LEDs of different intensity need to be the same brightness

[pomprocker]

Hi all,

I have a need for 8 Red LEDs

I have 8 Red LEDs, but they are different intensities

The dimmer 4 are from Radio Shack:
28mA (max)
Typical Voltage is 2.25, with a maximum voltage of 2.6V
Typical MCD is 5.0
http://www.radioshack.com/product/index.jsp?productId=2062552&tab=summary

The brighter 4 are from Sparkfun.com
I have no tech specs on them
http://www.sparkfun.com/commerce/product_info.php?products_id=533


I would like to make them around the same brightness if possible.

The voltage supplied will be either 2.4(if using rechargeable batteries) and 3v (alkaline - i expect this most of the time)

So what series resistor should I use with the bright one, and what resistor with the dim one to get them to equal in brightness

[Soeren]

Hi,

First off... You should get 8 new LED's of the same type!
Mixing different LED's where you want the same look and feel is a bad idea and buying 8 LED's won't ruin you ($1.49 is hilarious though and 50 cents are still way too expensive).

LED's 50 Bright Red 3mm 1.9-2.5 Volts 30mA for $5.99
http://cgi.ebay.com/LEDs-50-Bright-Red-3mm-1-9-2-5-Volts-30mA_W0QQitemZ300259652126QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item300259652126&_trksid=p3286.c0.m14&_trkparms=72%3A1234%7C66%3A4%7C65%3A12%7C39%3A1%7C240%3A1318%7C301%3A1%7C293%3A1%7C294%3A200

1000 red diffused 3 mm LED's for $10.99
http://www.besthongkong.com/product_info.php?manufacturers_id=&products_id=30


A rating in mcd (millicandela) is worth nada without the angle of half intensity. -Theoretically, you could make any LED give eg. 100,000 mcd, by simply reducing their angle of half intensity.
To get a comparable measure, you need both the mcd and the angle to convert to lm (Lumen), then use the lm to see how bright any LED actually is.

To get an understanding of the concept, use a magnifying glass and focus/defocus the image of the sun - the smaller the spot (i.e. angle), the brighter it will be.

[pomprocker]

Yeah radio shack and sparkfun are both too expensive for components.

This is for a hand held POV, so you're right, I should get 8 matching LEDs

I think I will drive up to this bigger electronics shop that has a better selection.

What LED and resistor specs to you recommend to get as bright as possible with 3v supply, 2v fwd drop, and 20mA?

[HDL_CinC_Dragon]

depends on the LED really. Typically the LEDs that take the most voltage will be the brightest. Since the LED packages typically give you the voltage and current rating for the LED you can very easily calculate the resistor value.

[Soeren]

Hi,

What LED and resistor specs to you recommend to get as bright as possible with 3v supply, 2v fwd drop, and 20mA?

Finding the resistor is purely Ohms Law, so I won't insult you by giving you an answer on that one

The LED should of course be the one fitting the purpose and having the highest intensity (in lm).
You need to decide which angle will suit your purpose best and then go from there.

To get a higher intensity, just up the current a bit, you don't wanna play with a POV toy for 100,000 hours anyway. Don't go much above around 50mA for one nominated for 20mA though, unless you're willing to sacrifice a few experimenting.

Those little key ring lamps with either 2 of CR2032 or 3 to 4 of AG13 or similar relies on the fact that you won't be using it for more than perhaps 10 to 100 hours during its lifetime, so they are heavily overdriven to get useable brightness.

[pomprocker]

arrrrrrrg


so i have 3mm red leds from radio shack, sparkfun, and my local electronics shop. none of em really came with specs, but the sparkfun ones are the best!   what are theyyyyy   they don't have the specs on them either cause i emailed them   

[HDL_CinC_Dragon]

I would assume 3v at 20mA for the LEDs... buuuut not really sure what your really asking here lol

[pomprocker]

I want more of the ones that sparkfun sells, but I want to get them from somewhere cheaper and not pay $0.50 each for them :\

[HDL_CinC_Dragon]

well what do you mean by theyre the best? Do you mean you ran the same current and voltage through them and the sparkfun ones were brighter?

[pomprocker]

Yeah I breadboarded all my different red LEDs, and figured out which resistor worked best 47ohm gave me 25mA current with a 3v supply. Then i did a persistance of vision test my waving the breadboard around in the dark LOL

the sparkfun ones were the brightest, diffused, and had the best viewing angle.

The second best were the NTE super bright deep red 3mm led, but its not diffused.

I think i might try sandpapering one of them tonight and see if that makes it work as well as the sparkfun LED for the purpose of a POV toy.

[HDL_CinC_Dragon]

You gave each LED its own resistor right? Cuz if you just put em all in parallel with just one resistor supplying them all then they werent all getting the same power

[pomprocker]

of course they all have their own resistor

[airman00]

Cuz if you just put em all in parallel with just one resistor supplying them all then they werent all getting the same power

Side topic: But if they are exactly the same specs and same LED , it would be fine, correct?

[pomprocker]

airman: Consult the wizard

http://led.linear1.org/led.wiz

[HDL_CinC_Dragon]

Side topic: But if they are exactly the same specs and same LED , it would be fine, correct?

no, if theyre in parallel they all need their own resistor. As admin had pointed out to me in a post somewhere on here, the LEDs will have extremely tiny internal resistance differences but just enough that if 2 LEDs were in parallel with the same source resistor then one LED could be taking 10 times the current as the other one. When in series it does matter

[pomprocker]

problem solved, i filed down the epoxy on the nte super brights with an emory board

[HDL_CinC_Dragon]

hahaha emory boards are freakin great, I have one on my workbench as well