With the nice small display, it would be nice to also have some buttons, so that you could at least navigate a menu. So I decided to make a shield with three buttons. The problem is, we only have one free pin left with the display shield on. I didn't want to muck with reusing the SPI pins for other functions while the CS pin is high, and other trickery, but I noticed that the free pin is an analog pin. Great, we can do the resistor ladder trick!
Basically, we will need a voltage divider, with different ratio of resistors switched by different buttons. Something like this:
When no button is pressed, the analog pin is pulled up to 3.3V with the 20kΩ resistor, and the analog pin reads its maxium value. When button A is pressed, half of that voltage is redirected to ground, and the analog pin only reads 1.65V. When B is pressed, ⅓ of the voltage is redirected, and the pin should read 2.2V. When both A and B are pressed, ⅗ of voltage gets redirected, and the pin reads 1.32V. And so on, adding button C for ⅕.
Now, looking at the voltage we can figure out exactly what combination of buttons has been pressed. It gets even easier if you only look at the top 3 bits of the reading.
The shield itself is designed to go on the bottom of the trinket — because the display goes on top, of course.