Powering 128 leds

Other Hardware General Electronics
1

First, hello, I am new here
I'm Aruban, web developer living in Tokyo, Japan.

I've been working on my free time with a small collective on different projects, and recently, one of them bring me to electronics.
The project isn't very original, and will certainly sound boring for most of you :wink: We are building objects where we put leds that react to sound...
This is the very first time I touch to something related to electronics... Obviously, the hardware and the theory is a totally new thing for me.

As a test, I build a small "frame" made of 33 pixels of 2 leds each.
this is the schematic of the circuit I came up with : Skitch | Evernote
this image is a bit small, but to explain it briefly:

  • 1 Arduino duelmilanove
  • 4x 74HC595 shift registers to drive 32 pixels
  • last pixel driven pin pin #13
  • each pixel is a 200ohm resistor and 2 white leds (dunno much about the leds, white 3V LLED-W501)
  • I have a 0.1uF capacitor at each Vcc and Gnd of the registers
  • Sound input plugged to the analog pin 0 (via RCA, L and R regrouped )
  • a potentiometer to adjust the sensibility on analog pin 5

The arduino code to drive this: #include <SPI.h>#include <ShiftOutX.h>#include <ShiftPinNo.h>/* * LED - Pastebin.com

I builded this mainly with ressources I found on this forum :slight_smile:

The circuit I made worked OK so far, and I could kept it running non-stop during a 9H event, without burning anything :slight_smile:
It was powered via the arduino USB plug, connected to a wall USB adapter (that one: http://goo.gl/hkYxj)

this is a small video showing the thing running (it can help to understand the code)
http://vimeo.com/38356233

As I said, this was just a prototype, a small thing to see if it's possible or not. We need now to scale this up for the primary project.
I know this is not a forum good practice, but I have 2 "questions", maybe someone can help me :

1/ When I plugged a 12V DC 2A adapter (this Skitch | Evernote) to the arduino power plug, the arduino got VERY warm quickly, and nothing worked (leds were all blinking super fast with low intensity). When I plugged it with a USB cable to a USB adapter, it worked perfectly. Can someone explain me why ?

2/ How should I power this similar circuit, but with 16 registers and 128 led (only one led per "pixel" this time) will the USB be OK for 128 leds too ?
Would it change anything to have 2x 8 registers (using 6 pins) instead of using 1x 16 registers (using 3 pins) ?

Hope my questions aren't too annoying... I somehow have the feeling I'm asking for simple solutions to complexe questions, if at least somebody could drive me to some ressources that can help me to understand, it would be awesome :slight_smile: !

A

2

I suspect that power supply is not regulated, so that the "12V" could mean anything from 10V to 14V at full load and the no-load voltage may be 20V or more (many cheap power supplies are like this). You need to measure its output voltage. Even if its performing OK 12V is a bit on the high side to power the Arduino - its regulator will get quite hot (and you can't run any significant loads from the 5V out without heating the regulator up even more).

Running all those LEDs off the 5V pin from the Arduino will probably be a no-no - with a 9V input to Vin I'd suggest no more than 200mA out of 5V pin - any more and an external regulator on a heatsink is required.

If your supply is 12V you are going to have to dump a lot of heat somewhere - a regulated 5V supply would suit your circuit far better.

3

As a test, I build a small "frame" made of 33 pixels of 2 leds each.
this is the schematic of the circuit I came up with : Best Note Taking App - Organize Your Notes with Evernote

That schematic will not work. The chips have to be connected directly to +5V and ground not through a capacitor. The capacitors should go across the supply tp each chip. That is one end to +5V and the other end of the capacitor to ground.

4

Sorry for my late follow up...

MarkT:
a regulated 5V supply would suit your circuit far better.

I am now reading about regulated supply, another new concept for me. Would using chained 9V batteries work as a regulated source of energy ?

MarkT:
If your supply is 12V you are going to have to dump a lot of heat somewhere

Is there any good practice or method to do so ?

Grumpy_Mike:
The chips have to be connected directly to +5V and ground not through a capacitor. The capacitors should go across the supply tp each chip. That is one end to +5V and the other end of the capacitor to ground.

You mean like this: Best Note Taking App - Organize Your Notes with Evernote ?
Just read your article about de-coupling (http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html) I think I'll have to read it another 10 times, but it's very instructive.

Thanks you so much for your time !

5

aruban:

MarkT:
a regulated 5V supply would suit your circuit far better.

I am now reading about regulated supply, another new concept for me. Would using chained 9V batteries work as a regulated source of energy ?

The voltage in the battery will drop over time, and I suspect it would not last very long powering that many LED's

aruban:

MarkT:
If your supply is 12V you are going to have to dump a lot of heat somewhere

Is there any good practice or method to do so ?

A heatsink and/or fan

aruban:

Grumpy_Mike:
The chips have to be connected directly to +5V and ground not through a capacitor. The capacitors should go across the supply tp each chip. That is one end to +5V and the other end of the capacitor to ground.

You mean like this: Best Note Taking App - Organize Your Notes with Evernote ?
Just read your article about de-coupling (http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html) I think I'll have to read it another 10 times, but it's very instructive.

Yes, but I think you're supposed to keep the caps as close to the power pins as possible.