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I'd like to connect my own device (a device that can record the trace of the camera within the exposure time) with my SLR camera (a Canon EOS 5D markII) through the hot shoe. I need a signal to tell my device the shutter is pressed. Can the camera itself can generate a signal when I press the shutter? If not, how does the camera trigger the flash from the hot shoe?

According to Wikipedia, the hot shoe and the contact point are isolated when the shutter is not pressed and are shorted when the shutter is pressed. Is this still true for the Canon EOS 5D markII? Do I need to add external voltage to have that signal or is the signal generated by the camera itself? If I need to add external voltage, then what is the maximum voltage I can add?

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The circuit will connect the shoe mount to the center pin to fire. It sounds like up to 6 volts is what Canon asks for, but it can apparently vary a lot depending on the flash. Other pins may contain other signals, but that portion is proprietary in most cases so you'd probably want to use the circuit completion and provide your own low voltage signal. The voltage is normally provided by the flash itself since it has to be the appropriate voltage for the flash's triggering circuitry. The camera only acts as a switch.

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For Nikon you can find out about the other pins here: – Unapiedra Apr 24 '13 at 21:26
You can also utilize a camera's PC Sync Port if your model has one. I know the Nikon D4 and Canon 1DX both have them. Nikon calls it the Flash Sync Port and Canon refers to is as the PC Sync Port. This type of connector has been around for a number of decades and provides a very simple open/close circuit that, if memory serves, does not utilize any voltage across the terminals. It should be more of a infinite/null resistance situation. Whatever you have connected to the camera, I highly suggest using something like a optocoupler device to separate the two circuits. – ISOTropic Pixel Apr 26 '13 at 23:17

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