NerdKits - electronics education for a digital generation

You are not logged in. [log in]

NEW: Learning electronics? Ask your questions on the new Electronics Questions & Answers site hosted by CircuitLab.

Basic Electronics » Which LED?

May 19, 2009
by mcai8sh4
mcai8sh4's Avatar

Sorry about the obscenely basic question, but I'm planning my design for the the finished LED binary clock. After playing with a few ideas for the design, I think I've got a quite a good one (and sadly I'm really excited about it)!

The LEDs I got for my marquee where unbranded, cheap onces with no technical spec on the package. Whilst they worked, I wish I'd have got different ones (not amber, for example).

I live in England and for some reason my local electronics store (Maplin) is very expensive - but since I only need 10 LEDs I think I can streach to their prices.

Basically, what voltage rating would you recommend. I've tried the ones that where supplied with the nerdkit and they seem perfect (the green and red) so does anyone know what they are?

May 20, 2009
by digiassn
digiassn's Avatar

Check Mouser.com. I think they will ship to the UK. I looked on Radio Shack, and they are selling your standard LED for about 1.34. Mouser has them for about .10 a piece (US dollars).

You just need any standard, not high brightness or high power, LED. They range from 1.8 V - 2.6 V max.

May 20, 2009
by mcai8sh4
mcai8sh4's Avatar

So I can use a 2.5V forward (max 5V reverse) led hooked up straight to an output pin, without the need for any resistors. I havn't used any resistors with LEDs up to now, and they seem to work.

Thanks for the help

May 20, 2009
by gzsfrk
gzsfrk's Avatar

I recommend DealExtreme.com for any DIY parts they have in stock that you need. Free worldwide shipping, and the starting prices are even super cheap.

Here's a link to 10 bright-white LEDs for about $1.50 shipped:

http://www.dealextreme.com/details.dx/sku.1105

I buy from DealExtreme all the time, and love'em. Cheap Hong Kong imports are the lifeblood of the poor electronics hobbyist. :)

May 20, 2009
by digiassn
digiassn's Avatar

You should be able to use 5 volts forward biased with no problem. You will have about a 2 volt drop across the led, and the rest across what should be the pull-up resistor in the micro, or any current limiting resistor you put in series.

Check this link, it explains how to figure current limiting resistors and how to read the data sheets: http://www.kpsec.freeuk.com/components/led.htm

May 20, 2009
by mcai8sh4
mcai8sh4's Avatar

@gzsfrk - thanks for the heads up - really good prices! Plus, after a quick look around, there seems to be a load of interesting stuff!

@digiassn - thanks for the advise. That link you've given is really good - just what I need.

Thanks guys!

May 30, 2009
by wayward
wayward's Avatar

Marktech used to have a huge deal on closeout LEDs, 500 pieces for $10 in various wavelengths, even some dual-color ones, but now it seems that only SMD ones remain. I'd still keep an eye out on them.

May 30, 2009
by wayward
wayward's Avatar

No, they still have 3mm and 5mm LEDs; check the menu on the left! They ship to USA only, though.

May 30, 2009
by mcai8sh4
mcai8sh4's Avatar

Those prices are great - shame I can't get them here. :(

I'll keep searching for a cheap (pref free delivery) one - until then I'll just have to keep on buying just the amount I need.

June 02, 2009
by mcai8sh4
mcai8sh4's Avatar

I'm getting (hopfully today) some of these LEDs :

blue

green

Will I bo ok to just connect them to the pins (mA looks high) or do I need resistors?

August 05, 2009
by BobaMosfet
BobaMosfet's Avatar

mcal8sh4--

How much current you can run through an LED depends on what the LED is made of (Gallium Arsenide (GaAs) for example), and what it's forward voltage rating is (which differs by color).

If you look on Wikipedia for LEDs (http://en.wikipedia.org/wiki/LED) you will find a valuable table telling you what the safe voltages are. I usually average the voltages given (like for Yellow it's between 2.10 and 2.18 Volts, so I apply 2.14Volts) and then I only run 20mA through the LED. Sure, many are rated for more, and there are those rated for less. 20mA is usually a pretty safe value that will light it without burning it up or shortening its life.

Whether or not you can connect an LED directly to your MCU depends on whether or not that pin is designed to sink or source the appropriate amount of current to drive an LED. Not enough and it won't come on. Too much, and you'll need to use a resistor to lower it. Just right, and it will work, without any additional resistor.

IMHo, I will avoid futurlec.com in the future. Some people have great reviews, but the majority I have read about have negative experiences similar to my own: I've been waiting weeks for a $100+ shipment that still hasn't come and no courier (Tawan, Austrailia, or US) even recognizes the tracking number they gave me.

August 05, 2009
by mrobbins
(NerdKits Staff)

mrobbins's Avatar

Hi there,

See also this thread about whether a series resistor is needed.

Also, a good vendor for LEDs we've found recently is abcTronics -- ultrabright red or amber LEDs for $3.99/100, blue for $9.99/100, and green or white for $10.99/100. We've had very positive experiences working with them for our LED Marquee project. They do appear to ship internationally if that is a concern.

Mike

August 07, 2009
by mongo
mongo's Avatar

Mike, Thanx for posting this company! I bookmarked the site. And to think, I remember when you could get any color you wanted, as long as it was red.

One question: Are these bright LED's? or just average?

DF

August 10, 2009
by BobaMosfet
BobaMosfet's Avatar

Mike, thanks for the heads up on the LED Vendor. I remember when these things were $10per (yes, $10 for one LED), so it's awesome to be able to afford them in reasonable quantities for awesome projects.

Post a Reply

Please log in to post a reply.

Did you know that a motor's no-load current at a given voltage is much less than it's resistance would suggest? Learn more...