[bertwert]

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OK this might not be the best forum to ask on, but hopefully someone will help me
I've got 100 5mm white LEDs to make a ring light to use for macro photography, here's some questions:
How many LEDs? I was thinking around 40-ish?
Should they be wired in series or parallel?
What batteries to power it?
Potentiometer to change brightness?

Thanks for any help!

[amoringello]

You're probably going to want to get a book on the subject.
First off, LED lights are generally not directly dim-able. Instead you need circuitry that actually flashes them on and off so quickly that your eyes perceive them as brighter or dimmer.
This might affect your photos if you use a high shutter speed. I would suspect with Pentax's slow sync speed that this may not be much of an issue.
A potentiometer could definitely be used to control their speed and in turn their perceived brightness over time.
Almost any batteries should be fine. You're going to need to adjust the voltage for the LED and of the controlling circuitry anyway, so the exact type of battery isn't such a huge concern. Just design the system to handle what you have. But probably at least 3 AA to get you up to around 5V is a good starting baseline.

I've read a few forums with people arguing benefits of using series and of using parallel, and a mix of both for long strings of LEDs. Sadly, I'm not qualified to speculate who is "correct". :-(

[bertwert]

LED lights are generally not directly dim-able

Controlling the voltage to them would change the brightness... no?

I would suspect with Pentax's slow sync speed that this may not be much of an issue.

It won't be connected to the camera in any way... so I could use 1/6000 if I wanted

I've read a few forums with people arguing benefits of using series and of using parallel, and a mix of both for long strings of LEDs. Sadly, I'm not qualified to speculate who is "correct".

That was the question I wanted answered most...
Thanks for your opinions.

And in case I wasn't clear this is a light that is on 'all the time' instead of a flash.

[bigdavephoto]

Watch this.

You Tube

You Tube

I have thought about building one but if I do, I would make a minor change or two.


He also has a video about building a light panel that shows the different way LED's work.

[amoringello]

Controlling the voltage to them would change the brightness... no?

It won't be connected to the camera in any way... so I could use 1/6000 if I wanted

That was the question I wanted answered most...
Thanks for your opinions.

And in case I wasn't clear this is a light that is on 'all the time' instead of a flash.

Generally LEDs are not dimmable. They are very digital, on or off. Some may have built-in circuitry to allow dimming, but if not, you will need to create your own circut.

As far as the high shutter speed, I was assuming a "constant-on" light.
Most (all??) LEDs need to flash to create a sense of differing brightness. This may lead to unpredictable result with regard to brightness or even total darkness if the shutter speed is faster than the dimming circuit.

I know LED panels are becoming more popular for photography. I assume more for portrait; i.e. in the range of 1/125 - 1/250 shutter speeds.
But maybe there is newer tech out there that makes LED feasible for higher shutter speeds. ????

Regardless, you will want to have details on your specific LEDs. Hopefully they will work.
Can't hurt much to try it. At worst they simple go out at lower voltages.

---------- Post added 01-22-16 at 07:07 PM ----------

... which by the way the next video on the LED panel mentions voltage modulation instead of pulse width modulation (PWW) which eliminates the flickering.
I learned something new. I will definitely look into building a few of these! :-)

[bigdavephoto]

---------- Post added 01-22-16 at 07:07 PM ----------

... which by the way the next video on the LED panel mentions voltage modulation instead of pulse width modulation (PWW) which eliminates the flickering.
I learned something new. I will definitely look into building a few of these! :-)

What he said.





That is what I was trying to say about the LED panels. .

[bertwert]

Generally LEDs are not dimmable. They are very digital, on or off. Some may have built-in circuitry to allow dimming, but if not, you will need to create your own circut.

Here for example he uses a potentiometer to adjust the brightness:
DIY Portable Ring Light
By increasing the resistance you are creating a lower current which then reduces the brightness of the LEDs.
(I may be totally mistaken somehow )

Most (all??) LEDs need to flash to create a sense of differing brightness. This may lead to unpredictable result with regard to brightness or even total darkness if the shutter speed is faster than the dimming circuit.

I'm going to need to go do some testing now, I never knew this before.

I assume more for portrait; i.e. in the range of 1/125 - 1/250 shutter speeds.

