LEDs and all the fuzz about them

[ment4lbre4kdown]

I'll put this simply.

I'm going to connect eight red (635nm) flashing LEDs (max 2.5V, 2500mcd intensity, 60ma) in a line. I will then connect a battery to my line of LEDs.

+__O__O__O__O__O__O__O__O__-

What battery do I need, and will I need any resistors? If I do, where should I place them? How would these values change if I put two more LEDs in the circuit?



EDIT: I changed all the values, sorry if anyone read the original post...

[deadastronaut]

i would start off with 4.7k resistor on the positive end...then a 9v battery..you wont blow em then...

you may be able to use a lower resistor but thats what id start with ...ok.


you could always use a  pot ...till u get the brightness u want then measure the pot value and replace with a resistor...

thts what i would do...others may say different though...

[ment4lbre4kdown]

Okay so if I add more LEDs, should I just gradually increase the resistor? Like, 500 ohm per LED?

[anchovie]

i would start off with 4.7k resistor on the positive end...then a 9v battery..you wont blow em then...

Or they won't even light up!

2.5V per LED x 8 LEDs in series = 20V.

Give this a try: http://led.linear1.org/led.wiz
You'll be able to try other quantities of LEDs too.

[deadastronaut]

i would start off with 4.7k resistor on the positive end...then a 9v battery..you wont blow em then...

Or they won't even light up!

2.5V per LED x 8 LEDs in series = 20V.

Give this a try: http://led.linear1.org/led.wiz
You'll be able to try other quantities of LEDs too.

whoops...yeah...sorry dude...

[ment4lbre4kdown]

A really cool link there, it helped me understand somewhat, but it shows parallell arrays with multiple resistors. I'm going to connect all my LEDs in a line (due to lack of space) and therefore only need one resistor. How big a resistor would you suggest and how should the size of the resistor change when/if I add more LEDs?

[ment4lbre4kdown]

2.5V per LED x 8 LEDs in series = 20V.

Heyyy...does that mean with a 20V power supply I don't need any resistors?

[anchovie]

You can still have your LEDs physically in a line, just wire them in parallel behind the scenes!

You simply won't be able to have all your LEDs in series off one 9V battery, I reckon that 2.5V spec is pretty rigid because of the inclusion of the flashing circuitry.

[anchovie]

Heyyy...does that mean with a 20V power supply I don't need any resistors?

The resistor is to limit current.
Ohm's law: R=V/I
LEDs are spec'd at 60mA max which is the same value for a whole line in series. 20/0.06 = 333, so 330R if you want to be right on the edge or 390R to be a bit safer. Increase the resistor to decrease brightness, though there'll be a point where the flashing part will be unable to function.

Good luck finding a cheap 20V battery!