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A Giant DIY LED Display
smf28 writes "Dheera Venkatraman has created a giant DIY LED display featuring 36 blue Luxeons in a 6x6 array on the windows of Simmons Hall, an undergraduate dormitory at MIT famous (or infamous, if you wish) for its design. Recent uses included welcoming students in September, Pirate Day, and others."
"We need babes now!" doesn't fit on a 6x6 array.
Being a crontrols system desiger, I hope some of the goofs of industry look at stuff like this. I am constantly yelling at my vendors that I don't need a super screwed up version of RS232/485/422 etc to network sensors around a machine......binary and ASCII protocols WORK GREAT for stuff like this. KISS (Keep it Simple and Stupid). A few micros some twisted pairs and your basic switcher......bingo big network of fun.....if you need some more bandwidth, throw Ethernet at it. I've been saying this for over 10 years and still all the big players want to sell you a "Field Bus". I hope some of the MIT guys move in with the big guys...and slap some sense into them.....rant off.
Thanks for the suggestions. The PIC should actually be able to drive the MOSFET, it's just that there ends up being some small amount of ringing, which then [in some way that I didn't bother to analyze] caused the entire thing to hang. Adding the resistors killed that and made it work especially for cases where a large fraction of the entire array is suddenly turned on. But yeah, redoing it with better power supplies and power circuitry would definitely be something to do when we get time. At the time it was built we were trying to finish as quickly as possible and used ATX power supplies which are essentially free at MIT (you find them lying around in dumped computers everywhere).
Oh, another thing. Do not put a resistor between the PIC and the MOSFET gate. Use a driver chip to translate the current levels. Cheap insurance.
You're kidding right?
A resistor for gate isolation is just fine, especially for a low side FET drive. A driver chip would cost as much as the FETs, and is overkill to the extreme. In a perfect world, where money and time are infinite for design, it's easy to make anything better. For something like this, a little realism is in order.
My $0.02 on the design:
I've done something similar as a proof of concept for a customer...256 RGB LEDs (50mA/color, ~38A at full bright/full white) with 64 custom processors controlled by a big Atmel. It ran off a standard 600W ATX supply, and it worked just fine, no voltage dropouts at all. I don't think the ATX supply itself was the problem, rather the layout of the circuit. A normal ATX supply has rather good transient reacitve capabilities. Using a single power supply for an entire floor is likely the culprit. It looks like the run on each floor was about 60', and I highly doubt that he used the right sized wire for that run (25A @ 60'-> #8). The accumulated coltage drop would be pretty extreme, making the PICs low voltage brownout inevitable. Combine that with an improper power supply arangement at each processor location and bam, crashes. The 6600uF caps are a band-aid, I agree. A fat wire feeding the high sides of the LEDs, and a secondary wire feeding the PICs would be my choice. Yes, they can safely be tied together, but ONLY AT THE SOURCE. That long run of wire will be all the isolation they need. Standard long distance bypassing at the PICs will keep them happy (10uF/1uF/0.1uF) and a nice fat ground return keeps it all under control. There were a few mistakes, but by no means is it fatally flawed.
This is not a sig. this is a duck. quack.
The Big Round Cubatron is a much bigger, much cooler DIY LED display. It was the cool thing at this year's Burning Man.
...well, you get the idea.
Videos here, here, here,