The system looks like a regular Bluetooth device. The BOSI connects to the computer or mobile devices through Bluetooth to control the scanning, switching, or selecting actions.

Each user is different in muscular functionalities and strength. This electronic design is flexible for up to 12 keys. However, in most cases, one or two keys are enough for using default apps. If using two keys, one key can be set as selection, the other one can be a home/start button or right-click menu.

The schematic is made from the Firtzing, an open-sourced electronics design software.

It includes the following main components:

1. Adafruit Bluefruit EZ-Key (Adafruit - $19.95). It is also sold on the Amazon, Mouser, or ebay.
2. A battery either Lipo (150mAh - $5.95), 9V (clip - $3.00) or AAA (holder with on/off switch - $1.95).
3. One or two switches. I found some old switches from an old machine. It does not have to be exactly the same as I used here. It can be any kind of push button switch. The buttons I found are NKK Switches LP0115CCKW01A (http://www.mouser.com/ds/2/295/lp01_pushbuttons-15697.pdf).

These components are optional:

1. Charger if using a Lipo battery (mini charger - $6.95).
2. A power switch to turn off power while not using it.
3. 3.5mm phone jack for additional switches or existing single switches. I include here to show there are plenty of room for adding more switches to the system. I got these phone jack from the radioshack for $4.49. But it is just $0.50 on the sparkfun.

The total costs only $20-30, which is much cheaper than the switches on the market ($180-300).