RPN-duino

[Chasfield]

I have turned out a little project to reawaken this thread with.

I wanted to build an Arduino based RPN calculator entirely from stock sub-assemblies but not be restricted to the ubiquitous 4x4 Arduino tactile keyboard. In my experience, a 4x5 key module gets you into the comfort zone for a basic scientific calculator but the only low-cost, readily available keyboard of this configuration that I have seen is a self adhesive membrane unit, which makes for a quick build but offers very poor usability. Then, I chanced upon a small 5 way switch module that is configured like a mini joy stick. These are available all over the place, PCB mounted or loose, for next to nothing and can be piggy-backed onto a 4x4 keyboard's column headers to add the functionality of a an extra row of keys. The common terminal of the switch becomes row header 5.

The result is a rather strange looking device (see attached picture), which turns out to be unexpectedly ergonomic - for right handers at least. I topped the 5 way switch with the stem from a miniature push button switch. This mini joy stick gives the user's left thumb super powers:

Right = Enter
Up = Shift
Down = Roll down
Left = Exchange X/Y registers

Shift Left = Convert X to degrees
Shift Right = Convert X to radians
Shift Down = Sign change

Which is all quite intuitive.

I met with a couple of snags. The switch's PCB was loaded with redundant pull-up resistors and these were leaking row scan signals directly to the column inputs, resulting in column one responses for all key presses. It took only 2 minutes to desolder them. A fifth switch function is available when the switch is pushed in but I found this action to be almost unusable. The actuation pressure is much higher than it is for the four directional movements, so there was a tendency to get an uncommanded directional actuation when trying to press the switch down in the normal way.

There is still a bit of key legending work to be done.