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Offline icoms8Topic starter

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LED sign
« on: August 22, 2009, 06:55:30 PM »
im making a LED sign for school and i am wondering if this would be a bright LEDhttp://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=C5SMF-RJS-CT0W0BB2CT-ND. and what resister would make good brightness but have it last a good time. i was think 345 ohms. and i am useing 20 of them. how many and what kind of batterys should i use. i was hoping to use 9v.

Offline SmAsH

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Re: LED sign
« Reply #1 on: August 22, 2009, 07:01:02 PM »
its kinda bright... not the brightest though...
are you going to wire them in parallel? series parallel?
the 345 ohms is right, but have you considered wiring them in series parallel?
like having sets of 3 in series and wiring the sets in parallel? this would waste less energy from the heat...
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Offline icoms8Topic starter

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Re: LED sign
« Reply #2 on: August 22, 2009, 07:42:39 PM »
thanks. oh and im make the poster sign kind of like the leds a stars. ther not in sereial or parrel.(sorry for spelling)

Offline SmAsH

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Re: LED sign
« Reply #3 on: August 22, 2009, 07:51:35 PM »
how can they not be in series or parallel? i didn't mean the arrangement i meant the way you hook them up...
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Offline Soeren

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Re: LED sign
« Reply #4 on: August 22, 2009, 08:18:27 PM »
Hi,

They would be fairly visible, but note that the horizontal angle of half intensity is 100° while the vertical angle is only 40°, so mount them accordingly.

When you have more than one LED, they will be either in series, in parallel or a mixture (unless you use a separate power source for each.
To reduce power consumption, you need to have 2 LEDS in series on 9V (3 LEDs are too many, considering the voltage variation of a 9V battery throughout its lifetime).
However, a 9V PP3 battery, if that's what you're hoping to use, is a very poor solution, as it will only run a single LED (@20mA) for around 20 hours, so 20 LEDs will only have power for about 1 hour, give or take - 2 hours if you put each 2 in series (it will probably only be around half that, since capacity drops with a rising load).
AA cells have more than 5 times the capacity, so would be a better choice.
A wall wart supply would be the optimum solution of course.

A 1.5V battery cell is actually closer to 1.65V when fresh and considered flat when at 0,9V. A 9V battery varies from 9.9V down to 5.4V over its useable life, which means that the light output from your LEDs will fade if a resistor is used to control the current.
Eg. with the closest E12 value from your 345 Ohm resistor, which is 330 Ohm:
from (9.9V-2.1V)/330 Ohm = 30 mA
to (5.4V-2.1V)/330 Ohm = 16.4 mA

Constant current devices are the key to a uniform light intensity over the battery lifetime.


Should all 20 LEDs be on at any given time or what?
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
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Offline icoms8Topic starter

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Re: LED sign
« Reply #5 on: August 22, 2009, 10:59:46 PM »
Sorry I miss understud you on the serial parralel. Which on would work better I need them to run as long as possible on AA batterys. Thanks!

Offline SmAsH

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Re: LED sign
« Reply #6 on: August 22, 2009, 11:03:11 PM »
well, how many AA batteries do you have to run them?
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Offline icoms8Topic starter

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Re: LED sign
« Reply #7 on: August 22, 2009, 11:54:41 PM »
How many is best. But don't want to have like 16 of them

Offline SmAsH

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Re: LED sign
« Reply #8 on: August 23, 2009, 12:01:28 AM »
nono, you could get away with 2... and then have all the leds share a single resistor and be wired in parallel...
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Offline Razor Concepts

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Re: LED sign
« Reply #9 on: August 23, 2009, 01:13:55 PM »
and then have all the leds share a single resistor

Don't do that, you will fry your LEDs. Put them in parallel, with each LED having its own resistor.

Offline icoms8Topic starter

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Re: LED sign
« Reply #10 on: August 23, 2009, 01:36:50 PM »
Ok get a resister for every LED. But how many AA batterys?

Offline Razor Concepts

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Re: LED sign
« Reply #11 on: August 23, 2009, 01:43:21 PM »
2 AAs should be fine, you may not even need a resistor depending on the LED. If you go 3 or higher, put a bigger resistor on the LED.

For more runtime you can put more AA batteries in parallel.

Offline Soeren

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Re: LED sign
« Reply #12 on: August 23, 2009, 01:59:19 PM »
Hi,

The LED needs 2.1V
2 AA's will fall to 1.8V near the end of their life.

Much to large a variation even with 3 cells, consider a Constant Current Device.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline icoms8Topic starter

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Re: LED sign
« Reply #13 on: August 24, 2009, 03:04:46 PM »
So will 3 AA batterys run 20 LEDs for 10 days?

Offline Razor Concepts

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Re: LED sign
« Reply #14 on: August 24, 2009, 03:13:23 PM »
Assuming the AA battery has a 2500mah (best case scenario), and the LEDs take 20ma of current each, you are looking at around 5 hours total life.

You should use a wall-wart  :)

Offline SmAsH

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Re: LED sign
« Reply #15 on: August 24, 2009, 03:25:01 PM »
but thats if they're all in parallel...
if you ran two sets of 10 leds in series you could have closer to 12hrs...

i must second the wall wart suggestion though... much better for this application...
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Offline Soeren

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Re: LED sign
« Reply #16 on: August 24, 2009, 04:32:26 PM »
Hi,

but thats if they're all in parallel...
if you ran two sets of 10 leds in series you could have closer to 12hrs...
End-of-life voltage for 3 AA cells is 2.7V
Each LED is 2.1V
(Gee I hate repeating myself)
Either do the numbers cerebrally or break out a calculator ;-/
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline Finnik

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Re: LED sign
« Reply #17 on: August 25, 2009, 03:51:36 AM »
Smash, if you'd put 10 LED's in series you'd have to supply them with at least 21V, seeing as each LED needs 2.1V and voltage adds up in series.
Think outside the box... inside is to crowded.

Offline SmAsH

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Re: LED sign
« Reply #18 on: August 25, 2009, 03:56:40 AM »
i meant wire up 2 sets of 10 leds in parallel and have those two strains wired in series...
so it would use up 4.2v and a bit less current.
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Offline icoms8Topic starter

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Re: LED sign
« Reply #19 on: August 25, 2009, 02:28:41 PM »
how do i put them in parllel?

Offline SmAsH

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Re: LED sign
« Reply #20 on: August 25, 2009, 03:46:31 PM »
wire all the anodes to battery + and wire all cathodes to battery-
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Offline Razor Concepts

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Re: LED sign
« Reply #21 on: August 25, 2009, 04:04:27 PM »
i meant wire up 2 sets of 10 leds in parallel and have those two strains wired in series...
so it would use up 4.2v and a bit less current.

I'm no electronics expert but I am thinking that will also lead to frying the LEDs.. if one LED has just a little off on its internal resistance it will change the current balance.

Offline icoms8Topic starter

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Re: LED sign
« Reply #22 on: August 26, 2009, 05:04:21 PM »
so i need to wire 10 leds to 2 AA batterys and the other 10 to another?

Offline Razor Concepts

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Re: LED sign
« Reply #23 on: August 26, 2009, 09:20:54 PM »
Just make sure each LED has its own resistor... never have LEDs paralleled and then have a resistor coming from that.

Offline blake

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Re: LED sign
« Reply #24 on: August 31, 2009, 02:36:26 AM »
Just make sure each LED has its own resistor... never have LEDs paralleled and then have a resistor coming from that.
what .......a resister for each led it would be easier if you had 10 leds and wired to 2 leds at a time

 


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