Good camcorder hack from Dan @ Instructables – “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.”Link.