Go Down

Topic: LilyPad how many LEDs per petal? (Read 7313 times) previous topic - next topic


(moved this post from the Projects section)

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: http://www.flickr.com/photos/leahbuechley/2596552004

But according to the specs on of the LilyPad Main Board the DC current per I/O Pin is only 40 mA

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. 


You're talking about this?
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.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

Go Up

Please enter a valid email to subscribe

Confirm your email address

We need to confirm your email address.
To complete the subscription, please click the link in the email we just sent you.

Thank you for subscribing!

via Egeo 16
Torino, 10131