Overview: We used the MIC jack of a camcorder to detect when the
camcorder is on. We built a low-voltage solid-state relay to
detect the MIC jack and automatically turn on and off a remote
device at the same time as the camcorder. The solid-state relay
is a general purpose device that could be used for a wide
variety of projects - it is a very simple and inexpensive
electronic circuit.  The Problem: We're using a camcorder for
marine applications and it needs to be totally waterproof. We
put the camcorder in a sealed box, and there's a small
waterproof remote camera that we plug into the 'video-in' jack
on the camcorder. we use a remote control for the camcorder
which is outside the box (it uses the Sony LANC standard to
connect to the camcorder). So, the camcorder itself is used just
as a the data-recording device, the waterproof remote camera
takes the video.  The waterproof camera uses its own battery, it
is too easy to forget to turn it on or off when we are out on
the water in a tense situation! we hit '