For the last couple of months I have been busy working on a rather special project. My own invention, in fact. Today though I can finally pull back the curtain and reveal my prototype!
A laser trigger takes a photo whenever something crosses a carefully positioned beam of light. If one wishes to photograph elusive nocturnal creatures such as badgers and mice, then it is preferable to set up an automated system and return home to a warm bed instead of prowling dark, cold woods at 4am in the morning. The mere presence of a human may frighten such creatures away.
Split-second triggering is also critical for capturing moments that are too fast for mere mortals to catch: Ice-cubes splashing into soft drinks, sprinters crossing a finish line, insects in mid-flight... Wherever precise positioning, timing and repeatability is necessary, automatic means are often the only reliable solution.
Unsurprisingly, this particular flavour of photography is rather specialised. A lot of the necessary technology is very expensive and built to order only. For this reason many people like myself roll their own. Not only is it a fun and educational electronics project, but you have complete control over every facet of design and operation.
The laser trigger consists of three main parts: The laser, the sensor and the control unit... I will be testing these over the next few weeks in the Highlands of Scotland.
The control unit connects to the camera via the cable release socket. A signal from the camera's x-sync socket - normally used for firing strobes - helpfully tells us if a photo was actually taken. Useful if you want to ensure that the battery and memory card are okay. Feedback also enables the use of auto-focus if so desired.
Not just one but two laser-sensor pairs can be connected. You can cross the beams for ultra-precise, pin-point triggering, or create a cage of light.
A flash can also be connected directly if the camera proves too sluggish. Although a good DSLR may fire off exposures as short as 1/8000s, it can still take as much as a third of a second for the internal mirror to flip up and for the shutter to begin opening. This is known as shutter lag. To get around this, it is sometimes best to rely on a short, sharp burst of flash instead. Since the flash is purely electronic and has no moving parts, it's response time is far quicker.
The laser module actually contains both red and infrared lasers. The infrared laser is completely invisible to us and most animals, making it perfect for covert use. But if you can't see the infrared laser, how can you possibly aim it at the sensor? That's where the visible red laser comes in: During setup, we can flick the weatherproof switch on the back and see more clearly which way we are pointing! A buzzer inside the control unit also lets us know when we are on target.
In order to automatically adapt to changing light levels, the laser is pulsed hundreds of times a second. This allows the system to measure both the ambient light and laser light levels. By comparing the two we can reliably detect sudden breaks in the beam. We can also detect when the beam has shifted out of alignment or been knocked over.
Every laser module is paired with a sensor. Theoretically these can be placed far, far apart - a laser beam may extend for hundreds of metres. Realistically though, one is limited by the ability to aim the laser at the sensor's lens. Five-to-ten metres is a more practical range.
My first attempt at a sensor was rather crude, consisting entirely of a single photo-transistor and nothing else. Trying to align the infrared laser with this rudimentry detector was practically impossible: The laser beam was little more than 2-3mm across; the sensor was a mere 1mm square. The beam was also invisible. Attempting to align the laser was like trying to thread a needle at five metres with your eyes closed! Additional optics were therefore required in order to gather the laser light and focus it on the photo-transistor.
The laser trigger is microprocessor controlled using an Arduino Pro Mini programmed by myself. It is the brain of the system, responsible for pulsing the lasers, reading the sensors and figuring out What It All Means. The delicate camera and flash electronics are protected from the rest of the circuit by opto-isolators.
And there you have it. This is just the start. Over the coming weeks I will be subjecting the laser trigger to the cold, harsh, unforgiving environment of the Scottish Highlands. At the moment it is undergoing a rigorous shake down, whereby the most obvious glitches, quirks and missing features are identified and fixed. Once I am reasonably confident it can be left for long periods alone, I will begin setting it up further afield in the nearby countryside...
