One Exposure Two Events


Hi Folks, I’ve been asked to explain how the above shot was created. Easy thought I, until I tried to do it. A practical demonstration would be far more effective but unfortunately SIGs are still a thing for the future (and I’m still trying to work out how to show it without hurting anybody). Anyway, I’ve taken a few shots which I hope will go some way to describing the process involved.

Here we see the home made crossbow used to fire the dart (previous ammo included pencils, ball bearings and more darts). The complications start when we try to get the release of the dart to trigger the illumination of the bulb. This type of shot relies on the exposure being provided by speedlights, and thus typically the camera shutter will be set to about two seconds (in near total darkness). If the bulb is already on when we open the shutter the exposure will be blown completely by the time the dart finds its way to the bulb. It’s impractical to try to press a shutter release at the same time as switching on the bulb and firing the dart.

Triggering is achieved by the inclusion of a spring switch which goes “closed circuit” when the bit of plastic holding the contacts open is pulled out automatically by the dart being fired. The closing of this switch turns on a relay (low voltage control) which in turn controls it’s own internal switch (240V rated) to allow mains voltage to flow through the dimmer control which ultimately turns the bulb on. Why not just wire the dimmer directly to the spring switch? Because we’re working with mains voltage and having 240 volts going through exposed contacts would be a bad thing. I’ve been bitten by electricity numerous times (ex electrician) and though it’s never nice I’ve managed not to be killed once. Best to keep it that way.

To the left we see the set up for the firing of the speedlights (two used). This is the brains of the operation. The blue box contains a programmable Arduino unit which is often described as a mini computer, (more accurately called a micro controller as it doesn’t actually compute anything). This unit is programmed to switch things on or off (action) as a reaction to an external event.
In this instance the event is the sensing of an impact by the little Piezo sensor shown in the image. This sensor creates an electrical signal when it senses a vibration (caused by the dart hitting the bulb). The Arduino unit responds to this signal by closing yet another switch, this time causing the flash to fire which is plugged into the back of the box (the second flash also fires at that is set as a slave).

Although a lot of this may sound a bit daunting It’s actually not that hard to set up, once you’ve made a crossbow etc. What may appear to be the most off putting would be the creation of the blue box of magic tricks. Arduino isn’t that hard to get to grips with, if you’re comfortable with Photoshop Arduino is a breeze as you usually don’t need to write any code yourself (stripped back version of C++).
I have found that I don’t have any original ideas which means that the creation of a device that will fire a flash on demand (vibration, sound, light, minimal use of Semtex) has been previously resolved by somebody else. As Arduino is designed to be “open source” these previous others are usually only to happy to share their code and schematics (to enable the physical building of the device itself).

As an aside, one thing for me that makes the original image special is the fact that the bulb isn’t illuminated when the dart first hits it. Pixel peep and the dart can be seen lifting the yet to be lit filament. The bulb illuminates immediately after hence the recording of the second event by the long exposure. To prove this shot wasn’t a complete fluke see below for a second attempt where the relay has been triggered fractionally sooner thus the bulb is illuminated on point of impact. Very technical adjustments where made, I shortened the piece of string holding the bit of plastic in the spring switch.