# How many LEDs on lilypad

Hello,
I tried searching for the answer to my question to no avail. How many LEDs can I hook up to a single lilypad board? Leah Buechley seems to suggest around a hundred are possible on this site: http://web.media.mit.edu/~leah/grad_work/diy/diy_tank.html. Also this project seems to suggest a lot are possible: http://lilypadarduino.org/?p=1514. Most of the answers concerning this question on the forums point toward charlieplexing, which I'm not interested in. For my project I'd like to hook up two sets of fifty LEDs that fade in and out. Is this possible?

For my project I'd like to hook up two sets of fifty LEDs that fade in and out. Is this possible?

The number of LEDs in the set is irrelevant, if all the Arduino is doing is controlling the brightness.

If you expect the Arduino to provide power, too, the number is far less than 50 in a set.

Yes I was hoping to power all the LEDs off the Arduino, but I suppose 20ma is too little power… So I presume I'd have to use some sort of transistor and external power supply? How do I get started figuring this out? Are there any tutorials? My application will be on a garment.
Here's a post of someone talking about hooking up 64 LEDs to her lilypad… Login
I'm a little confused as there seems to be contradictory information out there.

I just came out with this breakout board with a MAX7219, which can control 64 LEDs.
Connect 2 in series control up to 128 LEDs. Each has 8 registers which control 8 LEDs each.
I show it with female headers, and a IC socket. You could just as easily leave out the socket and solder wires leading out to your LEDs to make it lower height.
No resistors needed, the MAX7219 takes care of the multiplexing at 800 Hz and controls the current to the LEDs with a single resistor setting the max current. 15 levels of brightness control are available.
I have a short sketch that I will send out (& post when I get a chance) showing the setup and then writing to the 8 data registers.

Only 8 LEDs are on at time, so two boards would need 16 * 0.02mA = 320mA. Not a problem for an Uno, not sure what regulator is on a Lilypad (or which Lilypad, there are quite a few variations).

That is an interesting solution that may work for the garment I'm attempting to construct. On the other hand my fantasy is to glue surface mount LEDs directly onto a person's skin and paint the circuits on with a home-brew conductive body paint. Is there any way to control two sets of fifty LEDs using just four pins from an arduino?

Individually? Only if you have a chip that controls multiple LEDs.
MAX7219 needs SCK, MOSI, and SS (or use the slower shiftOut equivalent).

One big long string - sure, but need 100V from end to end and transistor to switch it on & off. Zap!

I don't need to control the LEDs individually. Just two rails of fifty each connected in parallel. I found this schematic Arduino Playground - HomePage
On this thread: http://forum.arduino.cc/index.php/topic,15683.0.html
On the third response the poster suggests it's possible to control fifty LEDs at once using a TIP102 with a small heatsink. The schematic shows a 5v rail, which I would feel comfortable running on someone's skin, but are you suggesting that the amount of LEDs I want to light up would require too much voltage to be safe? These would be surface mount 1206 LEDs.

You will be severely limited using conductive paint. It isn't that conductive, you see.

50 in parallel need 50 x 0.02A = 1A of current. 2 strings, 2A That's going to take one big battery.
That will likely vaporize conductive paint too.
Can you run from 12V battery, strings of 4 or 5 LEDS? Will drop your current to 1/4 or 1/5 of an Amp.
Then one Logic Level, Low Rds, N-channel MOSFET to switch all the strings in parallel on & off.