This blog is certainly not intended to be a 'how to build a DSLR setup' as there are plenty of great resources on the internet covering that, www.camtrappers.com being one of them. Instead, this post is intended to highlight some things that you don't obviously think about at the beginning of your build, as well as bits of kit that work well.
The case. While all components in the system are important, in my opinion the case is central. Get this wrong and the whole build could be ruined. Unless you live in a hermetically sealed world with no weather or dust your case needs to be weather sealed. It also needs to be strong and lockable. In the UK your case, containing your camera and unique images, is at most risk from weather and theft, rather than your subject matter. I use an adapted pelicase which gives ideal weatherproofing and allows me to lock it to trees via a thick security cable. This prevents opportunistic thefts and keeps an honest person honest.
You'll put a lot of effort into the creation of your case so think carefully about which camera you're gonna use because each setup is largely unique to the camera. At best you'll have to make adaptions to fit another camera into the case, but sometimes it's simply not possible. For example my case which was setup for a canon 5D will not take a 7D due to the variation in their dimensions. Try to think and build for the longer term.
Happily, most other aspects of the system can be updated without too much fuss.
PIR triggers. The Snapshot Sniper SSII PIR (sourced from the USA) is very good, but requires a bit of effort to connect it to a remote shutter cable, as well as to build a weatherproof housing, however, it does the job well. It does, however, lack flexibility, which is why I upgraded to the Camtraptions PIR v.2. My goodness the Camtraption PIR is flexible. It has numerous potential triggering modes which can all be customised. It'll take a huge amount of trial and error to get all these settings right but at the moment I only need a couple of modes for what I'm doing. The Camtraption PIR is a great off-the-shelf solution and probably the only sensor you could ever need, but it's more expense.
To avoid the necessity of retrofitting cables through my weatherproof case which would require the cutting and splicing of the supplied Camtraptions PIR-to-remote shutter cable, I opted to use the PIR in a wireless setup. This required the purchase of a second set of Camtraption trigger and receiver (see next) but adds a new dimension of flexibility to your setup.
Camtraption wireless triggers and receivers are great. They are so easy to deploy and totally reliable. The hahnel equivalent, while mostly good, were the source of many failures and were awkward to connect with absolute certainty. I ended up pairing the trigger and receivers each time I setup and was left the setup with fingers firmly crossed that they would work. I now use a Camtraption trigger and receiver set to link the PIR and remote shutter release on the 5D, and a second trigger attached to the camera hotshoe to fire the flashes (one receiver on each flash). The only thing you have to remember is to set the two wireless systems onto separate channels, which is very straight forward.
Flashes. I've used Canon speedlite 420ex flashes for a while. They work quite well for a 24 hour setup but they can be a bit slow in waking up, or that's my interpretation as the first image of a sequence was quite often black. I did make bespoke external battery packs for them which really boosted their endurance and sped up their recharge time considerable. Unfortunately something when wrong at one setup and the camera shutter fired seemingly continuously for a while, with the result that both 420ex flashes blew. I did replace the 420s from ebay and still use them sometimes. Eventually, I swallowed my Canon pride and managed to source a couple of Nikon ST28s from ebay which serious camtrappers rave about. In particular they are renowned for their standby duration and their wake-up speed coming out of standby. To date my experience is that they live up to their reputation although they don't appear to be as powerful as the 420ex.
Lighting the scene. It's pretty essential to use at least 2 flashes to light your scene to avoid heavy shadows. My gut instinct is that three or more flashes might be better, as you could reduce the power output of each flash and thus speed up charging. It might also allow for a low power backlight. Things to think about for the future. I try to deploy my two flash setup with one at low level (50 cm off ground) and one high (150 cm) and also use a diffuse flash (the lock'n'lock case does that job) to avoid harsh lighting. I think a high set flash provides a better depth of illumination. I'm also in the process of building some secure cases for the flashes (small peli-cases) as in theory the flashes are easier to steal than picking blackberries off a bush.
Composition. Perhaps the hardest part of the whole process. When I've grown used to catching technically nice images I've wanted to capture bahaviour or a more pleasing composition. Critical to this is observation for which I use a consumer trail camera in video mode so that I can see what the animals are up to. I recently noted that some foxes were using a fallen tree as a regular path to access and jump across a fence. I initially setup to catch the fox on the tree (see gallery). When they appeared to be comfortable with the flashes, I set up the system to catch the far more pleasing composition where the camera is at the end of the fallen tree. Using dog biscuits as bait gets the fox into the desired position or focal plane.
A small gallery of recent cam trap images