Mutiplexing additional pushbuttons

[kmcclary]

We've run out of input bits on the OI.

Anyone have a GOOD trick for multiplexing switch inputs so we can get a FEW more? I need the equivalent of either a bunch of pushbuttons, a "radio push switch" (one of N buttons lock down), or something otherwise akin to a Single-Pole N-Throw rotary switch to select at most ONE (or NONE) of something.

I'm not sure if a diode matrix is legal ("custom circuit running an input"), and I'm not sure if we WANT to have to "address" them with LED outputs even if it WERE.

I'm also not sure if using an R2R resistor ladder (or shorting out individual weighted resistors) to encode them in binary as a "pot input" would be clean enough from a debouncing perspective (we may get wrong values on some loops, but we COULD filter them).

So... What have YOU guys done in the past whenever you've run out of switch inputs to get a FEW more??? (Besides "change the design" ) Please be specific with values if you have something like a resistor trick.

- Keith

[rbayer]

Two years ago, we ran into the same thing. In order to get one more switch, I wired up an LED and a switch in series to an analog input. When "on", it read one value and when off read 127. I'm not sure how legal/smart this was, but it worked two years ago.

--Rob