If your making a keyboard, your gonna need plenty of keys right? This is an easy enough task, you just need to plop a load of buttons onto the pcb and your done. I didnt want to waste i/o pins reading the status of all those buttons so I have made use of a chip that was made for the job. 

The TCA8418 chip can monitor up to 80 buttons, using minimal power and storing the key events in a nice little buffer so that you can process them whenever you are ready.

Using proper keyboard switches would be the ideal for the most usability, but they are not really pocket device friendly. there are smaller tactile switches, but once you start adding many multiples of buttons, the cost starts to shoot up.

If you have ever taken any small mobile device apart, you might have come across buttons that are stuck to the pcb under a plastic sheet. These are 'snap dome' switches, essentially taking a standard tactile switch and stripping it down to almost nothing. The pcb has 2 contacts on it, and there is a metal dome sat on top. pressing the dome deforms it, making it bridge the 2 contacts. The domes are arranged on an adhesive sheet, so they can be installed easily during manufacture. These custom sheets are actually really cheap, but the minimum order quantity on them is normally in the thousands, so prototypes and changes can get expensive.

It is of course possible to buy a bag of the domes, and make your own sheets with some tape and some steady hands... and the help of a 3d printed guide

lay some tape over the top, and then peel it off and transfer it to the pcb

Its not perfect sure, but its close enough to test :)

The other part to the puzzle is the strength of the domes. The ones I have used in the past were pretty 'strong' so when they were pressed and deformed, they had a loud and tactile click (hence the name clicker)

I wanted to get away from the noisy clicking (can you imagine a classroom full of clicky buttons being pressed) so I went for the lowest strength domes I could find. at a mere 55g activation force, you can almost trigger them by blowing on them!! I think there is some more experimentation needed to find the sweet spot of tactile feeling and minimal noise.

The quest continues....