LED Wooden Sign

[Saimaster13]

Alright, so I have this wooden sign with a bunch of LEDs on it. All the LEDs are hooked up in parallel to the same type of LEDs, red is hooked up to red, green to green, orange to orange etc. I am having a real problem trying to get them all to light up to an acceptable brightness level off of a 9v. Is there a way to do this? Do I need more voltage and amperage? Would I have to deal with each LED individually or at least separate some?

note: The picture is running off of 12v 2A, but I still would like it to be brighter than it is (it only looks that bright since it was dark.)

I also have a video: Video of project at night

[Saimaster13]

Ended up finding out my own solution.

I connected all the anodes together, and all of the cathodes of the same color. Then I connected V+ to the anodes and resistors of the same value to each cathode. I then soldered all the resistors together and connected them to ground.


Why (I believe that) it works:

Before I had hooked up all the anodes to anodes and cathodes to cathodes. When power was applied, only one group of LEDs would light up. I believe this method had two problems that were solved by using one resistor for each group.

The first problem was that the LED groups took different amount of current. Some required more and had an increased resistance while some required less and had less resistance. But there was not enough current to go around. The current went the path of least resistance and therefore only lit up one group of LEDs. Using multiple resistors fixed this by making the resistance path to the LEDs almost the same. Before, the resistance of each LED played a major role in the total resistance, but when adding hundreds of ohms to the groups, the LED resistances aren't very important.

The second problem was the lack of current. There was not enough current to power all of the LEDs at once. With the added resistors, the amount of current that was going to all the branches before is now going to each group of LEDs, effectively sextupling (in my case) the amount of current going through all the LEDs, providing enough to light up each LED.

[brad]

Good job! Here's the reason that the LED's need their own resistors (If you want them all to be the same brightness - you actually need one resistor per LED)

Different coloured LED's have different forward voltage drops. for example a red LED might have a forward voltage drop of 2v whereas green or blue might be 2.5v

If you don't give an LED it's forward voltage drop - it won't turn on. Also, because an LED is a diode - once it has reached it's forward voltage drop - it will not drop any more voltage.

So if you have a red and green LED in parallel - only the red one will turn on because the red one will be fixed at 2v whereas the green one cannot turn on because it needs 2.5v

Connecting them like this will make sure that all LED's will turn on. Also, notice how the resistor in series with the LED's can have a different voltage drop (due to the LED's forward voltage drop) if you want them to have the same amount of current, you need to select different values of resistance to give them the required current.

screen-capture-10.png (12.73 KiB) Viewed 6845 times

for example let's say the RED led had a forward voltage drop of 2v, which leaves 10v for the resistor. if we want 20mA flowing through this LED then 10v / 20mA = 500 ohms. Now to make the green LED have the same current we need to apply the same principle:

2.5v on the LED, therefor 9.5v on the resistor so: 9.5v / 20mA = 475 ohms.

Hope that helps!