# Longer battery life for LED circuit.

Discussion in 'The Projects Forum' started by cbseattle, Apr 4, 2012.

1. ### cbseattle Thread Starter New Member

Apr 4, 2012
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0
I have 8 low voltage LEDs (1.6V) powered by 2 CR2032 batteries.

The LEDs: 4 sets of 2 (3mm) LEDs with 500K resistor, in parallel
2 batteries in series (6V) to supply the necessary voltage.

My (questionable) calculations suggested I could run the lights for approx 48hrs continuously. In fact they will be turned on and off so that will end up being a lot longer for actual usage (typical usage will probably be 1-2 hours per day).

My main question is: is there something I can do to extend that battery life other than a bigger resistor which will make it too dim. If I put anymore than 2 in series then they don't light unless the batteries are brand spanking new.

My side question: I often see LED circuits run by batteries and cannot understand why they work. My understanding is that the typical 5mm LED that I see used requires 10-20mA of current. Button batteries, I thought, cannot supply that kind of current at once, so why do they work?

Thanks in advance for any help. Those of you who take so much time to help all of us are saints - this stuff is really confusing for a beginner.

2. ### jimkeith Active Member

Oct 26, 2011
539
99
5mA is generally sufficient to get a good indication--perhaps less with efficient LEDs.

Eliminate the 500K parallel resistors--they are not necessary--only one series resistor is required to limit the current for each series string--two in series is probably best for that voltage.

You can reduce LED average current by an order of magnitude by driving them with a TLC555 timer set up as a flasher.

Button batteries can supply that level of current.

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3. ### #12 Expert

Nov 30, 2010
16,248
6,745
Mostly elaborating on what jimkeith said,

Keep poking around this site and vendor sites for ways to buy super bright leds. Those can get you more light for less current.

The timer circuit that pulses the LEDs works because of the human characteristic called, "persistence of vision". Anything over 30 or 40 blips per second looks continuous. Making the blips 2 or 3 milliseconds long saves 90% of the current. The TLC version of the 555 is an energy saver compared to the old style like the LM555.

Then, there are current regulated, fast switching supplies that eliminate the series resistor, saving more energy.

How complicated are you willing to go?

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4. ### cbseattle Thread Starter New Member

Apr 4, 2012
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Thanks for the advice. Very helpful.
I'll change the resistors and that will clean things up too.

As far as the super bright LEDs, in the huge morass of LEDs it's hard to find anything. Any chance you could give me some kind of spec. for what I'm looking for to help? I'm assuming it's low voltage, low current and high mcd. But I'm really not sure what mcd is good for an LED that needs to be easily visible in normal daylight conditions (i.e. not in the dark).

As far as blinking, I'd actually really like to add that. In fact I'm not even concerned with persistence of vision, I'd be happy to have them visibly blink and I understand how that could be a huge battery extender.

I've spent some time looking into blinking and it doesn't seem too easy to get them to blink. I'd like to keep things pretty simple just because this is all wired into a piece of clothing. 555's seem pretty simple though. I guess I'll have to hunt down the simplest blinking 555 circuit using the version of the 555 you suggested.

Again, thanks so much for taking the time.

5. ### #12 Expert

Nov 30, 2010
16,248
6,745
This site is loaded with 555 circuits. If Bill Marsden sees this he will overload you with circuits that will work. Find his name, click on it, and go for his blogs...or post a title about 555 circuits. He will find you!

Focus on the word, "superbright". That's the most millicandelas per milliwatt.

6. ### cbseattle Thread Starter New Member

Apr 4, 2012
4
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Will do, on both counts. Thanks

7. ### crutschow Expert

Mar 14, 2008
12,977
3,221
To extract the maximum energy from a battery you could use a Joule Thief circuit. it can drive an LED from just one battery.

8. ### Wendy Moderator

Mar 24, 2008
20,764
2,534
I sensed a disturbance in the Farce.

There is a different approach I have used, demonstrated on this post...

High Power LED Flasher

The idea is a buck puck is a converter, it will power the LED with maximum brightness until the battery voltages start to drop significantly. At this point the batteries are pretty well exhausted, and the LED will rapidly dim, having sucked the battery dry.

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9. ### Audioguru New Member

Dec 20, 2007
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But PWM is used as a dimmer because it reduces the duration of the pulses. A duration less than 30ms looks dimmed.

10. ### cbseattle Thread Starter New Member

Apr 4, 2012
4
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The LED flasher looks cool. I'll need to spend a little time seeing if I can really understand it. Thanks for the help.