LED cutoff point?

Using Arduino LEDs and Multiplexing
#1

I just got a 6 x 10 each pack of colored clear LEDs from CLEDs and was wanting to test them for color and brightness.

I loaded the blink sketch and tied GND and Pin 13 to the power rails on my breadboard. I stuck in a red LED and it flashed on and off and was very bright. I then also stuck in a blue LED but it remained off. I pulled the red LED and the blue one started blinking. I put the red one back in and the blue one stayed off.

Did I reach some sort of cut-off point where the blue LED just refused to start because not enough current?

#2

First off you must use a resistor in series with the LED. Pin 13 dose not contain a built in resistor despite what some web pages say.

Second use one resistor / LED combination. Do not connect LEDs directly in parallel.

Read this:- http://www.thebox.myzen.co.uk/Tutorial/LEDs.html

Each LED has a forward voltage drop and when you have a red LED in then there is not enough VOLTAGE (not current) to drive the blue LED.

#3

stonent: Did I reach some sort of cut-off point where the blue LED just refused to start because not enough current?

The red LED is lower voltage so all the current went that way (path of least resistance).

However, you're doing things hopelessly wrong and will end up burning LEDs, and possibly even the Arduino.

Electronic components can die. It's a good idea to learn something before connecting random parts together.

#4

I then also stuck in a blue LED but it remained off. I pulled the red LED and the blue one started blinking.

The red leds have lower Vfwd than the blue leds so the blue leds never got a chance to light up when paralleled with the red ones.

#5

Yep. The voltage drop of a Red LED is around 2V, a blue/white one 3,6V. You cannot parallel them without a resistor infront of each of them!

You will destroy the Arduino and the LED's if you are not using resistors

Resistor for a Red/Green/Yellow LED on the Arduino is 150 Ohm (5V supply - Vdrop (2V) = 3V / 0,02 (20 mA) = 150 Ohm)

Resistor for a Blue/white LED on the Arduino is 70 Ohm (5V supply - Vdrop (3,6V) = 1,4V / 0,02 (20 mA) = 70 Ohm) - Closest standard value is 68 Ohm

// Per.

#6

Might explain why my yellow LED lit up orange.

#7

Well there were no specs or instructions with this. It was 6 bags of 10 LEDs each and 60 resistors all of the same value.
Since they were all the same value I assumed all the LEDs ran at the same voltage.

#8

As a test. I put the yellow LED on the 3.3v line, it was orange. I touched it ot the 5v line and it was red.

#9

fungus:

Zapro: And yes, it turns orange just before the magic smoke comes out ;-)

Did you just burn a breadboard to prove a point about LED color? I like the cut of your jib. PS: Crocodile clips... :)

The breadboard is fine, there never was much current running flowing.

The LED and Resistor died though ;-)

// Per.

#10

Seems some magis smoke came out of the limiting resistor too! Must have had "limited" capability.

#11

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#12

That's why it is a limited "limiting" resistor.

#13

Zapro:

fungus:

Zapro: And yes, it turns orange just before the magic smoke comes out ;-)

Did you just burn a breadboard to prove a point about LED color? I like the cut of your jib. PS: Crocodile clips... :)

The breadboard is fine, there never was much current running flowing.

The LED and Resistor died though ;-)

// Per.

The most interesting part was that both parts died, not just one or the other. We assume that 1 part fails, opening the circuit, and the other parts in the circuit are are spared, but clearly, in your demonstration both parts failed.

FWIW, I have LEDs running w/out resistors, and they have been running for months without failure. I had originally assumed that the LEDs or the arduino would fail, but not both. I keep waiting for the cube to die, but its been a few months now, and they just keep working. I wonder how long it will take to break 192 LED running 24/7 with one arduino and no resistors. What will break first the LEDs or the arduino? who knows maybe both!

#14