The display shows up the battery status, Missed calls count and Unread SMS on 8 digits 7 Segment display module. The communication with the accessory is using Bluetooth and one Arduino board that interfaces with the display and the Bluetooth unit.
You need to run the program once (per boot of the phone), and even if the program is in the background, when the accessory and the phone are in range of Bluetooth communication, the display will show the status of your phone. Pretty much like Car hands free set, that automatically connects with your phone.
The code runs on Android phone and source code of the Arduino accessory and the Android phone code are shared here in step 4.
Step 1: What you need to assemble it?
2. Bluetooth module like this one, cost is ~$ 40 at Sparkfun. To use cheaper one ($ 10 on eBay) you will need minor code modifications which I tend to do for a new assembly of this.
3. 7 Segment display , cost is ~ $10
4. Wires for connecting and potentially a power supply for driving the accessory and enclosure. I did not complete this part yet.
Do yourself a favor and look for the items on eBay. It will save you money.
Total cost, if assembled by items you shop on eBay can be about $ 30. If you go for traditional on-line stores, cost will be higher.