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I occasionally use the RS60 cable release with my Canon T3i:
The Canon RS60 E3 replicates the functions of the shutter release button. The remote switch, on a 2-foot cable ... is compatible with the Canon EOS Rebel ...
There isn't any battery in the thing (unlike the wireless remote shutter release), and, unlike the ancient cable release, this gadget doesn't connect directly to the shutter button. How does this work?
Basically the cable release is powered by the camera.
The cable release doesn't send a signal to the camera, like the wireless remote. Instead the camera provides a current in one of the leads, and the cable release closes the circuit so that the current flows back to the camera.
In older cameras the shutter release buttons were mechanical, but nowadays they are electronic (in most cameras), so the cable relase works just like the ordinary shutter release button, only that the wiring extends outside the camera instead of going internally.
The details of how an electronic switch work aren't appropriate for the site, but its just like any other button the device. Imagine if the electronic connections on the bottom of the normal shutter button were very long wires...and there's your wired shutter button.
It makes the connection and then the software detects a shutter button press.
The Cable plugs into a port on the side of the camera which is connected in parallel to the normal shutter button. Now you have two buttons, one on the camera, and one that is at the end of the cable.
Note that it is possible for the camera manufacturer to design the plug in such a way that when you plug in the remote shutter button, the internal one will not work, much like when you plug a headphone cable into your laptop, the speakers turn off. I am not certain whether the T3i does this, but it does not matter in most cases.
There are a number of web sites that have instructions for building a DIY remote shutter release. Here are just a few:
http://martybugs.net/photography/remote.cgi
http://www.doc-diy.net/photo/eos_wired_remote/
All three of these sites contain schematic diagrams that show exactly how a remote shutter release can be wired -- it's essentially just two switches connected to the appropriate conductors of a 2.5mm 3-pole plug. As for the batteries -- none are needed. The switches just complete a circuit in the camera.
