Author Topic: Led tutorial or help  (Read 7267 times)

gnort_2

Led tutorial or help
« on: July 10, 2013, 12:06:22 AM »
Time to buy an led and hook it up to my booster pedal.
How do LEDs work?
How are they rated?
How do I know what resistor to use?
Are different colored ones different internally? Or just a different color casing?
How can I test all of this?

Thanks
(Still learning to crawl)

duck_arse

Re: Led tutorial or help
« Reply #1 on: July 10, 2013, 09:30:32 AM »
how they work? they are semicondutor junctions, etc etc. the experts are needed here. different materials internal produce different light colour external.

they are usually rated by their light output; how much of it they produce. also rated by forward voltage, light output colour, wavelength, lens dispersion, etc. get some data sheets, look through them.

you know what resistor to use by subtracting the led forward voltage, which might be 1V6, from your supply voltage, which might be 9V. then you pick a forward current, lesss than the max specified in the data sheet, less than will blind all the neighbours, and a bit less than will drain your battery (we'll assume) in an hour, so lets say 1mA. then you do ohm's law, R=V/I = (9 - 1V6)/0.001 = ? ohms.

this value resistor then limits the current by dropping a voltage, and the led doesn't self distruct. and that's how you test them. buy some leds, and some resistors, and a breadboard, and away you go.
".... just enough bling hardware to complement the quiet textured slip-proof pants ...." - customer appraisal of last pedal build.

gnort_2

Re: Led tutorial or help
« Reply #2 on: July 10, 2013, 10:00:24 AM »
What's does 1v6 mean?

duck_arse

Re: Led tutorial or help
« Reply #3 on: July 10, 2013, 10:14:49 AM »
1V6 means 1.6 Volts. many years ago, engineer types decided to replace the decimal point, when it occured, with the multiplier, in this case V for volts. this cut down on strings of '0.000" and typo errors in magazines. supposedly.

1V6 is also the voltage you might read across a red led when lit, with proper series resistor fitted. this is the forward voltage referred to in the data sheets.
".... just enough bling hardware to complement the quiet textured slip-proof pants ...." - customer appraisal of last pedal build.

Jdansti

Re: Led tutorial or help
« Reply #4 on: July 10, 2013, 12:11:10 PM »
A general safe rule of thumb would be to use at least a 680R (680Ω) resistor for most LEDs. 1.4K seems to be popular. The lower you go, the brighter the LED, but the risk that you'll burn it out is higher.

Someone on the forum, I think deadastronaut or Mark Hammer, built an LED test box with multiple resistors so he could quickly see how bright an LED would be with a given resistor.

You can do the calcs that duck gave you to start with as a minimum value resistor, but I'd have some larger values handy in case you think the minimum would be blinding. After a while, you'll learn what values you prefer.

Edit: premature post button activation. ;)
« Last Edit: July 10, 2013, 12:13:11 PM by Jdansti »
R.G. Keene: EXPECT there to be errors, and defeat them...

gnort_2

Re: Led tutorial or help
« Reply #5 on: July 10, 2013, 03:23:52 PM »
R=(Vs - Vf)/ If

I think Vs is the voltage reading were the led gets its power

Is Vf the voltage rating on the led package?

Jdansti

Re: Led tutorial or help
« Reply #6 on: July 10, 2013, 05:14:34 PM »
R=(Vs - Vf)/ If

I think Vs is the voltage reading were the led gets its power

Is Vf the voltage rating on the led package?

Yes. Vf is the "forward voltage" of the LED.  This is the voltage at which the LED turns on.  If is the current rating for the LED.
R.G. Keene: EXPECT there to be errors, and defeat them...

gnort_2

Re: Led tutorial or help
« Reply #7 on: July 10, 2013, 05:31:57 PM »
Thanks again, I'm gonna get this stuff yet...

aron

Re: Led tutorial or help
« Reply #8 on: July 10, 2013, 08:36:44 PM »
WaitAminute - first off congratulations on getting your pedal to work, but come on!!!!! There is a document that explains a LOT of what you ask:
http://www.diystompboxes.com/wiki/index.php?title=DIY_FAQ

It's on the top - the DIY FAQ. Please check it out.  ;)

Also this link:
http://www.diystompboxes.com/smfforum/index.php?board=10.0

Jdansti

Re: Led tutorial or help
« Reply #9 on: July 10, 2013, 09:13:56 PM »
^ Great info! 

I just want to add that you should keep in mind that the LED formula gives you the minimum size resistor for the LED. For example, if we look at a typical white LED with a Vf of 3.2V and If of 20 mA (use 0.02A for the calculation), using a 9V supply, the equation gives us 290Ω. That's going to be one freeking bright LED!   A resistor over 1kΩ would be more reasonable. That's why I was saying that is good to understand the equation, but after you get some experience under your belt, you'll know what size resistors give you the brightness you like.
R.G. Keene: EXPECT there to be errors, and defeat them...