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DIY 228 LED Panel, ~US$35

Last month, we attended a presentation on LED lighting for the Austin SmugMug group by Kirk Tuck (author of LED Lighting). It was the absolute neatest thing to see the real-time effects on the model of changing light source size (backlit diffusion panel) and its proximity to her.

I have long been eyeing LED panels but have not yet taken the leap to buy a commercially available one…

Last year, I dabbled with LED festoon “bulbs” off eBay, a four cell AA NiMH battery pack and an adjustable step-up DC-DC converter but I have not really been satisfied with the results (mostly the lack of an easy way to mount and use it). I also won’t bother posting pics of it because it was never fully completed. It is pretty bright, though with just 24 LEDs. They are cool white, so to make it more natural, I placed three thicknesses of 1/8 CTO over them.

After seeing the live demo, I set about to finally build my own large and bright LED array based on the LED strips as was done in this video. I opted not to use an RC throttle and servo controller because the brightness can be controlled more easily with a purpose-built PWM dimmer.

Here is the basic parts list:

  • 5 meter LED strip with 300 LEDs in 5050 package (three LEDs in a single package)
  • 12V PWM dimmer
  • Spigot for mounting on a light stand umbrella mount
  • 9×13 baking pan with plastic lid (from Walmart)
  • AC mains power adapter: 12V @ 2.5A or higher (or 12V jump starter)

I ended up finding better prices for the LED strip and PWM dimmer at eBay ($24 and $3, respectively, but I’ve since seen LED strips a little cheaper). I like the color of warm white, so that’s what I ordered. I already had the spigot stud and power supply. I bought the baking pan at Warmart for $4. I chose a 5050 packaged LED strip because there are three LED chips inside instead of a single chip, hence more light output.

The LED strip came with double sticky tape on the back, and while that’s probably insulating, I did not think it wise to directly mount the strip to the bare metal baking pan since the backside of the strips had bare copper showing. I used a sheet of Coroplast as a substrate. I laid down 11 strips of 18 LEDs (the strips can be cut into lengths that are multiples of 3) and two lengths of 15 on the outside edges to allow for the corners of the pan. The strips should be oriented the same way so that the positive and negative solder pads are all on the same side to make wiring it all together easier. I soldered jumper wires between all the strips.

I connected the leads that came with the strip to the output of the PWM dimmer. I added a barrel connector pig-tail to the input side for the AC mains power supply to plug into. The dimmer is mounted on the outside of the pan with the wires taped down to avoid snagging them on anything.

Here is the completed panel:

I mounted the spigot on the same end of the pan as the dimmer, off center, so there would be a convenient way to strain relief the power supply cable. The head of the 1/4-20 screw can be seen in the photo above.

At the moment, the sheet of Coroplast is taped to the back of the pan with gaffer’s tape. Because of the heat that was generated, I ended up cutting the back sheet and the fins off the sheet so the heat would better transfer to the pan. I did not pull the LED strips off the Coroplast because I don’t think they would have survived unscathed. I’d recommend a thin sheet of plastic contact cemented to the back of the pan or maybe a teflon coated baking pan. Once I get some cement, I intend to glue the sheet down for better heat transfer.

Here is the dimmer mounted on the pan:

During some initial testing with the video camera, I could see banding from the PWM at lower intensities at all shutter speeds except for 1/100 sec. The PWM operating frequency is somewhere between 300-400 Hz. To smooth out the pulses, I found that adding two 1200 uF capacitors inside the case and across the output filtered it nicely, removing the flicker. [Note: I think adding filter caps to the PWM is essential for video. From reading reviews, I think many commercial LED units don't bother including them.] The 228 LEDs draw just over 2 Amps when supplied a constant 12V.

You can see the three separate points of light in each LED here:

I saved the lid because it provides a nice front cover to prevent damage to the LEDs when packing it away. I don’t think it wise to keep the cover on when the lights are brightly illuminated even if holes are drilled it because there is better airflow when no cover is present at all. Here it is with the cover on but dimly illuminated:

Now that I’ve built a bright panel, I need to get some diffusion material and take some actual photographs with it…

[Note on the photos in this post: I took these using a grid made from Coroplast at fairly close range (about 2.5 feet). The flash was off camera at 1/16th power, ISO 100, 1/125 sec @ F7.1. The intent was to highlight the subject and eliminate the distracting background (without having to actually clean up, move stuff around, and set something up).]

 

 

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Posted in diy by david on April 22nd, 2012 at 4:54 pm.

8 comments

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8 Replies

  1. I’ve been hunting the web trying to find out about these dimmers for the strips. Have an idea identical to yours that I’ve been wanting to try out, so this was a nice surprise.

    Would you mind explaining how you added on the capacitors to the dimmer?

  2. Naturally, I don’t have any photos… I tend not to take any while I’m in the process of building something. And then, when it’s all done and works, I don’t feel like deconstructing it to show the inner details.

