# Can a circuit of 16 LEDs be made with 4 AA batteries powering it?

#### Sarah808

Joined Oct 18, 2017
3
Hi, I'm new to this forum, looking for help! please could someone tell me whether it's possible to make a circuit with 16 leds attached to 4 x AA batteries? Am attaching the spec of the LED's... man at maplin had very good intentions I'm certain but his suggestion hasn't worked. I'm a complete novice with electrics, the circuit is for an art project... please assume I'm an absolute idiot and explain as basically as possible if poss!! Thanks very much.

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#### philba

Joined Aug 17, 2017
959
No way to easily make a string but you can do it by running the LEDs in parallel. You will need a resistor per LED to control the current and the 6V (4X1.5) battery pack will be about 1/2 wasted. You could use 3 AA bats (4.5V) to the same effect. You will get a bit more than 4 hours of run time.

What color LEDs are you using? Different colors will need a different value of resistor. And the resistor also depends on the battery pack (3 vs 4 AAs). Answer those questions and we can tell you what resistors to get.

Solder the resistor in series with each LED. Connect each resistor to the longer leg of the LED. Then connect all the unsoldered resistor leads to the + terminal and the shorter LED leads to the - terminal. I would include a switch to shut it off when you don't need it.

If you don't know soldering, you'll need to find someone to help you.

#### dl324

Joined Mar 30, 2015
15,453
Welcome to AAC!

What color(s) LED did you purchase? How long do you want them to operate on battery power? What is more important, brightness or operating time?

If you operate at low enough current, you might be able to have 8 strings of 2 LEDs with 8 current limiting resistors. If you use a switching regulator, you can boost the 6V from 4 AA batteries high enough to operate two strings of 8 LEDs.

#### Sarah808

Joined Oct 18, 2017
3
Thanks philba,

Using white LEDs. The battery pack is 4 x AA. If at all possible it would stay that way because the shape of it has already been cast (plaster) for it to slot into.

Why is the run time so short? Can you buy switches to wire into circuits or would it switch need to be within the battery pack?
Do you mean each LED in essence has its own circuit? Am I ncluding an interpretive drawing of my understanding of what your saying...

#### Sarah808

Joined Oct 18, 2017
3
Thanks Dl324, I guess in terms of priority: brightnes;, they're being used to conduct light through acrylic rods. Could you tell me would it be the LEDs that burn out or the battery that runs out of juice? Thanks

#### dl324

Joined Mar 30, 2015
15,453
I guess in terms of priority: brightnes
Then you probably want to run them at 20mA. They'd only be marginally brighter at 30mA.
they're being used to conduct light through acrylic rods.
You'll get better coupling of the emitted light into the rods if you cut off the top of the LED lens and used an optically transparent epoxy to attach the LEDs to the rods.
Could you tell me would it be the LEDs that burn out or the battery that runs out of juice?
If you operate the LEDs at 20mA, battery life will determine operating time. If you operate them at 30mA, which might be the absolute maximum, you could affect LED lifetime. LEDs usually dim when they near end of life, as opposed to burning out.

If you can operate 2 LEDs in series with a small current limiting resistor, you'll waste less battery power in the resistors.

If you incorporate a boost switching regulator to increase the 6V from the batteries to 22 or 24V, you could run 2 or 3 strings.

#### philba

Joined Aug 17, 2017
959
For full brightness, your LEDs need to run at 30 milliAmps. Multiply that by 16 and you get 480 mA, Your batteries can provide about 2200 milliAmp Hours. Divide 2200/480 and you get a bit more than 4 hours though you never actually get all the "juice" out of a battery. No more blood from that stone! It's the batterys that die, not the LEDs,

OK to use 4 AA, just wasting 1. Not the end of the world.

Yes, each LED would essentially have it's own circuit.

For white LEDs and 4 AAs, you need 90 Ohm resistors. If 90s are hard to find, you could use 100 ohm, a standard value, at a very small loss of brightness but with a little longer battery life (about 24 minutes longer).

You can buy a switch to add to the circuit (between your battery pack and all the LEDs). Even your local hardware store probably has them.

#### philba

Joined Aug 17, 2017
959
Then you probably want to run them at 20mA. They'd only be marginally brighter at 30mA.
You'll get better coupling of the emitted light into the rods if you cut off the top of the LED lens and used an optically transparent epoxy to attach the LEDs to the rods.
If you operate the LEDs at 20mA, battery life will determine operating time. If you operate them at 30mA, which might be the absolute maximum, you could affect LED lifetime. LEDs usually dim when they near end of life, as opposed to burning out.

If you can operate 2 LEDs in series with a small current limiting resistor, you'll waste less battery power in the resistors.

If you incorporate a boost switching regulator to increase the 6V from the batteries to 22 or 24V, you could run 2 or 3 strings.
The LEDs Vf is 3.3V. 2 in series will have a 6.6V Vf. The 4 AA pack is at 6V. Not sure how well that will work and the current limiting resistor is going to tricky to figure.

I also thought of a boost regulator but unless there is an easy module to use, I think it's going to be a real challenge. That might allow the system to make it to 6 hours on full brightness. I did find this chinese module Not sure how good it is and it looks like it ships from china (2-4 weeks). Most of the ones I saw needed more than 6V input.