Building an A/B box - what resistor for the LEDs?

[hamuelo]

Hi guys, just got a quick (and hopefully easy) electronics question that I need some help with.

I'm building an A/B switch box, as found on GeneralGuitarGadgets here, though I sourced the bits myself.

So I'm using two 3mm LEDs instead of standard 5mm, but it's thrown me off when it comes to what resistor to use. This is my green and this is my red LED, with the values shown there. My problem is that the 1K resistor (as shown on the wiring diagram here) won't be right for my circuit now, will it?

So my problem is I can't for all my trying work out what resistor to use that will be right for both LEDs without either feeding one too much or making the other too dim, seeing as they have different values.

I'm not asking for someone to just slap an answer in front of me (though I wouldn't complain), I want to learn how to work this out myself too! I've tried a few different methods and come up with nothing solid so far.

Thanks a lot for any help! Cheers!

[bluebunny]

The size of the LEDs won't make any difference: the physics is the same.  And in any case, the 1K value isn't "right" (whatever that means...).

You can play with the following formula:

R = (V_S - V_F) / I

R is the resistor you're searching for.  V_S is your supply voltage - typically 9V.  V_F is the voltage dropped across the LED - the "forward" voltage.  I is the maximum current you want going through the LED.  You might typically see this as "I_F max" in the specs for the LED.  Note the "max" part.  (Indeed, you should recognise all these values in the data sheet.)

If you pump in your values, you'll find you "need" a resistance of less than 300 ohms for your two LEDs.  You will find this to be very bright!  Which is why you see 1K on the GGG schematic - it's easier on the eyes.  Try a few values out and see what you prefer.  I typically use 1K5 or 1K8 - whichever falls to hand first.

[hamuelo]

Thank you so much! You just taught a man to fish.

Cheers!

[bluebunny]

You're very welcome.  I'm just giving back a tiny fraction what I've learned myself in this very forum.  It's a great place, full of great people and you'll learn loads (as I have).  And build loads too (as I have!).   

[induction]

Also, notice that Vf is different for your two leds.  Using the same resistor for both might make the red one brighter than the green one. The difference may or may not be perceptible, but if that's the sort of thing that bothers you, it's worthwhile to test them out side by side on the breadboard (or with roach clips or whatever).  If they don't match, you can use different resistors for each led and swap them out until you're happy.

Whether this is overkill or not depends on how OCD you are. 

[bluebunny]

^^^ Yep, definitely try them out.  And if possible, in conditions where they're going to be used: so turn the lights out if you're destined for a life on a darkened stage!

[deadastronaut]

try 100r and sunglasses..

[bcalla]

... it's worthwhile to test them out side by side on the breadboard (or with roach clips or whatever)...

Whether this is overkill or not depends on how OCD you are. 

try 100r and sunglasses..

Try the roach clips and sunglasses