Dissecting a keyboard

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Cathleen Sanders
5 years ago
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1 Dissecting a keyboard The keyboard on Jacky s PC died. Another opportunity to take something apart and admire its design and construction. I turned it over and pulled out all the screws. The back wouldn t come off

2 Well, sometimes they hide the final screw under the label. The label, by the way, suggests this PC was made in Makes sense; we got it in November, Gives you an idea of the level of technology we ll be looking at.

3 I peeled off the label. No joy.

4 Ha! The final screws were under the stick-on pads, one on either corner.

5 With the top tipped back, we first see a small circuit board with the LEDs.

6 Here it is, right-side up. Some lint is visible here, but there s lots lots more! Immunizing keyboards from contamination is one of the things we ll be looking at.

7 Atop the LEDs, a clear plastic bezel whose function is to guide the light through the holes in the cover, thereby making it more visible. By plugging the holes, the plastic domes also keep some of the dirt out.

8 The plastic bezel half removed. These two photos were taken later in the sequence, after everything had been torn apart.

As well as the LED panel, there s another little circuit board. The date code on the IC probably means 1997 week 18. The cable to the LED panel plugs in to the left of the picture.

9 As well as the LED panel, there s another little circuit board. The date code on the IC probably means 1997 week 18. The cable to the LED panel plugs in to the left of the picture. The two ribbon connectors are part of the conductor matrix that forms the keyswitches themselves. And the clear circles on the metal backplate are bits of plastic, melted to clamp the thin sandwich that is the working part of the keyboard.

10 The bottom of this larger circuit board.

11 The tray that forms the bottom of the keyboard, with the keyswitch sandwich assembly rotated upward at the top of the picture. Notice all the black grounding wires. Good hygiene!

12 If we pull off a couple of the keys, we see that they simply clamp onto the top of their plungers. Both plunger and base appear to be something like nylon, which is not only impervious to spills, but has low sliding friction. The second line of defense against dirt and spills is the high conical surround, molded into the baseplate and completely enclosed by the keycap.

13 Now let s look at the keys themselves. This photo is after blowing out much of the lint and debris, but there s still plenty left! To the right, we the ALT key, a little larger than the others, but basically the same, a single key resting on a single pedestal with a single plunger. To the left, we see most of the space bar. To keep it aligned, it actually travels on three parallel guides, two of which are visible. The larger one, at the center of the key, is the plunger that activates the switch. And there s a wire spring that helps return the key to the neutral position. About a dozen of the larger keys had these springs, but only the space bar had multiple guides. The reddish dot is a spot of grease.

14 The spacebar removed, showing the three guides, the plunger in the center, the wire spring.

15 Usually the cap comes off, leaving the plunger behind, but sometimes the plunger pops out.

Looking into the switch assembly through the hole where the plunger isn t, we see a flexible membrane that yet again protects the electronics from spills and dirt.

16 Looking into the switch assembly through the hole where the plunger isn t, we see a flexible membrane that yet again protects the electronics from spills and dirt. This membrane is made of something like rubber, and has enough elasticity that it pops the keys back up to their neutral positions. As we observed, however, some of the larger keys require external spring assists.

The outer shell removed, along with all of the keycaps. Most of the lint and dirt has been blown free or wiped free. At the top right, we see the ribbon connectors that connect to the circuit board.

17 The outer shell removed, along with all of the keycaps. Most of the lint and dirt has been blown free or wiped free. At the top right, we see the ribbon connectors that connect to the circuit board. There is also a set of special pushbutton keys, a single piece of molded rubber. The actual switch mechanism is the same as the others, but these are not keys for touch typing, so they don t need and don t have rapid response characteristics necessary for the other keys. Saving money! Yet another vital attribute of the design. Until now, we could theoretically reassemble the keyboard into a working unit. But now we pop the sandwich apart, breaking all those melted plastic links sandwiching the layers together, and there s no going back.

The white dots are the melted plastic pins that used to tie the sandwich together. There are three layers in the keyswitch assembly itself. In the foreground is a layer for columns.

18 The white dots are the melted plastic pins that used to tie the sandwich together. There are three layers in the keyswitch assembly itself. In the foreground is a layer for columns. In the background, a layer for rows. Between the two, there s a third layer with no circuitry, just holes in the mylar where the row layer and the column layer can be shorted together by the pressure of a plunger.

Showing how the ribbon connectors come off the layers. The columns cable has 18 conductors; the rows cable has 9. That would theoretically allow for 162 keys.

19 Showing how the ribbon connectors come off the layers. The columns cable has 18 conductors; the rows cable has 9. That would theoretically allow for 162 keys. Of course, a keyboard is far from a simple rectilinear matrix I look at the keyboard on my current PC, and speculate that it would need about 21 colums but only 6 rows. To reiterate, however, it s far from a simple rectilinear matrix.

20 To the left, the nylon baseplate, plungers in some of the holes. Next, the flexible rubber dirt guard and springy return for the plungers. Then the column layer. The intermediate layer isn t that easy to see, but the row layer is toward the right. My hand isn t really bloody. It s just an artifact of the lighting!

23 An attempt to show the three layers, the intermediate one comprising nothing but holes.

25 Here s that set of special keys, designed for dirt protection and low cost. As we see, there s a hard rubber plunger in the center of each key, so the switch mechanism is the same as for the other keys. It s easy to predict that you couldn t touch-type on them.

27 The flexible sheet, combining dirt and spill resistance with spring return and low cost. Most of the keyholes still contain their plungers, but I show one plunger separately. Interesting to notice that the plungers have a definite E-W or N-S orientation, an orientation that has to match the keycap or else!

28 To summarize: here s the nylon top plate, the steel baseplate with the flexible springy layer, and the plungers, which just fall out once you take it all apart.

29 The same, with a few more of the bits and pieces. Very functional, very cost-effective. As well as being inexpensive to make the components, you could imagine something pretty close to robot assembly, as well.

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