

[rsbseb]

Great project. I need to tackle a similar one in order to develop some skills as well.. nice looking board BTW