I needed dimmable photography lights for a photo booth project I'm building for a friends wedding. Pre-built lights are quite expensive so I decided to build my own. I based the design on white LED strips which can be purchased quite economically when purchasing directly from Chinese distributors (Like aliexpress.com)
I went with the largest and densest LED strips I could find. The LED strips come in 5 meter lengths; I found for $12 (inlcuding shipping) I could get a 5M LED strip with 600 LEDs. The leds are the 5050 size which is the largest commonly available on LED strips. The 5M of LEDs draw 72W.
Building the Panel:
I build a quick panel out of some white hardboard and pine trim I had around from home renovation work. I sized the panel to fit 10 50cm lengths of LEDs. This made a nice ~60cm x 20cm panel
The strip is directly powered off of a 12V source (I used an old ATX computer power supply). Each segment of the strip was wired to a 18 gauge power supply wire. From here it can be connected directly to 12V or via a control circuit as I have done.
The photo booth I'm building runs on a raspberry pi embedded computer. Attached to the raspberry pi is a PCA8685 I2C PWM driver. Adafruit sells a nice breakout (http://www.adafruit.com/products/815
) board to make wiring it up easy. This way the photobooth control software can simply set the desired PWM duty cycle and be done with it. The PWM output of the PCA8685 is connected to a mosfet which switches the LEDs on and off at the right duty cycle to set the brightness.
Mosfet Dimmer Board:
I designed a simple four channel low-side mosfet switch board for connecting the LEDs (and other DC things) which needed control from the raspberry pi. The hardware design is open source and can be found here: https://github.com/scottjgibson/4ch_mosfet_switch
erm I'm not a photographer so not totally sure on how fast shutter exposure times are but I think there may be a flaw in this system. PWM works by flashing the lights on at full brightness for a fraction of a second then off for a fraction of a second and by adjusting those fractions it creates the illusion of dimming. If the shutter speed of the camera is fast then your exposure could take place entirely within the 'on' or 'off' cycle of the lights leaving you with full brightness or full darkness in the final picture and not the dimmed effect thats visable to a mk 1 eyeball. I had similer problems with mine as the flicker frequency of my pwm lighting controller matched up with the flicker time of my 3d glasses which ruined the ambilight style system id made when watching 3d films.
The PWM speed is many times faster than the shutter speed so it should not be a problem.
Let's say the shutter speed is 1/16th of a second and the PWM is 34k, which means that the LEDs will flash over 2000 times while the shutter is open. It would take some fast movements to get strobing.
Best regards, Jack
Update: the Adafruit driver board is only 1000Hz max so that is 1000/16 for the number of cycles while the shutter is open. That's still 62 flashes and that should be adequate.
Awesome job. I'd be interested in creating a 'box' using this strip idea to border (in an attempt to evenly light) a green screen. Any idea if recording video at 24fps or even 60fps show any strobing effect due to the PWM?
I'll have to take an updated video, I chaned to a diffuser from a overhead neon ligh fixture and it's much more even, works really well.
I really love to read out your blog, i guess you have made huge researches about led lighting strip and led lights. keep it up.
Do you have any of those extra 4 channgel Mosfet boards ready to sell? I'm interested in purchasing about 4 of them if available. All the boards I've found are in China or out of stock in the US.
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