A/B Box build - Newbie question

[springlux]

Hello folks,
this is my first ever post to DIYS so hopefully you guys will not be too tough with me 

So basically I want to build the A/B box from the Fullton website.
I also found one on the GGG website.

My question is this.
What is the significance of the 2 different LED resistor sizes in the Fulltone spec ?
i.e. one is listed as 10K , the other as 2.2K.
Why are they different values and not the same ?
Also it looks to me like they are connected (in series ?) , so why do you even need two and not just one ?

As you probably guessed, I know little to nothing about electronics and have never built a real pedal !!!
So far I have built a Beavis Board and tested a PNP and an NPN Fuzz Face, all of which seem to work pretty well 

Cheers guys.
Paul in Luxembourg

[duck_arse]

spring, welcome.

the two leds are different colours, and therefore have different forward voltages, the red being a little lower. the yellow probably produces less "light", and at a different wavelength, meaning the brain will perceive the 2 colours as different brightnesses. so, the series current limit resistor is calculated to allow more current to the yellow led, so it will produce more light output, in the hope that both will appear the same brightness.

taking the red led as Vf of 1V6, and a supply of 9V, the current in the led will be, as ever, I=E/R, in this case 9V (the supply) - 1V6 (the voltage the led will eat just to light) divided by 10,000 ohms (the series resistance).

and the led current? well, now you can work that out. if you look at manufactures data sheets, you can find the Vf for different colours.

[springlux]

Brilliantly explained so that even I understood 

Very many thanks.
Paul

[duck_arse]

but you didn't post your answer.

[tubegeek]

1. Welcome springlux to the forum!
2. Yes, we give homework here.

[springlux]

 
Haha, nice one.

Hmm , so is the answer is 9-1.6/10000 = 0.00074 Amps or 0.74ma ?

[duck_arse]

for the red led, yes. you'll need the data sheet for the yellow led, just googoo the part number in the build doc.

and to save all the decimal points and leading zeros, you are encouraged to use uA, as in 740uA. (I dunno where the mu key is.)

[springlux]

OK,
feeling a bit more confident now so looking at the datasheets, I found that the kingbright LED's actually report the following Forward Voltages:
red = 2v
yellow = min 2.1, max, 2.5 (I will average at 2.3)

So regarding current :
Red LED has the 1K resistor and a 2V Fv so: 9v - 2v / 10000 = 704uA
Yellow LED has the 2.2K resistor and a 2.3 Fv so: 9v - 2.3v / 2200 = 3045uA or 3.045mA

Regarding the min/max values from the datasheet, is there some way to determine the exact value for my yellow LED ?

Many thanks again.
Paul

[duck_arse]

are we both looking at this box?

http://www.fulltone.com/sites/default/files/manuals/fultone-a-b-switcher.pdf

once again, correct.

if you want to know the forward voltage of the part you have in your hand, simply put in on the breadboard (get one if you don't have one) with a resistor connected to its kathode, lets say 10k, just to be sure, and connect 9V to the anode, 0V or minus or black to the resistor. then get yr multimeter, and prove Ohm's law on the resistor, and read the voltage across the led terminals. it's that simple.

[springlux]

Yes, that exact one.

Once again thanks for the great explanation :-)
I breadboarded all my LED's tonight and checked the voltage, current and luminosity(visually).
All the values matched the theory perfectly.
I also did the test with a red and yellow led with the different resistor values (10k & 2.2k).
As you said, suggested the yellow LED was too dull with 10k. At 2.2k the luminosity was as near identical to the red (with 10k).

Great introduction to the forum for me 
Thanks you so much.
Paul