Hi, this may seem like a joke of a question to you all but i am looking to place 2 5mm 3.2v blue LED's in a costume and need to know what is the best voltage battery to use and what resistors are needed? I would like the LED's to last as long as possible as it is a fancy dress costume. If you have any suggestions i would be extremely grateful to hear them. (also i haven't purchased the LED's yet incase anyone has a suggestion as to better quality or different voltage LED's that would enable a longer battery life, and as this is for a costume i would like as small of a battery pack and the least ammount of wiring possible!)

Thanks so much to anyone who can help!!

 

LEDs need a constant current not a constant voltage. This is done by using a voltage higher than the LED voltage and using a series resistor to drop the current.

So your battery voltage needs to be higher than the LED voltage.

If you connect your 2 LEDs in series, you will need a voltage higher than 6.4V. Then you could use a small 9V battery and a single series resistor.

The value of the series resistor depends on the current required. More current = brighter light.

Start with 20mA (0.02A). Resistor = (9-6.4)/0.02 = 130 ohms.
Nearest standard value is 120Ω

 

We would have to know how many LEDs and the minimum required run time to be able to determine a suitable battery. We also need the complete set of specifications for the LEDs you're looking at.

 

We would have to know how many LEDs and the minimum required run time to be able to determine a suitable battery. We also need the complete set of specifications for the LEDs you're looking at.

he mentioned using 2 3.2V blue LEDs.

The current of which is 20mA, and I haven't seen a blue 5mm LED yet that isn't 20mA

So what JDT said is completely accurate.

BUT using a 120Ohm resistor will cause the LEDs to receive 53mA!! There is a good chance that much current will blow the LEDs.

At the very least it's going to make them run insanely bright, and significantly reduce the lifespan of the LED....if they survive.

a 130Ohm resistor IS a standard value....so use a 130Ohm resistor and a 9V battery.

 

It is not necessary to run the LEDs at their absolute maximum rated current. They will look very nice at half their maximum rating...try 10 milliamps. This will also cause the battery to last twice as long.

 

he mentioned using 2 3.2V blue LEDs.

The current of which is 20mA, and I haven't seen a blue 5mm LED yet that isn't 20mA.

Right. I totally missed the quantity in the original post. This may sound pedantic, but I'm unwilling to assume the current specification even though I agree it's very probably as you say.

For an application like this I'd just get two largish button cells and directly connect the LEDs each to one of the cells. If it's bright enough and runs long enough, you're done. To make it easy, try a couple of these sewable button cell holders with a built in switch:

https://www.sparkfun.com/products/11285

 

I've seen more than one retail LED product that depended on the internal resistance of the battery(s). I have a UV penlight with a coin cell and a 21 LED flashlight with (3) AAA cells. Each of them has no control circuitry of any kind. It's just, you gotta know what you're doing to get away with that.

 

I've seen more than one retail LED product that depended on the internal resistance of the battery(s).

+1 Almost all of them do, at least in my experience.

There are a ton of small, battery-powered LED gizmos available commercially; throwies, blinkers, even ice cubes. Any one of these will have solved most of the OP's design problems (battery, switch, LED type, etc.) and could be used with minor mods at most. Just start googling.

 

he mentioned using 2 3.2V blue LEDs.

The current of which is 20mA, and I haven't seen a blue 5mm LED yet that isn't 20mA

So what JDT said is completely accurate.

BUT using a 120Ohm resistor will cause the LEDs to receive 53mA!! There is a good chance that much current will blow the LEDs.

At the very least it's going to make them run insanely bright, and significantly reduce the lifespan of the LED....if they survive.

a 130Ohm resistor IS a standard value....so use a 130Ohm resistor and a 9V battery.

(9-6.4)/130=20mA
(9-6.4)/120=21.7mA.
Where did you get 53mA?
I agree that 130Ω is a standard 5% resistor value.

 

I've seen more than one retail LED product that depended on the internal resistance of the battery(s). I have a UV penlight with a coin cell and a 21 LED flashlight with (3) AAA cells. Each of them has no control circuitry of any kind. It's just, you gotta know what you're doing to get away with that.

The internal resistance of the coin cells prevents too much current from flowing.

The high LED count is just running them on the edge, they'll work, but won't last as long.

If the OP only needs 2, he could put the legs around a CR2032 battery (need 2 coin cells) and they'd stay on for a little over 24 hours straight (look up "LED Throwie").