I should mention for completeness I've thought of three other ways to get the Android phone/tablet to communicate with the Arduino. None of them is what I would choose to use, and I present them more for amusement value.
- You could program the Android to flash the screen different colors and have a color sensor on the Arduino to respond to a particular sequence;
- Likewise, you could program the Android to emit a particular sound, and have some sound sensor waiting for a particular tone, such as the DTMF tones used for dialing on wired phones;
- And silliest of all, put a motion sensor on the Arduino and attach it to the phone, then have the phone go into vibration mode to tell the Arduino to do its thing.