# Battery powered LED project

Discussion in 'The Projects Forum' started by TexAvery, Jul 12, 2013.

1. ### TexAvery Thread Starter Member

Oct 7, 2009
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Hello everyone!
I have a parking meter project that will require 40 to 60 5mm LEDs that need to be battery powered for 30 days, but only at night.
LEDs can be any color but red.
I am thinking a Joule Thief circuit with a darkness sensor (photo diode)?
The challenge is I only have two 1"x1.5"x4" space for the battery/batteries.
Yes, this would be a paying project.

2. ### wayneh Expert

Sep 9, 2010
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No solar power or recharging?

I think if you do the math for that much battery volume, and assume the energy density of a modern Li-ion battery, it cannot be done. I could be wrong but that's my gut hunch.

3. ### blueroomelectronics AAC Fanatic!

Jul 22, 2007
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Lets do some math.

40 LEDs (the minimum described) x 10mA = 400mA.
30 nights of runtime is 12x30 = 360Hrs
That's .4 * 360 = 144Ah

Last edited: Jul 16, 2013
4. ### Bernard AAC Fanatic!

Aug 7, 2008
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Decimal point slipped, try 14.4 Ah; still a big battery.

5. ### blueroomelectronics AAC Fanatic!

Jul 22, 2007
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Opps missed that, thanks.

6. ### Bernard AAC Fanatic!

Aug 7, 2008
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Two genuine NiMH D's & bright yellow LED's would come close.

7. ### Bernard AAC Fanatic!

Aug 7, 2008
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There is a post for, on @ night, with optional flasher, output would need beefing up.

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Last edited: Jul 12, 2013
8. ### John P AAC Fanatic!

Oct 14, 2008
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I think that under conditions of subdued ambient light, 10mA would be pretty generous. You might be able to run small white LEDs with 1mA, but it depends on whether you want to create something visible, or actually light up the area around the meter. Could there be some interaction with a user, where if someone were actually near it, the lights would become brighter, or more of them would come on?

Still, I also think it's ambitious in terms of battery size versus the amount of energy to be delivered.

9. ### wayneh Expert

Sep 9, 2010
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I think the calculation must be done on power.

Let's see, 40 LEDs at 10mA and 3Vf equals 1.2 watts. Total for 30 nights is 432 watt-hours. Given circuit losses and such, let's say we need 500 W•hr in the battery.

Battery volume given is 6 cubic inches or 0.098L. State of the art Li-ion batteries are at 800Whr/L, or just 78 W-hr in the given space. And that's assuming no space for anything but battery guts.

No can do.

Flashing and running for much shorter hours would help.

10. ### TexAvery Thread Starter Member

Oct 7, 2009
58
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I can fit four 4000mAH 3.7V Li-ion batteries if that helps.
The LEDs are not for lighting... more for visible effect , so 10mA will work.
Adding a PIR sensor for interaction would be a bonus!
Great idea John P ...Interaction with a user, where if someone were actually near it, the lights would become brighter, or more of them would come on is exactly what the job calls for.

11. ### wayneh Expert

Sep 9, 2010
15,551
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That's 59 W-hrs of energy (assuming steady battery voltage, which it isn't).

You need strategies to dim or turn off the LEDs to save power. There's no way around it. Full-on while dark isn't going to work.

Flashing at 2-5mA with a 10% duty cycle when nothing is detected by the PIR would save an enormous amount of battery. With multiple flashing strings instead of all at once, you could have a more pleasant effect, perhaps. Goose up the current and the duty cycle when the PIR sees an audience.

12. ### John P AAC Fanatic!

Oct 14, 2008
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I was recently visiting a shelter for hikers on the Appalachian Trail which is actually a small house, with beds upstairs. They had battery-powered lights on the stairs, each of which was a bank of white LEDs (maybe 9 of them, or 12, in a rectangular pattern) and there was a motion sensor which made the lights come on when anyone was on the stairs. I don't know how much power it takes to run the motion sensors, but I assume not very much. I expect the lights were just purchased at Home Depot or somewhere, not specially designed.

13. ### TexAvery Thread Starter Member

Oct 7, 2009
58
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OK~ I have found more space for batteries, about the volume of four 9v batteries in the parking meter. Let's reduce the number of LEDs to 20 and add PIR interaction so the LEDs are on standby mode (dim) with no movement.
Is this possible?

14. ### wayneh Expert

Sep 9, 2010
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Yes, I think so. You need a few basic functions.

First, use PWM control over your LED brightness. It's more efficient than other methods (e.g. linear control over current) that waste as heat the energy not used by the LEDs. Think about what you want the "quiet time" effect to be. You could have a dim steady glow (fast PWM with very low duty) or a flashing bright blip every few seconds (slow PWM with very low duty cycle.

Second, you'll need a way to control the PWM duty cycle when the PIR triggers. I'm not familiar with these devices so someone else needs to help here. I think you want full-on (100% duty cycle) for the "awake" status?

Third, you want the whole thing to do nothing unless it's dark out, right?

15. ### TexAvery Thread Starter Member

Oct 7, 2009
58
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Correct LEDs only function during night time.

16. ### TexAvery Thread Starter Member

Oct 7, 2009
58
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Correct LEDs only function during night time.

17. ### blueroomelectronics AAC Fanatic!

Jul 22, 2007
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That's still 384 hrs of runtime a month. How bright do you need them? Might help if you described what its purpose is.

18. ### TexAvery Thread Starter Member

Oct 7, 2009
58
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It will be a functioning single head Parking Meter that will have 20 holes drilled in the enclosure with recessed LEDs that light up at night to attract attention. To save power the LEDs can be dim/blink/fade (ON)-(OFF) whatever. When motion is detected the LEDs get brighter/change effect to give a "AWAKE" indication. Every month the money is collected the attendant can change the batteries.

19. ### blueroomelectronics AAC Fanatic!

Jul 22, 2007
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We have all sorts of parking tag meters here in Toronto, Canada. They'll all solar powered and some include RF telemetry. No batteries to replace which would be a pain in the... No reason it could't be shrunk down to a single head unit.

Why aren't you using solar to charge the cells, gotta have plenty of sunlight in Texas.

20. ### Bernard AAC Fanatic!

Aug 7, 2008
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Thr R-S PIR has a hemispherical IR lens, about 7/8 in dia.;is there cpace for it?