Saturday, July 4, 2009

Simmons LED Display Part 1: Architecture

This is the first post of a series documenting the Simmons Hall LED Display I worked on in September 2008. The idea for the project came from an earlier project by Dheera Venkatraman. Over the years, Dheera's display fell apart. I decided to reimplement the 6 by 6 Luxeon LED Display as a way to learn a little bit more about electronics and microcontrollers.

The display spans two floors of Simmons Hall, an undergraduate dormitory at MIT. Housed in the student gym, the display is visible far into Boston at night and is capable of scrolling most alphanumeric characters to display messages.

Just a little warning before I get into the architecture of this project: I know very little about electronics, so most of the decisions I made about the implementation of this display were uneducated guesses and probably represent the worst way to do most things. Now, onto the design.

I wanted the display to be very easily programmable. This meant that anyone should be able to change the message without needing to know how to program a
PIC or rewire the circuit. I also wanted to eventually build a web-interface for the display, so people could change the message in real time without having to even see the circuitry.

I also wanted to keep the electronics simple and expandable in case someone wanted to maintain it later. With these things in mind, the general architecture I arrived at is shown to the left.

I would have a computer connected to multiple microcontrollers, each controlling multiple LED's. The computer's role would be to decide which LED's needed to be turned on and for how long to print a given message. The computer would then tell the microcontrollers to turn pins on and off to form messages. The microcontrollers would act as slaves, listening to the computer for instructions.
Cracking open my copy of The Art Of Electronics, I found a small problem. The Luxeon LEDs I planned on using drew a full amp of power each. No microcontroller I could find could source that much current. I got some help from a couple electrical engineers and designed a power circuit for the LEDs that could be operated by a TTL signal from a microcontroller. The architecture then changed to what is shown to the left.

The microcontrollers toggle MOSFETs on the power boards which light up the LEDs. I wanted to place a power board on each floor to avoid running my own power cables too far.

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