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."

Comments

Re: Slashdotted

[dheera]

oh god. mirror: http://web.mit.edu/dheera/www/simdisplay.php.html

Blinkenlights?

[Animaether]

http://www.blinkenlights.de/index.en.html

And I *know* there was a Dutch team that did much the same as well, and a Dutch commercial venture (was it KPN - Dutch telecom?) has one still up and running, I think.

But I guess they didn't use the Ooh! Shiny! blue LEDs :)

Re:Potential Income Opportunity

[JoshJ]

I'd much rather they not corrupt the project by doing that. It's awesome as is, if it turned into yet another kind of lame advertising, that'd suck.

Too bad

[fuckingsound]

"We need babes now!" doesn't fit on a 6x6 array.

Re:Too bad

[smf28]

It is animated and scrolls text or any 6x6 animation; see the video on the link.

Re:Microchip PICs

[Anonymous Coward]

Fine use for any uC.

Check out Atmel AVRs. They're especially good if you've ever needed to do I2C and really didn't want to write a bit-banger system. They also have on-die oscillators, which could have removed the external crystal in their schematic.

~AVR Fan Boy

Something like it has been done, and it was cooler

[BeeBeard]

Make the array bigger and put Tetris on that thing, kind of like these guys or even these guys.

Take some notice.

[nazera]

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.

Circuit design error

[Flying pig]

The diagram shows a simple, basic mistake in circuit design. The Vcc of the microcontroller should not ever be directly connected to the Vcc of the power bus. Adding extra capacitance to the supply is not smart, it is quite unnecessary.

The microcontroller should have a separate supply, and as the consumption of the PIC is so low this could be derived simply by passing the LED supply through a small low voltage drop diode (Schottky diode) and preferably a suitable inductor, and then decoupling it with electrolytic and ceramic capacitors (say 1000uF and 100nF) in parallel as close to the Vcc pin as possible. With this arrangement, the LED Vcc can even momentarily drop to zero and the microcontroller will just keep running.

(In fact, if you are thinking of doing this from scratch, you do not need an expensive supply at all. Rectify the output of a transformer directly to provide pulsating DC (100Hz Europe, 120Hz US.) This is the LED bus. You can do that with a 35A 50V bridge rectifier bolted to a nice big alumin(i)um strip. Then pass the output through a diode, a suitable resistor, and stabilise it with a 5.1V Zener. Assuming a peak of about 8V from your transformer, a 1A Schottky, a 10 ohm 3W wirewound resistor and a 5W 5.1V Zener will do just fine, with maybe a 1000uF electrolytic and a 100nF ceramic to stabilise the voltage at the PIC and provide enough surge capacity to drive the MOSFET gates. That way, you avoid the major disadvantage of switching power supplies, which is that they do not like rapidly varying loads.

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.

Re:Circuit design error

[dheera]

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).

Re:Circuit design error

[dpaton.net]

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.

Re:Slashdotted

[dheera]

I looked at my logs. It's probably the initial rush of visits in the first couple of minutes. A potential "slashdotting-protection" system that could be implemented by large sites is to select different IP ranges at random and serve up the site with a 0, 30, 60, 90, 120, 150, or 180 second delay based on the incoming IP range (i.e. if you're in the IP range corresponding to the 120 second delay, the article doesn't even show up for you until 120 seconds after it has been posted. I don't know if this has other moral issues, or if it's Slashdot's responsibility to care, but it's just a thought about a hypothetical solution.

That's nothing compared to the Cubatron

[neuro.slug]

The Big Round Cubatron is a much bigger, much cooler DIY LED display. It was the cool thing at this year's Burning Man.

Videos here, here, here, ...well, you get the idea.

Delft University of Technology was first

[Thijs van As]

In 1995, Electrical Engineering students of Delft University of Technology did this, with playing Tetris on their 100m high building.
Link: http://www.etv.tudelft.nl/vereeniging/archief/lust rum/90/english.html

In 2001, they used their building as a big SMS display.
Link: http://www.etv.tudelft.nl/vereeniging/archief/lust rum/95/english.php

In 2006, a huge 8x4x2m LED MatriXX was created.
Link: http://www.etv.tudelft.nl/vereeniging/commissies/e lco/matrixx/