I need a little help from some LED experts, please.

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twebster
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I need a little help from some LED experts, please.

Post by twebster »

Hi y'all :D

I've been experimenting with some macro photography after dark, http://www.photomacrography1.net/forum/ ... php?t=3791. It's about to drive me nuts trying to focus with a flashlight illuminating my subject. :x I got to thinking last night that it would be so much easier if I had some sort of light that would attach to the flash. I think I would like to make a collar to slip over my flash head that would hold 4 LEDs, one in each corner of the flash head, and a small battery pack that could be velcroed to the flash head or to the flash bracket.

Now, I'm an electronics dunce. :D I don't know where to start to design something electronic like this. If someone could draw up a simple circuit diagram with the necessary electronic components I could take it from there and adapt it to my needs. Can anybody "out there" give me a hand with this :?: :idea:

Thanks, :D
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nephiliim
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Post by nephiliim »

Uhm i think there was a post a couple of weeks earlier about a home made ring flasher made out of cardboard and a few led's.. that one is a good guideline. I'm using the URL it is referring to too.(?)

haha

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Post by rjlittlefield »

Teva's LED ring is shown at http://www.mobisux.com/album/data/4340/3602led_ring.jpg

It appears to consist of 10 LEDs wired in parallel, with a single dropping resistor for all of them.

Radio Shack's white LED 276-320 is rated at 20mA max, 3.6V.

With a 9-volt battery, the dropping resistor would have to be at least R = (9-3.6)/(10*.020) = 27 ohms, giving a dissipation of (9-3.6)*(10*0.020) = 1.08 watts.

50 ohms at 1 watt or more should be safe in that case, e.g. Radio Shack 271-133, 50 ohms 10W wirewound.

Maybe teva can fill in exactly what he used.

For 4 LED's, you need more resistance but can get by with less power: at least R = (9-3.6)/(4*0.02) = 67.5 ohms at (9-3.6)*(4*0.02) < 0.5 watt.

100 ohms at 1/2 watt (Radio Shack 271-1108) should work OK for 4 LEDs.

--Rik

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Post by nephiliim »

Dude, you surely payed attention in physics didn't you?

LOL

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Post by twebster »

What's physics :?: :shock:
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Post by MikeBinOKlahoma »

twebster wrote:What's physics :?: :shock:
Tom, you've got it wrong. It is "What's Fizzix?"

:D :D :D :D :D :D :D :D :D
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Post by rjlittlefield »

nephiliim wrote:Dude, you surely payed attention in physics didn't you?
More years ago than I like to think...

Truth is, I don't trust calculations. I'm never comfortable until I actually build something and see it work.

But I haven't had time to build this one, so I thought I'd risk just "working the problem on the board" and hope that somebody else would catch any mistakes.

I laughed too, at how funny the posting looks. :D

--Rik

Remember: smoke is what makes electronics work. If the smoke gets out, the circuit's dead. :( :D

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Post by nephiliim »

twebster wrote:What's physics :?: :shock:
Physics is a educational thingy at school where they try to teach you tha laws of nature, the physical laws.. ergo; Physics

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Post by Ken Ramos »

Well, I don't recall much about physics or know a whole lot about LEDs' but I remember a cute little sweetie I used to sit next to in Biology class. We shared a microscope and a few frogs together. :wink:
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Post by nephiliim »

Ow.. is that how you call that lol....
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Post by GreenLarry »

rjlittlefield wrote:Teva's LED ring is shown at http://www.mobisux.com/album/data/4340/3602led_ring.jpg

It appears to consist of 10 LEDs wired in parallel, with a single dropping resistor for all of them.

Radio Shack's white LED 276-320 is rated at 20mA max, 3.6V.

With a 9-volt battery, the dropping resistor would have to be at least R = (9-3.6)/(10*.020) = 27 ohms, giving a dissipation of (9-3.6)*(10*0.020) = 1.08 watts.

50 ohms at 1 watt or more should be safe in that case, e.g. Radio Shack 271-133, 50 ohms 10W wirewound.

Maybe teva can fill in exactly what he used.

For 4 LED's, you need more resistance but can get by with less power: at least R = (9-3.6)/(4*0.02) = 67.5 ohms at (9-3.6)*(4*0.02) < 0.5 watt.

100 ohms at 1/2 watt (Radio Shack 271-1108) should work OK for 4 LEDs.

--Rik
Hey that looks really neat, and with qualifications in electronics Im sure I could put that together.
Question is, would it be ok photographically? That is as a light source itself?

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Post by rjlittlefield »

Here is some more info and useful links about LED illuminators.

1. For good info on powering LEDs, see http://wolfstone.halloweenhost.com/Tech ... esParallel. Important points are that a) it's really not best practice to wire LED's in parallel with a single resistor, and b) for powering large numbers of LEDs, a series-parallel arrangement makes best use of your battery because it puts more power into the LEDs and less into the resistor.

2. I have never bought from these folks, but http://www.ledsupply.com/led-catalog-5mm-leds.html seems to sell white LEDs for a good price and in a variety of emission angles. The ones from Radio Shack work fine, but they're a bit pricey.

3. As the only light source, you're liable to find that inexpensive LED illuminators are not as bright as you would like. Aside from microscope use with long exposures, I have only heard of LEDs being used for focusing and fill-in.

4. There is also a possible issue with color shifts that cannot be removed by white balancing. It all depends on how spikey the various spectra are (illumination, subject color, sensor response) and how the spikes happen to line up. As I understand it, white LEDs are actually miniature fluorescents (http://wolfstone.halloweenhost.com/Tech ... iodes.html), so I would naively expect the same kinds of problems. But I have not seen a careful study of this.

--Rik

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