Modding LED Brightness

This may seem like a funny questions but can someone help me choose an appropriate LED brightness assuming the LED is run at the typical voltage on the specs.

I have a bunch of superbright blue leds - 2000+ mcd. I even have some around 500mcd. But all are too bright for what I need. I plan to use them for a HD VU Meter and would prefer I didn't go blind or see spots when looking at them. I also need some for a keyboard led mod. I'm not sure what is too low of a mcd value.

I had thought 500mcd blue leds wouldn't be overly bright but apparently I still see spots if I look directly at them.

If anyone can give me some good ranges to go for that won't be obxiously bright - that would be great!

 

You're talking about the resistor values right?

 

No I'm talking about the actual rated brightness of the LED. Yes, I would decrease the brightness with a higher resistor but to a point it wouldn't work if there isn't enough forward voltage getting to the LED.

I mean the actual rated brightness of the LED - in MCD

 

I use 650mcd LEDs in my keyboard and on the front of my case and they are fine to look at. Although, none of them are just open, they all have some kind of plastic diffuser in front.

 

Exactly - looking at my 500mcd blue leds bare, it hurts. Funny thing is the 2000mcd didn't seem that much brighter - maybe i couldn't see at that point - i was seeing spots soon after.

 

why don't you put in a resistor to reduce the brightness? or even a potmeter.. then you can adjust the brightness...

 

Like I said above, its not that simple. Yes I could do that under normal circumstances but only to a certain point. If the voltage is not enough due to the resistance being so high, it will not light up. Plus for a keyboard mod, there isn't much room for resistors and for the HD VU Meter - using Macromans complex circuit, there are no direct led resistors in line so it would be complicated.

 

Not sure if something like 80mcd, 16deg viewable, water clear would be bright enough or still too bright?

 

umm... just put in a resistor small enough for it to light... but not fully... as for the keyboard: normally there are already resistors in there... just change them... on the keyboard i have the led don't get mote than about 10mA with the standard res...

 

The thing is - the circuit is complicated and theres no direct place I know to place an extra resistor to limit the voltage. This is for the HD VU Meter.

Can't someone just recommend a lower mcd rating? That would be must easier. Just not sure what is low enough to too low. This will be for indicator lights - 10 of them.

 

The brightness usually used for panel displays is around 30mcd.

 

I'd go the resistor route myself also. They will actually go quite dim before they go out.

 

Well, I have some at about 2 mcd, usefull in low light conditions, but in bright daylight you can't see if they are on

My bargraph blocks are at 3 mcd, mostly ok, but a bit too dim.

The feet on my shuttle are illuminated with yellow/green bi-colur LEDs at 15/10 mcd, a nice value

So, my advise:
<10 - Ok, for most, not good in bright daylight
10-50 - Ok
>50 - Getting brighter...
>1000 - Bright
>10000 - VERY bright (got some of those )

 

The other problem is that the human eye is not linear.

A 2500mcd Near UV LED looks like a dimm purple.
A 500mcd Blue looks brighter then a 2000mcd RED

 

Oh, and the spread is just as important. It's a big difference looking straight on a LED with 5 degree spread vs 45 degree spread..

 

nope... i think the brightness (in candela) is how much light per square 'something' you have..

but if you have the same chip i two different LEDS (different angle) you will get the light more consentrated so that you get more light per square cm... for instance..

 

And good thing too, or I'd have the wrong avatar.

First misconception to correct: it's more useful to talk about limiting the current delivered to the LED than about driving them at a lower voltage.

Okay tok3n, the thing you're building *does* have a feature to limit the current to all the LEDs with a single resistor, since it's based on the LM3914.

Quoting from page 7 of the excellent datasheet

Macro's voltage divider that sets the reference voltage is made up of R7 and VR3. So the total resistance between pin 7 and ground can change, but the datasheet tells us that the LED current is approximately 1/10 of that. I'd try substituting a larger resistor for R7 right off the bat. No harm will come of that.

 

I'll share a though I had a little while ago conncerining brightness control. Would a current mirror configuration with a pot to adjust the reference current be a good idea?

 

There's a mini LM317-like circuit inside the LM3904 generating the reference voltage. So you treat the voltage at pin7 like a reference voltage (that's how they refer to it in the datasheet) and program your current by changing the resistor.

 

For PC modding, 3000mcd is fine, just to light up a case, you would need around 5-7 LEDs some pointing up/down from different parts of the case.

If you want brighter, like me (I love light), you can go with 6000mcd LEDs, they are more money however, you can use less LEDs, or more.. hehehe, and have your system pretty bright.

Or. you can always use the green ones I have that are true green and are 16,000mcd, if you ever have seen the stop lights that look dotty, not one bulb, those are them. So if 50 can light up an interestion, heheheh, imagine what you can do in your PC. I use them in my car, have them on a dimmer, nice alt. from neon, and it looks like a million bucks.

So, recap. 3000mcd is good for cases, 6000mcd better if you want it bright. The LEDs that are in microsoft's optical mice are 2500mcd, just if you need to see how bright they are.

Yeah, so I don't know if that helped or whatever, cheers.

-emu