The display may work as a stand alone device driven by a microcontroller (Arduino stuff) or may be driven by a PC / laptop (interfaced by some discrete digital logic).
Design constrains / requests:
-to be big enough to be readable from 20-30 meters
-to be bright enough to be visible / readable in sun light
-to use energy efficient light emitting devices (bulbs, LEDs) / (low electrical power consumption)
-to use feasible and long lasting light emitting devices
-to withstand to harsh environment (some contests may be during fall, winter times, during rain, strong sun)
-to be light and sturdy in the same time (it has to be carried from point A to B)
-to have a reasonable size in order to fit on regular vans or on the roof of a car - it must be carried from a location to another
-to show hours, minutes, seconds and miliseconds (hundreds of a second) as a chronometer
-to show as well the temperature (negative and positive), local time and current date
-to separate the pair of digits by ":" - 2 separation dots
-to allow multiplexing of segments and digits and separate control of minus and separation points
-to have a reasonable cost
-to involve a reasonable amount of work
-to look nice
Based on these requirements, my choice was to use red LED strips for 12Vdc (these are comming in reels and you can cut the desired length), polycarbonate panels and a wooden frame. All the LEDs were connected using cheap network cable (8 wires inside, full copper).
The LEDs were connected to form 7 segments per digit and 8 digits (6 digits are big size and 2 digits are small size (these 2 digits are used to display hundreds of a second)). Also in front of the first digit is a big minus sign (for negative temperatures) and between each pair of digits are 2 separation dots.
-wire cutter (wire desoldering tool is nice to have)
-soldering iron and related tools for electronic parts soldering
-drilling machine and drill bits
-wood saw (any, you need to cut wood a few times)
-brush to paint
-hot glue pistol
-printer to print A4 sheets
-red LEDs - 2 reels at 12Vdc with adhesive back tape, one reel is 5 meters or 300 red LEDs (update 2014: you can look for LED reels with 120 LEDs per meter, the result is improved, see here another nice project: Giant Two-Digit Countdown Clock)
-polycarbonate panels for constructions - roofs (transparent preferred - mine was on sale and it is light brown)
-wood to build the wooden frame
-network cable 20 meters (UTP cable)
-sanitary silicone 2 tubes
-hot glue sticks 4-5 sticks
-thick paper pattern to align the segments and the digits - 6-8 A4 sheets (2 types - small and big)
-transparent packing tape
-plastic crosses for ceramic tiles 100 pieces pack should be more then enough (the thickness matter, so 3-4mm is ok or use 2, each one on top of the other)
Unfortunately at the moment of building this huge 7 segments 8 digits LED display I didn't take too many pictures, I had no intention to share that experiment, but now I realized that it may be interesting for others as well.
Due to this reason, the instructable did not documented with pictures each detail, some of the existing pictures are not the best or the most explicit, so you need to understand my English or to imagine another way of performing the same operation. Or simply to ask where you don't understand :) !
2012/12/10 Edit: LED stripes -> LED strips - see comments for more explanations