An interesting thing about this project is that the two displays collectively have 16 pins which are used, but the Arduino is able to control both using only 9 pins thanks to a technique called mulitplexing. This technique allows only one light to be on at any given time by connecting them together and then letting the Arduino control which display gets connected to ground. Even though only one light can be controlled at a time, thanks to the phenomenon of persistence of vision, if both lights are flickered on and off in series fast enough, we perceive them to both be on all the time. While this may seem complicated, this is actually a commonplace technique for controlling LED displays.
Get experimenting and see for yourself by building your own!
Step 1: Materials
(x1) Arduino Uno (Radioshack #276-128)
(x2) 7-segment display (Radioshack #276-075)
(x1) SPST momentary pushbutton switch (Radioshack #275-646)
(x1) M-type coaxial power plug (Radioshack #274-1569)
(x2) 220 ohm 1/4 watt resistor (Radioshack #271-1313)
(x1) 10K ohm 1/4 watt resistor (Radioshack #271-1335)
(x1) Grid-Style PC Board (Radioshack #276-149)
(x1) Piezo element (Radioshack #273-073)
(x1) 9v snap connector (Radioshack #270-324)
(x1) 9V Battery (Radioshack #23-853)
(x1) 5" x 2.5" x 2" project enclosure (Radioshack #270-1803)
(x1) 22awg solid core wire (Radioshack #278-1221)