So I have a LilyPad prototyping kit I’ve been messing with and I started snapping things off to try a few things out.
I’ve seen a couple posts about hooking up LEDs in parallel but they all sort of seem conflicting. There’s a large comment on the main page of the LilyPad indicating that you could hook up a whole bunch of them and they heard it straight from Leah!!!
But there’s also a couple posts I’ve seen on here that recommend against any more then 2… which seems to make sense to me given… 3.3 V across a 100 ohm resistor with a max 40mA current draw on the pins…
Given those numbers it seems like you should only be able to do 1 LED since that figures out to a 33mA draw right?
Well I hooked up 3 LEDS in parallel. Stuck a multimeter in there to complete the circuit between the positive side of the LEDS and pin 5… And the little needle only goes up to about 30 mA. Which seems like there’s even a little more head room 
I ordered a whole bunch of stuff today so I can start messing around with this more completely … But does that all seem sound? It seems like I’ll be fine hooking up 3 and a couple more LEDs on one pin…
You shouldn’t exceed the 40mA current per pin, or the maximum total current of 200mA for the ATMega chip.
If you supply more than 1 LED in parallel from the same resistor, the total current will essentially be the same as if you’d just used 1 LED, since the voltage across LEDs doesn’t change much with changes in current. If you want the total current to increase you’ll need to decrease the resistor vaue, but watch the max current figures mentioned above.
Some people say paralleling LEDs isn’t a good idea, but if they are the same type then they tend to share current pretty well.
Well I’m using the lilypad LEDs which have a 100 ohm resistor each
https://www.sparkfun.com/products/10081
So shouldn’t that cause the current to double with each LED/resistor combination that gets added in parallel?
(R1R2)/(R1 + R2)
(100*100)/(200) = 50
3.3/50 = 66mA
PureWeen:
Well I’m using the lilypad LEDs which have a 100 ohm resistor each
https://www.sparkfun.com/products/10081
So shouldn’t that cause the current to double with each LED/resistor combination that gets added in parallel?
(R1R2)/(R1 + R2)
(100*100)/(200) = 50
3.3/50 = 66mA
Your thinking is basically correct but the calculations are a bit off because you're not accounting for the LED properly. The LED, like all diodes, has a voltage drop that's largely independent of the current flowing through it. You can see what that drop is vs current from the datasheet. For the moment let's assume there will be 20 mA and that would mean a 3.3V drop. Let me further assume you're running the LP off a 5V source and so the current would be ...
(5v - 3.3v)/100ohms = 17mA ; pretty close to the original estimate of 20mA. So there will be slightly less current in real life than this 20 mA, which will result in a slightly lower LED drop which will tend to increase the current so … as you can see there’s an iterative process here and you have to decide when it’s close enough.
The above all said when you parallel the LED+resistor you should get 2x the current. If you haven’t my guess is that you’re running the LP off a lower voltage than my assumed 5V above. If you’re using 3.3V then you’re operating the LED(s) in a very non-linear region where a little current difference can encompass a wide range of voltages. Nothing wrong with this (though the LEDs won’t be very bright) but it may be that 1 LED+resistor is not much like another in this region. It’s also to be noted that the output pin won’t give you quite the input voltage and has some finite output resistance as well.
(FWIW: from the datasheet I’d estimate running at 3.3v gets you 5mA through the “LED” with a 2.8V drop across the LED proper)
http://www.sparkfun.com/datasheets/DevT … 6QWF-D.pdf
Awesome Everything you said is mostly what I’m seeing… Thank you for the detail… I figured I was missing something with the LEDs but I wasn’t quite sure how to account for those in the system…
And I’m pretty sure it is a 5V source because all my tests were done while plugged into the FTDI breakout into USB and (AFAIK) that will act as a 5V source
I’m not quite seeing a 2x current as I add LEDs in to the array though … My current configuration is a little hacked together so it’s possible I’m doing something dumb. I ordered a bunch of stuff and it should be here today or tomorrow so I’ll be able to get a better setup together to vet more of my numbers.
The first LED was at about 17mA but after that I was only really seeing like 5mA or so increases in current as I added on LEDs and they were all plenty bright.
Thanks again for all the info that definitely gets me on the track I knew was derailed from