    I opened the plastic case by removing the 4 screws in the bottom. There was enough empty space inside the box to solder the capacitors across the output terminals inside the box and still put it all back together without anything shorting out. I don’t recall exactly whether I had to wrap the capacitor leads with something or not.

    During testing, I had the capacitor connected at the green plastic screw header where the wires for the panel are. I did not try moving the caps (it was two in parallel, I think, or at least when testing there were two. I don’t recall if it ended up being just one.) inside the box until I was satisfied with the results.

    If you’re using these for just LEDs and for video, then you will probably need to add caps. If you’re going to use this dimmer for a motor, you need to add a protection diode for back EMF. If you’re going to use this for still photography, you don’t really need to add any caps at all.

    I also have two dimmer like these that I bought later:
    https://www.amazon.com/dp/B007RFMY1O/ref=as_li_ss_til?tag=goliyowo-20&camp=0&creative=0&linkCode=as4&creativeASIN=B007RFMY1O&adid=0TW1N0KA8F18W5XFJ4FN&

    I am not so pleased with these since the frequency is around 69 Hz; they noticeably flicker. Adding a capacitor does not help too much because you either still see some flicker or the cap smoothes it out so much that the LEDs end up being pretty near full brightness over the whole range of adjustment. I’ve not tried to see if I could change the PWM frequency and I’m not sure it’s worth bothering. The other factor is that the panel I was testing with draws a whole lot less current than the original, so it will naturally need less filtering capacitance. (This panel was made with the left over strip from the panel described above.

  3. Thanks for the reply, David!

    I’m intending on building it for video purposes mainly. I was planning on getting a roll of pure white and warm white and similar to what you did, aligning it so it’s PW WW PW WW, and wiring up all the PW to one dimmer and all the WW to another, and hence being able to blend the two to get whatever desired colour temperature.

    I’ll have to look into this more I suppose and figure out how to dim it, as I think I want to use up all 16 ft of each colour/roll.

  4. I’ve thought about doing the same thing but what holds me back is what to use for a substrate for the LEDs that’s not flimsy and can travel well. We don’t have the luxury of a studio and often pack everything up and go on location, inside or outside, somewhere else…

    I already have some 6500K panels I bought from someone that does outdoor signage, but they run at 24 volts. I’ve not used them much because they also don’t travel or set up well. I just ordered one or two more dimmers that look like this for them since the little ones won’t work. I am hoping the innards will be equivalent to the original in the surface mounting box. I’m still not sure how I will box these up, but I at least want to be able to use them at something other than full brightness.

    One thing that I think is not well addressed with the sticky LED strips is the heat dissipation. I’ve not seen any failures yet but then I’ve not run them for very many hours. I think the panel above has less than 10 accumulated on it. We’ve only just begun delving into video…

    Here’s one instance where I used the baking LED pan-el. The hair light in the back is a three LED E27 base warm white LED screwed into one of those light socket on a light stand things. Unfortunately, I did not bother taking any BTS shots as we were pressed for time to shoot all the kids for this…
    https://vimeo.com/58595806

  5. Been caught up and have just started thinking more about these panels; that footage looks pretty great.

    You were mentioning that the problem with dimming was the PWM Frequency. Making the boards myself would be way over my head so I’ve been looking around. I saw one big name LED panel company saying that they use a 20khz frequency to dim their product, and judging by the price tag I don’t think that it flickers at shutter speeds (at least I hope not!)

    In lieu of that, I found this. 13khz PWM frequency. Do you think this could work?

    http://www.ebay.ca/itm/12V-40V-10A-Pulse-Width-Modulation-PWM-DC-Motor-Speed-Controller-Switch-24v-36v-/230933316357?pt=LH_DefaultDomain_0&hash=item35c4b2c305&_uhb=1#ht_4563wt_889

  6. 13kHz is significantly higher than 300-400 Hz, so it should do well.

    At 300 Hz and 30 fps video, that’s about 10 flashes per frame of video. But then what is the shutter time of the video camera? It depends on how much light there is and the sensor sensitivity, etc. So some frames will get fewer blinks than others and that results in flicker.

    At 13kHz, there are 433 flashes per frame at 30fps. Even when the shutter speed of the video camera is fast, there should still be, on average, about the same number of flashes per frame, so the flicker should not be apparent.

    Ideally, the best way to control the LEDs would be to use a current source and then vary the current. I’m not aware of any cheap/easy ways to make or buy those, so they’re kind of out of the question.

    One nice thing about this dimmer, if there is indeed no flicker, is that it goes from 10-100%. With the capacitor with the dimmer I have, the minimum brightness is fixed by what the capacitor stores and what the panel(s) draw. If more panels are added to what I’ve done, I’d expect the minimum to change some.

  7. Wow, that’s pretty fascinating actually, it’s cool understanding how these things work.

    I’m going to try making a panel with these dimmers and will post you my results/keep you updated.

  8. I know it’s been a few months, but I just wanted to let you know that I got that dimmer and it does work. No flicker noted at all. It’s currently being used along with the LED strip as a replacement for the fluorescents in our kitchen.


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