Resistive switching

[Daniel Beardsmore]

According to Devlin, Alphameric's sealed military keyboards used "resistive switching", which is possibly described here, for the electronics gurus (people like HaaTa and hasu, not numpties like me):

http://www.scholarpedia.org/article/Resistive_switching

That might explain why they have both conductive domes and capacitor pads:

http://imgur.com/a/1raE6

I'm not 100% convinced though that the keyboard uses the same technology as described in that article — there might be something else that falls under that term. Hopefully someone here understands this all better than I do; maybe someone with such a keyboard lives near someone who has the skills to decipher how it works.

The rubber sheet kept out contaminants, which is why it was used for military keyboards. It's not their only switch, and it's not the one that is made today …

What I do find curious is that that keyboard appears to have another plastic layer above all the slider guides, because from the top you see a single slider guide plate, but from below it's discrete plate-mounted slider guides like Key Tronic or Topre.

[Daniel Beardsmore]

Apparently it is indeed capacitive (according to a former Alphameric employee), even if Alphameric did refer to it as "resistive". One day I'll go back and re-read the patents and see if I can make sense out of them.

[002]

What a strange creature... At first glance is looks capacitive, but the domes scream conductive.

[Muirium]

Belt and braces, perhaps? This is just the idlest of speculation, of course, but perhaps the idea was to make extra sure that come what may: this thing would always work? Or it could be a spot of dome reuse from an other design? Or perhaps I should shut up now before I give Daniel a headache?

[JBert]

It can't be a conductive circuit, the whole PCB is covered in green soldermask. jIt could be a matter of cost reduction to reuse existing rubber dome sheets, as long as the black stuff is a proper dielectric material the whole thing should work.

Note that model F hammers are also black and sparkly (possibly from carbon in the plastic, accoring to Ripster), yet they weren't conductive. Of course, those hammers needed to be strong whereas pure carbon is fragile so a composite material was needed. Here carbon-infused rubber seems like a cost-effective alternative.

[Daniel Beardsmore]

Belt and braces, perhaps? This is just the idlest of speculation, of course, but perhaps the idea was to make extra sure that come what may: this thing would always work? Or it could be a spot of dome reuse from an other design? Or perhaps I should shut up now before I give Daniel a headache?

I rarely get headaches but I've given myself one today.

Looking briefly at the patents on [wiki]Alphameric resistive[/wiki]:

http://www.google.co.uk/patents/US3940578
"The block 20 may for example bridge two conductive areas on the printed circuit board to form a conductive, possibly resistive path between these two areas or to provide capacitive couplings between these two conductive areas on the printed circuit board."

(What's a "resistive path"?)

http://www.google.co.uk/patents/US3797630
"Such keyboards however are expensive. In order to try to reduce the cost, proposals have been made to use flexible conductive elements, e.g.; a gold plated plastics film which can be manually depressed to complete a resistive circuit."

"Preferably capacitive coupling is used with the conductor on the key forming a capacitor bridge between two conductors of the printed circuit board."

"The conductive elastomer is used as a plate for a dual parallel plate capacitor."


I don't know for certain how capacitive keyboards work. I'm always hoping someone who understands them will split off that part ({{main|Capacitive keyboard}}) on the wiki and write up the subject in detail, with circuit diagrams etc.

One thing I learned — with a radio, if you don't shield the tuning capacitor, your finger will alter the radio's tuned frequency significantly while you're turning the dial. I seem to recall that capacitive keyboards run a high frequency signal through the matrix. I also recall someone saying that each key position has (in effect, I forget how it was worded) a sense plate and a reference plate. Maybe these ideas are related. I don't know. This is a whole side to the wiki that needs the electronics people to write up (all the electromagnetic switch designs).

However, there seems to be more than one way to do capacitive sensing, and it looks like Alphameric found another approach to the problem, one that gave them both tactility and a sealed assembly suitable for military use. How it works, I don't know — the patents don't appear to go into great detail of the electronics concepts, but I still need to scrutinise them for clues.