I just found something on the internet which has put a few thoughts in my head. Below it is copyed.
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"I took a small Palican watertight case, (this will carry the whole IR setup), inserted a sealed (gel type) lead/acid 12v battery, the output wires x2 go through the case wall via watertight connector posts. I'm using a 12v infra-red transmitter/receiver, that I got at Tandy (Radio Shack) in a sale for £12 UK, this comes with a small reflector to bounce the beam back to the unit, and interupting the beam activates the unit. The IR unit itself only has three connections, two for 12v power, and three for the switched output (one common, and one each for normally open and normally closed circuits). The common and normally open output feeds are used, and via 10 yards of cable, output via a 3.5mm jack plug into a converted Canon release adapter T3, the camera end of which obviously goes into a Canon camera. I can also feed this input from a radio release I made from parts similar, including a car alarm receiver.
Back to the IR beam break, the battery is quite large, and I'm not sure of it's maximum usage time in this situation, but all night and more seems likely. You could always fit a larger battery. On the end of the output lead to the T3 socket, I connected the metering and shutter release wires together, so they will both activate together.
I recently got a Canon T50 from ebay for £28 UK and that's about as cheap as they get. The setup with the T3 will also fit my T90, and my EOS 5, which when set to single frame AF will not fire until the object is in focus, so no worries there.
If you're still interested, or want to find out more, get in touch for more exact details, and maybe photos of the setup, which incidentally (without the Pelican case) could be built for about £40 UK, the price varying with the size of battery, although you could use a 12v car/ motorcycle battery from a junk yard."
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Think I might try it at somepoint. Whats the point in leaving a £500 camera out in the middle of a forest?! I think there may even be easier ways to do this for those who dont really understand this stuff and would still like to hear what any electric heads may think about this.