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Hey guys.  I know this question has been asked time and time again, but I want to get absolute clarity to make sure I don't burn down my house or something.

Basically, I'm building an LED sign for a performance I'm doing.  It's two cardboard boxes with letters cut out.  I'll be lining the top and bottom of the boxes with LEDs (100 in total - 50 for each box).

I'm interfacing with my performance software over MIDI - I've got that all figured out and it works fine with a single LED.  What I need is a good way to power these LEDs and turn them on and off, with PWM (so it can fade in and out to beats).  I'm currently just running the single LED off of a PWM pin.

Each LED consumed an avg of 20 mA with a max of 50 mA.  This is 2-5 A for all the LEDs, which is an awful lot.  What I was going to do is get a large 5V power supply, power the Arduino off of it unregulated and use a transistor to turn all the LEDs on and off (all through a single pin).

My question is this - what's the best way to do this, and what kind of transistor am I looking at?  It'd be nice for one PWM pin -> transistor -> all LEDs (with resistors) in parralel + 5V power supply.

Thanks for any assistance.

Drew
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Also, 2A is a lot to be running through cardboard, yes?
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Are all your LEDs the same color? What color?

I would use two power supplies - one for the Arduino one
for the LEDs.

I would use a 24V supply for the LEDs. Series up LEDs until the sum
of the Vf's is within 2 or 3 volts of 24V (for white LEDs this
is usually 7). Use a FET to turn the LEDs on and a resistor
to limit current. This will reduce your current my a factor
of 7. Parallel the LED strings as required.

If you wire all the LEDs in parallel I would get an adjustable
supply for the LED voltage. Adjust the supply so it is just above
the maximum Vf spec of the LED. Use a series resistor to limit
current. Your maximum current hasn't changed but your
efficiency is improved.

(* jcl *)

http://www.wiblocks.com

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Thanks for the replies.  They're all white LEDs, voltage drop across each is 2.7V, average drain 20 mA.  I'm going to go with a 12V supplt and follow that calculator posted.  Fantastic.

TIP102 or TIP120?  Any reason for those in particular?

I guess the transistor would have to be able to handle 600-1000 mA of current running through it.
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Cool, thanks.  I got a 12V 1000mA adapter and am using sets of (4 LEDs with a 68 Ohm resistor in series) in parallel.  This works great, but the only transistor I could get without doing another Digikey order is a TIP29c.  I know that it's NPN and that it'll handle 1000mA at the collector but I'm not sure what the gain is.  How might I figure out what resistors to use?
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Okay perfect.

Well, math tells me that the average current by the entire array will be 400 mA (4 LEDs in each string, 20 strings).

By the way, I'm in Victoria, Canada.  I'll update the profile, heh.

Thanks.
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Bump for help on this.

Can anyone help me calculate the base resistor voltage for a transistor with collecter current ~400mA?

edit:datasheet:

http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00001984.pdf
« Last Edit: January 18, 2011, 02:15:04 pm by smocksturr » Logged

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