LED`s ?

TamDB

Hi all, i`m probably in the wrong section again, but here goes. I`m trying to build myself a kill switch with an LED in it, so got hold of a bunch of LED`s, but it turns out they don`t like 9 volts, anything over 4.5v kills them stone dead, so i`m assuming that I need a resistor of some description, but havn`t a clue what to use, so any pointers in that sort of direction would be most helpful, thanks in advance, cheers, Tam. 

steamabacus

This should tell you what you need to know. You need to know the forward voltage and forward current for your LEDs (it doesn't need to be accurate, so long as your series resistor is large enough to drop the voltage enough to avoid frying the LED)

randomhandclaps

Unless you're really fussy about exactly how bright the LED is, a simple and common place to start is somewhere around 2.2 - 2.7k up to 4.7-5.1k. Whatever you have laying about or you can get hold of easily. You can use increasingly larger values and the LED will just get dimmer.

Jaden

LED's usually take around 20-30 mA

use the equation - R=V/I

V=9
I(amps) = 0.020 - 0.030

gives either
450 ohms
300 ohms
as an inline resistor

TamDB

Thanks guys, I wasn`t actually expecting any replies tonight,so well done that man, (all 3 of `em), i`ll go and check that out, and have a rummage in my electric spares box and see what I can find, cheers, T.

TamDB

I`m also looking for a bezel / housing of some sort to hold the LED as opposed to having it simply peeking through an orifice in the top of the pedal, any ideas anyone ?

randomhandclaps

Dependent on LED size -

http://spiratronics.com/electronic-components/optoelectronics/leds/led-optics-and-mounts/metal-led-bezels.html

steamabacus

I like the metal bezel type (above) as well. Maplins do them (I have a Maplins just round the corner from me - mind you, they never have anything in stock these days!)

TamDB

Thanks Mr handclaps, that`s just what i`m after. 

and Mr abacus, is your Maplins in Wood Green high road by any chance ? 

crunchman

steamabacus said:

I like the metal bezel type (above) as well. Maplins do them (I have a Maplins just round the corner from me - mind you, they never have anything in stock these days!)

And when they do it is grotesquely overpriced.

I needed a single resistor a while back so I took myself down to Maplin.  It cost well over 30p.  There are online sites selling singles for about 5p.  If you buy in bulk from somewhere like RS then you get even cheaper than that.

The exact value of the resistor probably isn't that critical.  There are sites that give you a calculator for it but anything between 1k and 3k should be fine.

TamDB

Thanks for all the replies guys, I`ve found a random resistor in my bitsa box, no idea what the value is, so i`ve soldered it to to one of the LED`s and stuffed it up a 9v PSU to see what happens, it seems to be working OK so far, so fingers crossed.

Once again thanks all, cheers, T.