Diecimila & TLC 5940

I'm using a Deicimila and 2 TLC 5940 to light these LEDshttp://www.newark.com/jsp/search/productdetail.jsp?SKU=87K7011 for a sign I'm making.

Bulb Size: T-1 3/4 (5mm)
LED Color: Yellow
Luminous Intensity: 2.5cd
Viewing Angle: 25°
Forward Current If: 20mA
Forward Voltage: 2V

My question is, what is the maximum number of LEDs I can use?
I know this board has an over current protection of 500mA. http://arduino.cc/en/Main/ArduinoBoardDiecimila
Does that mean I need to stay under 500mA?
I probably only need 21 LEDs, but may need more,so to play it safe, no more than 24 of the LEDs (20mA x 24 = 480mA)?


How will you be powering the LEDs? From a USB port? Dedicated power supply?

My question is, what is the maximum number of LEDs I can use?

With an appropriate power supply: 16 outputs per TLC5940 * 2 TLC5940s * (16 volts per output / 2 volt drop per LED) = 256. You would need a 17V power supply capable of delivering 650mA. (Fair warning: I ignored the TLC5940 power limits!)

The Arduino will be powered by a 9 volt wall wart.
I will power the LEDs from the 5 volts off the Arduino.
How many LEDS can I use then?

Are you planning to have multiple LEDs connected to each TCL5940 output (a "chain" of LEDs)? Or one LED per output?

No chain of LEDs. Just one per TLC5940 output.

500mA is only on the 5V line connected to the USB Port. If you are powering from a DC jack, the current limit is less obvious. It depends on how much heat the regulator generates, which will depend on the Input voltage.

At 9V, you can easily get over 500mA out of it.

Of course, you could also look at using the 9V raw on the LEDs, moving some of the power dissipation to the TLC instead of the Arduino's regulator. That might work better if you were putting more than 1 LED (in series) on each output.

Thanks for the input everyone. I've got it now.