Beginner Question About LED's

Forum 2005-2010 (read only) Hardware Interfacing
1

Now i am trying to use an LED on a pin other than 13,

I goggled but come up with two different connections

#1 -> uses pin10 connected to resistor (330 ohm) connected to + of the LED and - of the LED is connected to ground.

#2 -> uses pin10 connected + of the LED - of the LED is connected to the resistor and it is connected to the ground.

From my limited electronics knowledge #1 must be the correct one we decrease +5v to not damage the LED. Am i wrong here or are both these setups correct?

Thanks.

2

So to put it simple, the resistor is swapped from the cathode to the anode of the led. I dont see any issue with either of the two.

You can easily test these by modifying the pin number on the blink example. Also check out the pull-up and down examples in the playground.

Hope this helps.

3

for a basic led setup, it does not matter which side the resistor is connected to, its still restricting the flow of electricity from point A to B

and therefore limiting how much power the led has to consume

4

OK so my misconception is it does not matter which side we put the resistor it will still reduce the voltage on the whole setup.

I was thinking, i need to reduce the voltage before it hits the LED, that's why it seemed weird to connect the resistor after the LED.

So i should think of the more like a water pipe no matter where you slow it it will slow down the whole flow.

5

pretty much, but focus more on current and the voltage difference being a side effect

6

The consideration should be on whether you are using the I/O pin to 'sink' or 'source' the current to drive the LED.

If you are using the I/O pin to source(ie. where you connect the + to the I/O pin and - to ground), then you don't need a current limiting resistor in series with the LED, because the I/O pin has a pull-up resistor inside the atmel chip, and protection diodes for kick-back.

If you are using the I/O pin to sink(ie. where you connect LED + to VCC and - to the I/O pin), then you need a current limiting resistor, because the I/O pin can only sink so much current before you blow the channel's driver.

7 
If you are using the I/O pin to source(ie. where you connect
 the + to the I/O pin and - to ground), then you don't need a current
 limiting resistor in series with the LED, because the I/O pin has a pull-up
 resistor inside the atmel chip, and protection diodes for kick-back.

That is flat wrong. No matter if you are sinking or sourcing current from an Arduino digital output pin, you need an external current limiting resistor when driving a LED.

The software enabled optional pull-up resistor is a feature used for digital input pins only , nothing to do with digital pins that are set up to be output pins.

Lefty

8

That is flat wrong. No matter if you are sinking or sourcing current from an Arduino digital output pin, you need an external current limiting resistor when driving a LED.

The software enabled optional pull-up resistor is a feature used for digital input pins only , nothing to do with digital pins that are set up to be output pins.

Lefty

Well, I stand corrected. My apologies. I should have read the data sheet more carefully.