You can use this to even control two cameras at a time! Just make a second cable and there you go, control two cameras :D
Step 1: Well, Collect All the Parts Needed
*One RJ11 phone line cable
*one switch, two push buttons
*one phone line RJ11/RJ12 splitter
*one 2.5 mm TRRS connector
*one CANON EOS 450D
*cutter and the usual other tools
Step 2: Connecting the Wires
#1 will online be connected for FOCUS to the FOCUS switch
#2 will be connected to SHUTTER RELEASE and LONG EXPOSURE SWITCH
#4 will be connected to SHUTTER RELESE, LONG EXPOSURE and FOCUS SWITCH (GROUND)
Your phone line cable has 4 colors.
TRRS connector | RJ11 phone line cable
#1, black color | black color
#2, green color | green color
#4, copper wire | yellow color
part #3 of your TRRS connector and RJ11 red color wire will not be used.
If you have other colors or you want to be sure you have the right wires out of your TRRS connector use a digital multimeter to test it.
Put your digital multimeter on (usually a volume sign or a small speaker or something similar) and touch the red wire with the black wire of your multimeter. It will produce a beep sound that the two wires are connected.
So now put one probe (black) on one part of your TRRS connector. Put the other probe (red) on a wire that sticks out from the behind. If it beeps, it means you have found a pair, otherwise just put the probe on another part or wire until you find a match.
Step 3: Connect to Wires in the RJ11/RJ12 Splitter
I have removed the red wire already inside of the splitter to get more room.
The yellow wire looks like it's connected to the red wire, BUT IT'S NOT! The yellow wire is connected to the brown wire.
*The yellow wire of the cable that comes inside of the splitter is connected to the brown wires.
*The green wire of the cable that comes inside of the splitter is connected to the red wires.
*The black color of the cable that comes inside of the splitter is connected to the black wires.
*The red wire is connected to one part of the shutter release button and the other red wire is connected to the long exposure switch ( not the other side, but the other wire! the other side of the red wires are connected to the yellow wire ).
*One of the brown wire is connected to the shutter release button, the other one is connected to the long exposure switch and the last brown wire is connected to the focus button.
*Finally the single black wire is connected to the other side of the focus button.
I have used a hot soldering iron to make the holes for the buttons. Don't breath it in, and do this in a well ventilated room.