Available hot shoe plugs to connect the camera’s hot shoe to another device are bulky (1) and do not fit in an underwater camera housing.
I have built a simple plug with an open-market hot shoe cover (2). First, a 5 mm hole is drilled, aligned with the hot shoe’s large round contact (Fig. 1). A spring of a ball point pen, cut to the proper length, with an insulated wire soldered to it, pressure connects to this center contact.
The plug’s other contact is to the hot shoe rail, made from paper clip wire, bent at right angles, with its bent end secured with ABS glue in a 1 mm hole drilled in the hot shoe cover (Figs. 1 and 2). To provide space for this wire on the hot shoe cover, a bit less than one millimeter is trimmed lengthwise from one of its sides, using a fine tooth hack saw blade.
ABS plumbing glue secures the spring, paper clip wire and battery holder to the hot shoe adapter.
The electric circuit configuration depends on the plug’s intended use. For example, in a simple circuit, an electric wire is soldered to each contact, such that when the camera is fired, the hot shoe center button is connected to the ground frame and “flashes” the connected device.
Or, a 3V coin battery holder can be mounted on the plug, with insulated #22 wires soldered to its terminals and to the center spring and paper clip wire (Fig 3), such that when the camera is fired, the battery’s 3V is “flashed”.
Two other circuits, proposed by ajay and by bvanant (3), trigger the device using transistorized circuits.
With a hot shoe plug mounted on an underwater housing’s camera, one should be able to flash the device of one’s choice.
References:
1. The Vello hot shoe plug (adapter) sold by B&H PhotoVideo is a bulky example.
2. https://www.amazon.com/hot-shoe-cover/s?k=hot+shoe+cover
3. This post is a continuation of this one with comments by ayay and bvanant: https://wetpixel.com/forums/index.php?/topic/69864-do-it-yourself-led-strobe-trigger-question/
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