Three-leg switches

[Daniel Beardsmore]

It's well-known that some switches have three legs, but I never gave any thought to why. Here's a KPT keyboard with three-leg switches:

http://kbtalking.cool3c.com/article/13939

Notice how all three legs have separate traces. No leg is connected to any other leg on the same switch.

It looks to me as though they're increasing the rollover by probing switches from multiple angles. i.e. instead of putting diodes onto the PCB, they've put two probe lines on each switch. (Some switches do only have two legs soldered.)

Incidentally, he's got that down a 1984 keyboard, but the FCC claim it was registered in 1991. That's more than a small discrepancy!

[Findecanor]

If you look at the switch taken apart, there is one piece of metal that has two legs. I think that it is a clever way to incorporate a jumper into the switch design.

[Daniel Beardsmore]

Yes, I can see that the two outer pins are connected.

The jumper suggestion is a disappointing one, but more in line with the sort of credit that one would expect them to be due.

I don't know jack about electronics.

I still wonder why the FCC dates are so different to what Alps.tw believes is the timeframe for those keyboards and switches. But then, the 84- and 87-key keyboards are missing from the FCC database — either the FCC lost records, or KPT lied about having legitimate FCC IDs.