The way this remote works is that the decal has a little springy plastic area (the button) and inside of this little bubble is conductive material. When you push the button down you are pushing the conductive material onto thin metal fingers on the circuit board, essentially shorting it out, closing the switch. Ignore the copper zig-zag below, it's an optical illusion, we are interested in the interleaved gray fingers.
What we are going to do is hook up some wires to the chip in the same place that the button is hooked up. This lets us add an external button. There are two ways you can do this. If you are really talented at soldering or have a buddy who is and have some very thin wire you could attach the wire on the top and route the wire out the side. Those dark areas on the board and the two through holes are where you'd connect.
I chose to modify the component side of the board and have a little connector go out the side. Here's a close-up of the Nikon IC showing where I soldered the wires. Before I did that I first super glued a piece of a chip socket to the side of the IC. This will be the remote's plug. If you look closely at the plastic case to the left you'll see where I cut out a notch for the plug.
As you can see, pin #1 and pin #5 are used to trigger the remote. Double check for solder blobs and shorts when you are done. Here's a picture of the socket material I used for the plug. It's actually an IC socket, something you can pick up at radio shack or most electronic stores for a buck or less. You'll also want a switch. In this case a momentary N/O (normally open) switch.
Once you or your buddy have glued and soldered things into place it's time to put the remote back together. Actually, before you do, you can pop the PCB board back into the case (where the battery is) and try shorting the two plugs together and see if your camera fires. If it doesn't, make sure your battery is making contact and double check your wires and look for shorts.
Here's how my modified remote control looks. Fit and finish is a little off, mostly because I've taken it apart a few times to photograph it for this article. The good news is that it still works like a normal ML-L3.
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