On the left is the Raspberry Pi, which connects to the outside world via a multi-way GPIO interface,
here connecting to the home-made switching circuit via a ribbon cable. Signals from the Raspberry Pi
switch on transistors which send the necessary voltages to the solenoid valve for the two water drops,
and then to the camera to trigger the shutter. The home-made switching circuit is based on the design
by David Hunt (see http://www.davidhunt.ie/water-droplet-photography-with-raspberry-pi/ )
There are two camera outputs - to accommodate two sizes of jack plugs, 2.5mm and 3.5mm.
(Some of my remote shutter cables have one size, some the other, depending on the camera!)