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.
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Step 1: The Parts
Here's some photos of our waterproof setup with camcorder, box, remote waterproof camera, LANC remote control button, etc.
Step 2: The Parts - Solid State Relay
Here's what we need to build a solid state relay. Just by selecting the proper FET's you can tailor this solid state relay circuit to many uses: it can be very small, it can switch voltages up to 20V, it can work with a control signal voltage between 1V and 20V, it can handle up to 100A current or more with a big FET. I've chosen a FET package which has both an N and P fet in a single chip. This type of device is only available in a surface mount package so I also got a little Surfboard to mount it on and connect the wiring and resistors to. Below I discuss the FET parameters a bit more if you want to use different devices. All the parts are available from digikey
<ul><li>R1: 22k ohm resistor</li><li>R2: 22k ohm resistor</li><li>Q1: N-type FET with low threshold voltage (irf7309 or fds8958)</li><li>Q2: P-type FET with low threshold voltage (irf7309 or fds8958)</li><li>Surfboard 9081 to build on (digikey part 9081CA)</li><li>MIC plug to connect to camcorder</li><li>power plugs to connect to camera and battery pack (or you could hard-wire)</li></ul>
There are lots of choices for the FET's depending on how much power your solid state relay will be switching on and off. The FET's listed above are rated for 5A current, 20V max voltage (checking both Vgs and Vds), 4.5V full-on operation and on-threshold around 1.5V. I usually find it easier to go directly to the FET manufacturer site to find the right FET's, try http://www.fairchildsemi.com/parametric/ss.jsp?FAM=MOSFET or http://www.irf.com/product-info/hexfet/
Selecting Q1: the N-FET is only used to turn on and off Q2, it can be a low-current device and the only important choice is the gate threshold voltage. For our project we need a low threshold device to turn on and off from the 2.0V MIC jack. The most common FET's are rated for a 10V "on", and have an actual gate voltage (Vgs threshold) around 3-4V. low-threshold FET's are common though, you just need to check the spec sheet to make sure you have the right thing. You can get FET's with a threshold as low as 1V. You also need to check that the Vgs absolute maximum rating is higher than your main battery voltage.
Selecting Q2: This is the P-FET that switches the main power. Select an FET that can handle the current you need (usually you will pick one with 5x the actual current rating so that it does not waste power and no heatsink is needed). If you are switching less than a 6V power source then you will need a low-threshold FET here also. As before make sure the absolute maximum Vgs rating is larger than the battery voltage.
Step 3: The Circuit - Solid State Relay
- IN+ and IN- connect to the 8-AA battery pack powering the waterproof camera
- OUT+ and OUT- connect to power input of the waterproof camera
- SENSE+ and SENSE- connect to the MIC jack on the camcorder. The MIC jack is actually a stereo MIC jack, just connect one of the two channels as the (+) and ignore the other.
Step 4: Done!
I covered the finished circuit in hot-glue to protect it.