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 'record' on the camcorder, and it happily records blanks from the waterproof camera because we forgot to turn it on! Adding to the problem - the waterproof camera uses a lot of power! It runs on an 8-AA battery pack and lasts about 90 minutes while it is on - ouch! Usually we'll be out on the water for half a day at least, so even if we remember to turn on the waterproof camera, we often forget to turn it off so it runs out of power long before the camcorder runs out of tape.
Ideas: What we need is a way to turn the waterproof camera on and off at the same time as the camcorder in the sealed box. we are using the remote control to turn on and off the camcorder, how do we get the same remote control to turn the remote camera on and off? Initially we thought we'd hack the remote control unit, but after a bit of digging we learned that the Sony LANC standard it uses is complicated - we'd need a microcontroller to do anything with it. Isn't there an easier way?
The Solution: We did a bit of probing with our multimeter, and discovered that the MIC jack on our sony camcorder supplies a small amount of power for the microphone, and most importantly - it turns this power on and off at the same time that the camcorder is turned on and off with the remote control! on our camcorder, we found that the MIC jack provides a 2.0V power source when the camcorder is recording. This is not enough voltage or current for a conventional magnetic relay. We need to make a sensitive solid state relay. - a very simple and inexpensive electronic circuit using only 3 or 4 components. The circuit will function just like a conventional relay, but it will work using the 2V signal and draw far less power than a microphone would.
Step 1: The Parts
Here's some photos of our waterproof setup with camcorder, box, remote waterproof camera, LANC remote control button, etc.