Keypad for Arduino (Library Overview and Electronics Details incl. Oscillograms)

I wanted a keypad for my Arduino requiring no parts but switches … ↓↓↓ Complete description, time index and links below ↓↓↓ When the time had come to build a user interface for my UV exposure unit (link below), that included a keypad of course. Using an Arduino Micro as controller I had more than enough pins for the job. So I was searching for a solution that needed besides the switches no additional parts. I wanted it to be easy to use as well as to have a small footprint. And I found one. But then I wanted to know how it worked. 00:00 Intro – it works, but I want to know how 01:53 Wiring – just a matrix of keys 05:24 Oscillograms 1 – who measures measures rubbish 08:56 Library – easy to use, but a bit misleading 16:41 Documentation – nothing to write home about 22:32 Oscillograms 2 – are high and low Arduino pins really shorted 26:11 Arduino pins – they’re not three-state, but four-state 28:34 Measuring – four-state pins with an oscilloscope 30:04 Oscillograms 3 – the truth this time 35:47 Theory – of operation, of course