Paul Stoffregen of PJRC created this big 7-segment countdown timer for DorkBot PDX meetings:
Big 7-Segment Countdown Timer
A few months ago I was feeling inspired to create a nice countdown timer. With the next Dorkbot open mic only days away, I finally had the motivation to actually put it together.
The main challenge was driving the LEDs with a constant current, because they need about 10.5 volts across the several series LEDs. I wanted to run from 12 volts, so there wasn’t much voltage left over for the normal current limiting resistors. Instead, I used this opamp circuit.
The project runs from a Teensy 2.0. The code is very simple, using the SPI and Bounce libraries for the hardware interfacing.
PaulStoffregen has shared his board design on OSH Park:
Shared Project: big7seg 
Big 7-Segment driver. A ‘595 shift register lets you chain any number of these to build a large display.
Perfect for keeping Open Mic talks at hackerspaces on time!
OSH Park 