By Vlad Rex
The first image shows the complete system. Cameras are sitting on battery packs, so everything looks a bit bigger then it should, but I save a lot on lithium battery costs. Note, that only three cables connect both cameras through available remote release electrical connectors. Camera on the left side is the master. Once the release button is pressed there, both cameras fire simultaneously (of course each camera must be turned on first, unfortunately that must be done for each camera separately). Connection cable has three wires: ground, preset and release signals.
The second image shows a close-up of a connector on master camera. The plug is "double-ended". This way the remote release Pentax cable can be still used, when needed (as shown, but it is never needed in free-hand photography). On the lens you can see a piece of a ribbon cable socket. Two pieces of it were used to create the plug that connects to Pentax remote release outlet.
The third image has all cables disconnected. To make a thinner part of the plug, three openings were removed, leaving only one row that fits to camera connector. Visible black shrink wrap hides diodes isolating the cameras electrically in one direction. Original Pentax release cable can be connected to the socked via angled header (normally used electronic boards as terminators for jumpers or connectors for ribbon cables).
Now, how do you select a master camera. Each camera has it's own power converter transforming battery voltage (that goes down with use) to a stable source for it's electronics. Most likely one camera will not have the same internal voltages when compared to another camera. The difference will be in a fraction of 1 Volt. Measure voltages in both cameras. Connect a digital voltmeter to ground and release (or pre-set) pins. (make sure it's not an old analog type using coils - they may induce some high voltage and damage the camera). Use as a master the camera that has a higher reading. This is done to prevent a parasitic battery drainage when both cameras are connected with our cable.
Cable connecting two cameras should have diodes placed as shown below:
(right camera; master) (left camera; slave) __ / | | o | o--------------------------o ground | | | | |/| | o | o--------------| |---------o shutter release | | |\| | | |/| | o | o-------| |----------------o preset (exposure setting) |___| |\| socket
With this implementation, pressing a button on a master camera will release both, because the ground level will be applied to control pins on both cameras (directly from the push button on the master camera and on the slave the level on pins will be pulled to "0" through the diodes); pressing the button on the slave camera will release only the slave one, since the diodes will prevent ground level propagation to the master camera.
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