transistors and the infrared light spectrum

it's been quiet on the blog front lately. whenever that happens, you can be assured there's something new being developed behind the scenes. this time is no exception.

this probably won't excite the masses, only being appreciated by a select few i presume, but i'm pretty excited about it nonetheless. i had a problem that needed a solution: i've been planning a collaborative shoot that involves having a particular liquid-type substance being thrown at the model(s). i can't tell you anymore than that right now. i had been practicing the timing with my wife of many patience in our backyard and found that i could only catch the shot one out of every ten or fifteen times. even when i caught it, it wasn't necessarily the exact moment i wanted. i needed my flash to trigger automatically at the exact moment i wanted it to.

figuring there was already something out there, i went to ebay and started searching. the best i could find was some home-brewed stuff that cost $150+. that was a little out of my $5 budget for this. i put on my electrical engineering hat and came up with this:

the concept is simple: an infrared transmitter constantly emits infrared light and the infrared sensitive phototransistor receives it on the other end. when something gets between the two, the "beam" is broken and the flash triggers. painful details can be found at the end of this post.

i call it bluetrigger, mainly because i couldn't think of a name and also because it's easy just to put the word blue before everything.

here's a couple photos of me testing out the circuit. in the first one, i'm slamming my hand toward the breadboard and you can see that the flash fires before my hand can even reach it. the second shot is me flicking my finger across through the IR path. the reason my finger is blurry is because for testing i was firing one of my studio strobes. a speedlight has a much shorter flash duration which would reduce blur significantly. i'm waiting for a hot shoe adapter to get here in the mail that i need to connect to a speedlight.

one thing i forgot to mention - to do these shots you need to be in a semi-dark environment, put the camera to a shutter speed of two seconds or so (or more), open the shutter, then trip your flash with the IR beam break. this lets the flash expose the shot rather than the shutter. this is the basic premise of high-speed photography.

ok, that's enough nerding out for one blog post. if anybody out there finds this helpful or decides to make their own, i'd love to hear about it and any modifications you might make.

some details for those who care or would like to reproduce it:
the output of the phototransistor is an analog signal. sunlight and some ambient indoor light contains some infrared emissions, causing some interference with the desired signal. for that reason, the collector of the transistor is connected to an input on a comparator. the other input is setup with a voltage divider that lets you set the threshold that you want to trigger the rest of the circuit. this allows you to account for ambient to an extent and also lets you adjust the sensitivity. now, in order to actually trigger a speedlight, the center terminal must be shorted to the ground terminal (the ring above the shoe mount). for a regular studio strobe, you just short the two signals that come off the 1/8" jack. the comparator output runs into an optical relay which shorts its two output terminals. this is where you connect the speedlight or strobe signals.
the schematic shows an LED with a current limiting resistor (D2 and R6 respectively) that can be populated only if the flash is not connected. this is helpful to do while building the circuit in order to test it. D2 acts as the substitute flash, so you can easily see if it's working or not. it's also nice to have it there while setting the trigger up.

possible future upgrades and modifications:
adding an oscillator and a counter IC would allow the output signal to be delayed by however long you want. run the comparator output to the counter, then hook up whatever bit output from the counter that corresponds to your desired delay to the input of the relay.

put a little cylindrical piece of material with a reflective surface around the infrared emitting diode to concentrate the 'beam' and maybe extend the range.

add a switch to enable or disable the indicator LED (D2) rather than having to manually connect or disconnect it.

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