It's for macro, so with the lens stopped down a lot, I'll probably be using shutter speeds a bit slower than that 1/100 roughly - depends on LED brightness.

Thanks for the advice (again)

[excanonfd]

Why bother building one when you can buy one for the same price, ready to use: Meike FC 100 LED Macro Ring Flash Light With Ring FOR Canon Nikon Digital Camera 847567020078 | eBay

[Ray-uk]

Here for example he uses a potentiometer to adjust the brightness:
DIY Portable Ring Light
By increasing the resistance you are creating a lower current which then reduces the brightness of the LEDs.
(I may be totally mistaken somehow )

No, you are not mistaken. LEDs can be made to illuminate over a small range of different currents, however it is quite a small range between the light being just visible and burning out completely.
Using a potentiometer as in the link will enable you to vary it over this range but there are many problems.
The effect will vary wildy with battery voltage so although the chosen value of potentiomer will work when the battery is fresh then it wouldn't work once the battery starts to age.
The variation in brightness will not be linear and will probably not cover a large enough light difference to make it useful for photography.
The largest problem is that the colour temperature of the light will vary dramatically at different current settings.

[T Evergreen]

Rather than dimming LEDs, one could simply turn on or off individual LEDs in order to vary light output. With 40 lights, one could turn on either 10, 20, 30 or 40 to achieve 1/4, 1/2, 3/4 or full power and not have to worry about altering the color temp of the LEDs.

[bertwert]

Why bother building one when you can buy one for the same price, ready to use: Meike FC 100 LED Macro Ring Flash Light With Ring FOR Canon Nikon Digital Camera 847567020078 | eBay

Because the parts cost me $5 and I already have them...

[gavinhw]

Because the parts cost me $5 and I already have them...

I know it'd be cheaper and more fun to DIY. If you want an out of the box solution, there's this: Aputure Amaran Halo LED Ring Flash announced

I bought the HC100 model for Canon which works fine for any Pentax in continuous mode. It's dimmable though it might be too bright at its lowest setting in some situations. The Nikon version should also work. I bought mine off eBay, if I recall.

I suspect the dimming is achieved digitally. The LEDs are turned on and off faster than the eye can see. Not sure what the implications are with respect to shutter speed. I am currently helping my son with his science project which uses LED strips. In order to control the output (I had 120 LEDs in parallel, 2.4A current), the only solution I found was a $15 digital dimmer. Not sure how difficult it would be to build one yourself.

Good luck. Aputure Amaran Halo LED Ring Flash announced

[anemone]

About led control... Those are more current controlled devices. Still light output isn't linear. It is possible to get good results with PWM. Trick is to phase the ring some nice way and use high enough frequency.

LEDs also suck at color. White leds are blue leds which have yellow phosfor so it gives some kind of white light. As human eye is stupid it can be fooled to think not so continuous light in spectral domain being white. As camera is composing images from red, green and blue, discontinues in wavelength can give rather interesting results.

And last... I took a picture in "white" led light and automatic exposure underexposed. When I manually slowed shutter speed, I was seing +5 blinking in my K-3. The reason is that metering and sensor sensitivity wavelengths doesn't match exactly and I had peak in metering but not in sensor. Also naked eye guessing doesn't work in these situation. Worst leds are stage lights cheap ones being worst of the worst.

I haven't given this idea much thought about but I saw something in grocery store. One euro for optical fiber flower. Only thing it would need is the ring and then some duck tape to catch light from the flash. Might work or not. But I bought orange shoot through umbrella for warmer light. But it was actually just an umbrella with one euro price tag. That worked.

[maltfalc]

No, you are not mistaken. LEDs can be made to illuminate over a small range of different currents, however it is quite a small range between the light being just visible and burning out completely.

you're thinking of voltage. current can be adjusted from 0 to the led's rated amperage with a fairly linear change in brightness between the two. doubling the current gives a bit less than double the output because the led gets hotter and less efficient. there's also a slight shift in colour, which is why pwm is sometimes used.

[csa]

Bert, it's admirable that you are attempting to build this; I wish you success. If not, there are many very inexpensive ones available on the internet. I bought a used one here from a fellow member.