LED Flash

I was able to come back home for the long weekend and assemble the PCBs I ordered from OSHPark. I designed an LED flash driver and a constant current portable LED light thing (that I really don't know what to call) several weeks ago and only now have the opportunity to put them together.

Some background on why I made these: For EE lab in school we were asked to order free op-amps from TI, and I decided to just get some more free parts while I was at it. I wanted to make a simple LED flash for high-speed photography, and wanted to test out the UCC27324 MOSFET driver. And since I was already using LEDs, why not learn a bit about boost circuits and constant current LED driving with the LM3410 constant current boost LED drivers? All of this is good design practice in the end too.

Back to the LED flash. Here are the PCBs for the flash and constant current driver:

Purple PCBs!

Close-up of the flash.

Here some screenshots from KiCAD:

Schematic

PCB

Now all assembled:

All put together.

The design of this circuit is based off of a circuit found in this blog. and this paper. I'm not sure how critical it is to have gate resistors (actually looking around they seem important in preventing ringing oscillations on the gate), but I think this is fine. In fact, I probably don't need the driver at all but it's fun to use an IC just for practice. Also, here is an application note from Cree LED on pulsed over-current LED driving. I need to test out what is the optimal duration and voltage to get peak brightness out of the LED. Here are some preliminary test photos (the LED supply is at 12V, and the camera was set to ISO100, f/5.6, 3.2"):

10 us	50 us
250 us	500 us
1000 us	2000 us

I had another shot at 5000 us, but the apparent brightness of the picture didn't change much.

Things to do:

• Test the apparent brightness of illuminated objects, not the LED directly. 
• Try LEDs in series
• Different voltages to get different currents
• Test the sense resistor for measuring current
• Test rise and fall times and relative brightness with photodiode detector
• Test efficiency and find peak brightness

I'll update this with more info when I have more time to test. I need to assemble my portable LED next.

Resources mentioned in this post:

• Blog post from which my flash is based off of: https://petermobbs.wordpress.com/2015/02/06/experiments-with-led-based-flash-gun-for-high-speed-photography/
• Paper with similar flash design: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.474.6388&rep=rep1&type=pdf
• LED over-current application note: http://www.cree.com/led-components/media/documents/XLampPulsedCurrent.pdf