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).]