I'm duplicating his instructions, with his gracious permission, here for the sake of clarity (it falls across a few separate threads there).
My wife and I love doing self portraits of ourselves on vacation and when we are camping because we can't trust others (family or friends) to take to photos for us. You know how it is, person smack dab in the middle of the photo without any composition and likely blocking the landmark and scene. So normally do the 10 sec self timer and hopefully get into the scene before the camera fires. Once I was running to the edge of a cliff to get a photo of us on the overhang and show the valley below and the side profile of the cliff. Needless to say this photo was attempted many times since the I ran out of time before the camera fired. Of course I didn't want to run to fast fearing I would slip over the cliff. After that I buy an IR remote from ebay for my XT hoping I could use this instead of the 10 sec timer, but I was greatly disappointed by it's range. No more that 5 metres if I was lucky and pointing exactly at the sensor.
I've been reading on this forum about DIY/homemade remotes and I was intrigued. I had a couple of mini two way radios or walkie-talkies lying around and decided maybe I could convert this into a homemade Zap-shot. I'm no engineer but it seemed simple enough. My friend who is an engineer said all I needed was a relay switch. So he got one for me. I decide the most simple way to do this was to hook up the switch to the speaker of the radio and when the confirmation beep went off, it would fire the camera. Presto. It worked. If I pressed the ring but, since it is a series of rings, it would it cause the camera to fire a few photos. And the neat thing is the range is 3 kms. Although currently don't know when I need that sort of range.
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First here's a link to how to make a wired remote: http://www.chantalcurrid.com/remoteControl.htm
Using the same principles as the above link. I used a relay switch instead of a physical switch:
Radio Shack (I think I used the 0.5 AMP one.)
Then I connected the relay switch to the two-way radio speaker so that when the confirmation beep sounded, the current would flow through the switch causing the switch to turn on long enough to fired the camera. The speaker volume of the radio should be on the loudest to give enough current to the relay switch.
My design is a very simple one since I'm no engineer. The confirmation beep length gives enough current to fire the camera but not long enough to AF and fire the camera. So I AF via the camera and compose the photo behind the camera and switch the lens to MF to bypass camera AF when I signal the camera to take the picture with the two way radio which is fine for my purposes.
Neat how there is a confirmation beep. This way I can confirm the photo was taken. If I press the ring button, since there is a series of beeps, the camera takes multiple photos which is neat for group shots.
If you have a two-way radio with a earphone jack, likely you can use that instead of connecting to the speaker directly. Even cleaner design.
Since nothing is dismantled from the two way radio, it can be still used as a two way radio. :)
I would be interested in seeing how others make theirs. Maybe some even better innovations and ideas I could use for my design.