LED question

[Randall]

While putting a true bypass switch in a friend's Cry Baby, since I had extra terminals on the switch, I added a LED on/off indicator. I had run out of my supply of LEDS, so I pulled one out of an old bike light. This light has 3 super bright red LEDs, some kind of chip under a glop to make it blink, etc., and is powered by two AAA batteries. It didn't occur to me until it was finished that maybe I should have considered that it is now being powered by 9v, with no others to share the load. I currently have (see what I did there?) it in series with a 1.5K resistor. It has been on for an hour now with no trouble, but is it OK like this, or do I need a bigger resistor. And just how would I know?

[eschertron]

What's the voltage drop across the resistor? Since the resistor and the diode are in series, they must pass the same current. Calculate the current through the diode using the 1.5k resistor voltage drop. Then determine how much current you really need. Adjust dropping resistor as desired.

[The Dude]

Since you can't know LED specs (robbed form a bike light), it's anybody's guess. I'd start with a large resistor and work my way down in value until the LED was bright enough for your purpose. This will be the least likely to harm the LED and use the least amount of battery.

[Leo_Gnardo]

Since you can't know LED specs (robbed form a bike light), it's anybody's guess. I'd start with a large resistor and work my way down in value until the LED was bright enough for your purpose. This will be the least likely to harm the LED and use the least amount of battery.

This ^^^ . And those super brights are way efficient, you'll be burning your retinas I'm sure with just a couple milliamps. Some local musos have asked me to put them in pedals so they can see them in daylight. On a reasonably dark stage, they shoot a beam of color light onto the ceiling - woooo, psychedelic !

Back in the early 90's when super brights just became available, I swapped out the LED's in my Peterson strobe tuner. Now plenty visible in daylight without having to use an annoying makeshift cardboard cone gaffer taped over the display. H'ray for science!

[nosaj]

While putting a true bypass switch in a friend's Cry Baby, since I had extra terminals on the switch, I added a LED on/off indicator. I had run out of my supply of LEDS, so I pulled one out of an old bike light. This light has 3 super bright red LEDs, some kind of chip under a glop to make it blink, etc., and is powered by two AAA batteries. It didn't occur to me until it was finished that maybe I should have considered that it is now being powered by 9v, with no others to share the load. I currently have (see what I did there?) it in series with a 1.5K resistor. It has been on for an hour now with no trouble, but is it OK like this, or do I need a bigger resistor. And just how would I know?

Take a pot wire it up to the resistor and the power source, rotate the pot till you get desired brightness( since you might not like being blasted by a bright light on a dark stage(kind might have you seeing spots) Measure the ohms of the pot setting and there you have the resistor size you want. At that point you can figure current draw.

nosaj

[The Dude]

This ^^^ . And those super brights are way efficient, you'll be burning your retinas I'm sure with just a couple milliamps. Some local musos have asked me to put them in pedals so they can see them in daylight. On a reasonably dark stage, they shoot a beam of color light onto the ceiling - woooo, psychedelic !

Back in the early 90's when super brights just became available, I swapped out the LED's in my Peterson strobe tuner. Now plenty visible in daylight without having to use an annoying makeshift cardboard cone gaffer taped over the display. H'ray for science!

Another thing you can do to make LED's more visible in sunlight is to switch to blue led's. Red is much harder to see in direct sunlight. I used to work for a company that built digital dashes for street rods. They used vacuum fluorescent displays and you could choose your lens color. The sales people always tried to steer people away from red lenses because red is difficult to see in sunlight. But, there's that guy who says, " I have a red car- I want red lenses". More often than not they'd return the cluster and have the lense color changed.

[Randall]

The voltage drop across the 1500R is 10.8v, so 10.8/1500 = 0.0072, or 7.2mA. That seems relatively safe, doesn't it?

Or does it? I put in a 3K resistor for half the current at 3.6mA. Not as super bright, but probably safer.

[The Dude]

IMO, figuring current is pointless since you don't have LED specs. There are LED's rated from 1mA to 70+mA. The best you can do is eyeball it, which should be close enough.

[SoulFetish]

The voltage drop across the 1500R is 10.8v, so 10.8/1500 = 0.0072, or 7.2mA. That seems relatively safe, doesn't it?

Or does it? I put in a 3K resistor for half the current at 3.6mA. Not as super bright, but probably safer.

I would trust The Dude on this, he's been a technician much longer than I. I've never seen an led rated with a max operating current as low as 1mA, but they usually can't handle much current at all. Most of the leaded, through hole leds will burn out with anything over 15/20mA (although, I have some red LEDs that can handle a continuous 100mA).
So, I would take the advice above and start with a large resistance–working your way down until you can see a brightness you can live with at the lowest current. Do this in low light, for the reasons the Dude and Leo said. The big one is LEDs are really directional, and on a dark stage, they can be annoyingly bright. My guess is that you should see a reasonable brightness at 3-4mA. But if you can do it at 1-2mA, even better.

[Randall]

It's pulling 3.6mA, close enough for now. If it burns out, I will know to order some with specs I can refer to, and replace it. It's been on for 2 hrs now, and all is well.

[The Dude]

I would trust The Dude on this, he's been a technician much longer than I...

Well thanks, but I'm no authority on anything. Just offering my advice. Anyone is free to take it or leave it.

[frus]

Some local musos have asked me to put them in pedals so they can see them in daylight.

Reminds me of Boss pedals manual: "features a LED indicator for reliable effect status even in the darkest on-stage situations"

Sure, but in daylight they are impossible to see

[loudthud]

I ran across an old Boss pedal that had an external supply jack, but it wouldn't run on the typical 9V pedal board supply. It wanted a special Boss 12V floating supply. The LED wasn't very bright.

[J M Fahey]

7 or even 3.5 mA will wolf your battery down, since they eat more than the wah circuit itself.
Try 1 mA; it should be more than enough with those high intensity leds.

[Randall]

I'd like to stock up on some general purpose LEDs. Looking at Mouser, which is more important, forward current rating, or forward voltage? Interesting, they don't offer 9v, it jumps from 7.5v to 12v. They don't have to be high intensity.