A basic arduino voltmeter connected to the android with bluetooth. Arduino code: .
IOS control arduino with HM-10 bluetooth: Simple way to turn low and high state the arduino pin13, with .
Android receive data from arduino via Bluetooth. In the arduino code, i write 500ms delay, maybe that too much, you can change to 2-300ms. Arduino code: .
Very basic Android wireless keyboard. Not need any software at pc/rpi side. Just plug in the arduino with usb, and working. Tested with arduino pro micro.