The Simmons LED Display is a home-made two-story high 6x6 array of windows outfitted with Luxeon LXHL-LR3C Royal Blue LEDs. It is located in the exercise room of the architecturally infamous Simmons Hall, a residence hall designed by Steven Holl that opened in 2002 for MIT's undergraduates. (Each floor is three windows high and the overall design of the building is inspired by a sponge.) It can be seen from most of Briggs Field and Dorm Row at MIT, thus making for a convenient announcement board. It follows from a vision that many Simmons Hall residents have had in the past: to turn the waffle-like arrangement of windows into working pixels. This project implemented that with under $150 of components, funded by the Simmons Rush budget. This was also done just because blue LEDs that bright (possibly the world's brightest commercially available blue LEDs?) are just cool. Below is a view of the display as seen from the inside:
How it works
Since we were trying to get this done in time for Dorm Rush, we put this together with whatever I had at the time. Each floor has a controller that consists of a PIC16F877A that switches each LED individually using IRL520N power MOSFETS. Each LED draws upto 1 amp (in this case using 2 ohm resistors for the LED's they draw somewhat less), so the power supply needs to be able to handle upto 20 A or so for each floor, since each floor is a 3x6 array. I used ATX power supplies which are happy sourcing 25 A at 5 V, but as they are switching supplies, they are bad at not browning out for a few microseconds when the load changes suddenly. This often results in the microcontroller crashing. This is resolved by adding a few large capacitors to the input (about 6600 uF are used here). Not the best power filtering but it worked for the quick hack this was at the time. Also note that the microcontroller can brown out if it is connected directly to the gates of 18 FET's, so the design here uses 1K resistors between the microcontroller and each gate. A schematic of a single controller board is shown below:
There is one of these controller boards for each of the two floors. Both listen on an RS232 "bus" through the wire labeled "Data in" above, at 9600 bps. A separate box (also powered by a PIC16F877A) sends the signals to both floors on the same wire to coordinate them to behave as as a single display.
This project was mostly a quick hack for Dorm Rush and much of the duct tape came apart within months. I'm glad to know it has apparently been rebuilt by Dan Lorenc and later residents of Simmons Hall in green LED's with a much better interface and more robust electronics.
Thanks to everyone on the Simmons Rush team that helped with the installation including soldering, duct taping, and cutting. Thanks to Simmons Hall for providing the funding for this project.