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.
Hey Dan!  On which model of Sony are you using the remote control ?  Can you provide any other model numbers? 
I'm trying to use this for the same application you made it for, with the exception that I'm hooking it up to my ipod headphone jack, which only provides 1.1V. This is for a school project, but I'm totally illiterate when it comes to circuitry. What all would change if I used a transistor? What kind of transistor should I use? I tried searching on Digi-Key, but its all greek to me. I'll try to draw up a schematic of what I'm thinking of and maybe you could tell me if it might work?
any small NPN transistor should work to replace the NFET, and will let you use 1.1V input. eg: 2n2222 or 2n5088
Is there a way to turn the sony camera on using this system without needing the LANC?
Do you by any chance know the model number of a N-type FET that has a 1v threshold, I can't seem to find one through the listed sites?
it doesn't exist. the lowest that is made is 1.8V. however, a 1.8v fet wil probably be mostly on by 1v, so it might work for you.
Do you know what the model number is for the 1.8? Do they make it as a dual n/p like the one that you used above?
you can get PFET's down to 1.8v, i've only seen NFET's around 2.5V - which might work for this application, it is worth trying. you can get a dual N/P with 2.5V threshold. i haven't looked really hard for a 1.8V NFET so it might exist. try searching at www.irf.com or www.fairchildsemi.com, digikey and mouser then sell most of it if you find one. or use an NPN transistor instead of the NFET, since that will turn on at 0.6V.
Regarding the FET's Are the numbers in () the part#'s for digikey? Do you need to buy one chip or two? Why are they both the same part if the requirements are different? Q1: N-type FET with low threshold voltage (irf7309 or fds8958) Q2: P-type FET with low threshold voltage (irf7309 or fds8958) (Sorry I'm a bit of a newbie and just want to make sure I'm getting the right parts)
those parts have both an N and P in a single package. you will find them in digikey if you search for the part number.

About This Instructable




Bio: Dan Goldwater is a co-founder of Instructables. Currently he operates MonkeyLectric where he develops revolutionary bike lighting products.
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