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Can anyone help with a simple LED Design? Answered

Im looking to make a very simple circuit to add some lighting to a project. I want to create a circuit, powered by AA or AAA batteries that will power 6 LED lights (clear/white light) for up several hours. i know I need a power source (hopefully a simple battery holder with an on/off switch built in, a resistor and the set of LEDs (and wiring) what am I missing? which resistor am i looking for? How can I calculate battery life? Thanks for helping a new guy to electronics out!

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Josehf Murchison

4 years ago

For your battery amp hours so if you want it to run for ten hours and the LEDs are in parallel at 3 volts and 0.02 amp each making a total of 0.12 amps you will need about 2.4 amp hours. resistor + LED, beside resistor + LED 6 times.

amps x hours x 2

If you have them in series you need a higher voltage Depending on your LEDs 18 volts could be as little as 9 volts but you will only need 0.4 amp hours on the battery. Resistor + LED + LED to 6 LEDs.

And if you have them in a series parallel circuit 6 to 9 volts 0.06 amps 1.2 amp hours. Resistor + LED + LED beside resistor + LED + LED 3 times.

To determine the resistor it is voltage divided by current.

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Jack A Lopez

4 years ago

I think the most simple implementation for powering LEDs is just a constant voltage source (or series stack of battery cells), a single resistor, and a series stack of LEDs

That's the design logic used by linear1.org's well known LED-calculator, here:

http://led.linear1.org/led.wiz

For a beginner, I think the hardest part about using that calculator is comming up with some appropriate numbers to feed to it. So I'll give you some examples.

e.g. A stack of 3 alkaline cells, powering white LEDs with 3.6V characteristic forward voltage, at current of 20 mA, and total number of LEDs is 6. (Vsource=4.5V;VLED=3.6V;ILED=20mA;N=6;)

Feed this to the calculator and it puts each LED in series with 47 ohm resistor, then parallels that six times, for a total component count of: 6 LEDs plus 6 resistors.

e.g. A stack of 6 alkaline cells (or a "9 volt" size battery), powering white LEDs with 3.6V characteristic forward voltage, at current of 20 mA, and total number of LEDs is 6. (Vsource=9.0V;VLED=3.6V;ILED=20mA;N=6;)

Feed this to the calculator and it puts two LEDs in series with a 100 ohm resistor, then parallels that three times, for a total component count of: 6 LEDs plus 3 resistors.

By the way, this choice of a series stack of three 1.5V cells, e.g. three AA (or AAA or button) cells, seems to be pretty common in cheap designs, like for flashlights, closet lights, etc. In really cheap designs, like those powered by button cells, the resistor is often omitted. Guessing the reason why that works is because the button cells have sufficiently high internal resistance, and internal resistance limits current just as well as an external resistor.

If you have a DollarTree(r) store near where you live, they've got some of these artifacts, for a dollar per, which might be cheaper than separately buying white LEDs, resistors, and battery holders, and also maybe worth it just to open them up and see what their designers were thinking.

See:
http://bulk.dollartree.com/search#w=led%20lights

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Jack A LopezJack A Lopez

Answer 4 years ago

I forgot to mention battery life. To get an approximate estimate of battery life, divide the battery's current capacity, measured in ampere*hours or milliampere*hours, by the current it must supply, measured in amperes or milliamperes.

e.g. if a stack of AA batteries has current capacity of 2400 mA*h, and it must supply 120 mA, get:

t = (2400 mA*h)/(120 mA) = 20 h

This page, from Wikipedia,

https://en.wikipedia.org/wiki/List_of_battery_size...

has a table which lists current capacity for several different battery sizes and chemistries.

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iceng

4 years ago

Been a while since the Joule Thief reared its circuit.

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supertechey

4 years ago

you need step up circuit from 1.5 to req. led voltage of 5+

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iceng

4 years ago

Look in your junk box, design from there are so many many ways to do this as Downunder points out.

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Downunder35m

4 years ago

Get some dirt cheap solar lights.
Use the electronics ;)
They have ultra low current LED's in them andthe electronics provide all you you need.
Instad of the solar cell you can connect a charger for the battery if you want, just make sure it has the right output voltage.