Arduino and a lot of LEDs

Forum 2005-2010 (read only) Hardware Interfacing
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I'm in the process of making a banner for a friend's band, but not any kind of banner, one where the letters are LEDs. The leds will be the outline of the letters so they won't be filling up the whole letter.

I'm not sure about the exact number of LEDs to be used yet, but it should be somewhere around 450-500.

Now, I'm not much of an expert when it comes to hooking up lots of LEDs to an arduino so I went to my local electronics store to ask for some advice and the guy suggested using the ULN2803, but I don't think 2 of those are going to cut it (1000 mA per 2803 x 2 = 2000. At 20mA for each they could only drive 100 LEDs so that's not anywhere near enough).

I guess I would have to add some kind of IC to the whole thing, but I honestly have no idea what kind of IC this would be. Could anyone give some advice on how to acomplish such a thing?

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(1000 mA per 2803 x 2 = 2000. At 20mA for each they could only drive 100 LEDs so that's not anywhere near enough).

It is worse that that, you can only switch about 570mA from them so you will need more than two, see:- http://www.thebox.myzen.co.uk/Tutorial/Power_Examples.html
so you need a lot. If you want to just flash the LEDs you don't need an arduino just a DC SSR (solid state relay) and an NE555 timer.

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It is good practice for long to connect equally behaving LEDs serially. This depends on the voltage you have available. Popular is 12V = 3 to 6 LEDs (depends on the color) and 24V = 6 to 12 LEDs. This generally reduces the current need considerably.

In case of a banner you might want to switch-on LEDs separately (a "chaser") which reduces power needs further...