The keyboard PCB should be protected from accidental movements as most times it’s a cheap piece and will break easily and the soldered contacts will be easily detached, so I protected it on a small box.

This box should be tied to a wristband, to fix it in the arm, and it should have a way to secure the cell phone above it. I used a popsocket which is easily detachable and will hold the phone secure.

If the keyboard PCB is wired or not, then an OTG connector should be used to connect the cable to the cell phone’s micro USB port. If wireless, the keyboard will bring a little dongle which should be connected to the OTG.