LilyPad how many LEDs per petal?

Other Hardware LEDs and Multiplexing
1

Hi all, newbie here trying to figure out how many LEDs I can run off a lilyPad petal for a small project.

If one LilyPad Micro LED takes 20mA of current, but one petal outputs 40mA, then how does Leah Buechley manage to string 7 LEDs onto one petal in her tutorial?

According to her design, you can string 7 LEDs onto one petal: design: back close-up | Leah Buechley | Flickr

But according to the specs on of the LilyPad Main Board the DC current per I/O Pin is only 40 mA
http://arduino.cc/en/Main/arduinoBoardLilyPad

Assuming that each LED will consume around 20mA of current, how is it possible to string 7 of them together onto one petal?

Unless each LED is eating way less than the 20mA's?

Also, not sure how I would use Ohm's Law to calculate anything here. I know the current (I) as 40mA, and I know the resistance of the thread as 28 ohms per foot (lets say 2 feet are used, so 56 ohms). So I can use Ohm's Law to calc the volatge needed as 2.24 volts. But not sure how that helps me? I plan on using a 3V 250mAh coincell battery to power the lilyPad itself.

2

You're talking about this?
http://web.media.mit.edu/~leah/LilyPad/build/turn_signal_jacket.html
The LEDs are all in parallel.
The LEDs have a voltage drop, say 2V for Red since you using a 3V supply:

The current per LED will then be a max of: (3V -2V)/100 ohm = 10mA
Plus there is the resistance of the thread, and the voltage out of the arduino will be less than 3V since the current is going thru a transistor.
The 28 ohm/LED junction will never be lower than 2V, so max current thru the 28 ohm resistor is (3V -2V)/28 = 35mA, and it gets shared among the LEDs, so 5mA/LED with 7 LEDs. Above 20mA, the Arduino output will droop, so the voltage at the 28 ohm/LED junction will likely be lower, See Section 29 of the datasheet:

VOH Output High Voltage( Note 3). Condition: IOH = -10mA, VCC = 3V
Minimum output at 85C: 2.3V
Sub in 2.3V for 3V above, and the current available drops even more.

If the coin cell was actually good for 250mAH, you would not have very long life from 1 battery.