Thread: The RamWall...
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Old 01-08-2006, 02:23 PM
Grant Stockly Grant Stockly is offline
Join Date: Jan 2005
Posts: 447

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From: "Tim Williams" <>
Newsgroups: sci.electronics.basics
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Subject: Re: Wall of RAM
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NNTP-Posting-Date: Tue, 03 Jan 2006 07:50:02 MST
Date: Tue, 3 Jan 2006 08:49:29 -0600

"logjam" <> wrote in message
> Anyway, I gave away around 1,500 to friends. I still have MANY of them
> left.

Say, do you have a range of colors? I'm probably stupid to ask for it, but
a couple hundred LEDs could be nice for a whacky project. Probably is
stupid since about the only indicator I use regularly is my oscilloscope...

> I thought a "fun" project would be to create a "Wall of RAM". Now it
> would be fairly easy to emulate the ram and display the results on the
> wall using a CPLD, but what about actually making real ram?

Ooo, nice.

> Does anyone think its possible to use capacitors and high current
> drivers to use the LED+capacitor combination as ram?

Not that I know of. LEDs are exponential, not negative-resistance, like
neon lights are. If you had gotten a couple ten-thousand neon lights, you
could do it (although clock would have to be slowed to the 10kHz range!).

> What do you guys think? It seems that with some RAS/CAS decoding logic
> that it shouldn't be too hard. The cons of this project are the cost
> of 4,096 capacitors and PCBs. The pros of the project are, well, it
> would look really neat.

You'll need at least 8,192 transistors and 16,384 resistors, to make yea
many RTL inverter R-S flip-flops. To burn LEDs, I suggest using a red to
indicate OFF and a green to indicate ON for each bit. A negative or
positive pulse to the base (supplied by resistor or capacitor) sets state,
while electrical state is read by collector voltage. RTL with typical
transistors will switch in a 100-200 nanoseconds, so should be able to keep
up with a standard clock. I'm not sure how you're going to address it,
since you need essentially eight 9 to 512 decoders!

Lesse, at 5mA per LED, that's a good 25 amperes supply current. 'Gonna want
that as low as possible, i.e., 5V or so. Total dissipation will be on the
order of 125W, not too bad.

For the wiring, runs several feet long will want to be coax or at least laid
against a ground plane or something. Terminating resistors might be a good


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