Question about LED resistors

[unrise_lyrical]

So I have picked up a bunch of these buttons for my mbSID:

They have an integrated 1.8mm LED in them which looks pretty cool.

ANYWAY.

I want them to light up as soon as the mbSID is powered up...and remain on regardless of the button's function. I know I will need a resistor before the + of the LED...but what I dont know is what kind/strength of resistor? if someone could fill me in it would be greatly appreciated :)

[Wilba]

The resistor can go on the anode (+) or cathode (-) side, it's limiting the current through both the LED and the resistor.

Great reference site: http://www.kpsec.freeuk.com/components/led.htm

You'll need to check the current limit on the LED, then use Ohm's Law to calculate the required resistor for the voltage.

Most LEDs typically will work under their limit (25mA typically) with a 5V supply and 1K resistor. You might use a lower value resistor for more current, but check the specifications and don't push it to the limit, it's better to use a slightly bigger resistor and not burn out your LEDs.

[unrise_lyrical]

ok, I asked on the forum where I bought them and this was the guy's response:

on the resistors, if they are always on as you say, then you need to know what voltage is coming in and plan on a forward voltage of 2v for Red and green and 3.5 to 4v for Blue and white. also the output is needed so go for around 20ma for regular leds.

I DEFINATELY don't want these burning out....they would be a bitch to replace...I really don't mind if they are fairly dim...I am not going for max brightness.

With that in mind....do you still reccomend 1k resistors? Sorry, I would do the math myself but...its all foreign to me :(

EDIT: I was a bit confused with what that guy mentioned re. different colours. I gather from that that different colour LEDs require different power?  I guess that makes sense as red is on the other spectrum from blue, and the longest wavelength. These are red LEDs by the way :)

Going by the website you linked me to (you are right, very informative!) and the formula on there R = (VS - VL) / I

VS = supply voltage

VL = LED voltage (usually 2V, but 4V for blue and white LEDs)

I = LED current (e.g. 20mA), this must be less than the maximum permitted

Does this sound right?:

R=(5-2)/0.02

R=150

...does that mean i need a 150 resistor? My calculations seem way off compared to the 1k resistor you reccomend? :/

[Wilba]

Yeah 1K is really limiting the current (you can do the math)... but its a fairly safe to use for any LED, since at most it's I=3 Volts/1000 Ohm = 3mA.

Believe it or not, that is a good value for the red LEDs that SmashTV sells, even when used in an 8 column LED matrix!

Most of the time, a 220 Ohm resistor will work well and keep the current under the typical maximum (I=3/220=13mA)

150 Ohm in this case is pushing the LED to the limit, and might even be too bright for this context - remember you're staring at these switches all the time. Note also that all that current adds up... if you have 10 LEDs all drawing 20mA that's 200mA on top of LCD backlight, SIDs, etc.

Try 1K first, if it's too dim, try 560 Ohm (or use two 1K in parallel to make 500 Ohm), still too dim, try 220 Ohm.

[unrise_lyrical]

As I dont have a protoboard to test out different things (And knew I would be driving past jaycar today) I figured I would just pick up some 560 ohm resistors :) seemed like a good tradeoff between min and max mA usage :) will post a sexy pic one they are all soldered in.

[unrise_lyrical]

Just wanted to check if the way I am setting this up is correct. The image is horrid cameraphone quality with even worse drawing skills sorry

So the way i went about it is power is coming from that red lead to the board...then the gray line from it represents a solder train on the back of the board. The 5 resistors send this power into the anode side of the LEDs....but what I wasnt sure about is afterwards with the cathode side... Should I do the same sort of thing and have just 1 lead coming off the board...or 5 separate leads as drawn?

Also, having one wire providing the power that runs to the 5 resistors is ok yea? I dont know much about electricity but once I had soldered it I suddenly wondered if that was ok, or if the first 4 would mean the last 1 in the line may not recieve enough power or something...I have spent more time around cars than circuit boards and was picturing it like a fuel rail supplying juice to the injectors...

[Wilba]

You can connect up the "-" pins on the board, like you show for the "+" pins.

Basically you can put LEDs in parallel, but each LED needs it's own current limiting resistor.

You will (probably) never need to worry about parts being closer to the supply not receiving enough power... just assume one wire connecting things is the same as many wires. It is only when you start dealing with audio circuits that wiring topography has some effect. (I think, I'm no expert on that topic either).