Newbie question: TSA LED

[nosamiam]

Hi folks,

I'm very much new to this game but enjoying it immensely.  I'm a ways off from understanding much of what I'm doing, but some of it is coming together.  Anyways, I've got a problem:

I'm building a Total Sonic Annihilation clone based on this layout: http://www.geocities.com/austenfantanio/weedingout/Sonicdeathtutorial.htm

I've already built one successfully, but without the LED and I definitely want one this time around.  I've also done some stuff on this second build t like adding volume control to the feedback loop only, as well as making it switchable from feedback loop to effects bypass loop.  I'm very happy with it so far.

On to the question.  Is there a certain type of LED I should be using?  Do I really need two, like the schematich shows? I bought a bunch of these: http://www.mouser.com/index.cfm?&handler=data.listcategory&D=*3515500*&terms=351-5500&Ntt=*3515500*&Dk=1&N=0&crc=true from Mouser simply because I've used them before.  They keep blowing if I don't use a resistor.  So if I use a resistor, what value should I use?  Or should I be using a different LED altogether?

Any help would be appreciated.  Even better would be an explanation of the math to figure it out myself.

Thanks!!!!

[niftydog]

no, you don't need two if you don't want two.

All LED must have there current limited in order to stop them from blowing up. The way it's wired on the link you provided is wrong, as there is no provision for limiting the current.

Some LEDs come with integrated limiting resistors, but they are not common.

First, you want to work out what current the manufacturer recommends the LED be operated at. Then you use OHMS LAW to get a fairly accurate value for a limiting resistor.

(LEDs have a voltage drop accross them, this can usually be ignored in non-critical applications such as this)

OHMS LAW is a relationship between voltage, current and resistance.

Volts = Amps x Ohms

re-arrange;

Ohms = Volts / Amps

So, 9V battery, lets say you want 10mA for your LED;

Ohms = 9 / 0.01

you need a 900ohm resistor.  This value is not common, so go with the next best thing which might be 820ohms or 1kohm.

More info about this is easily found by searching this forum. A better explanation of ohms law and related concepts is easily found via google.