Monday, January 19, 2009

Nest Box Preparations

To prepare the screech owl nest box for the possibility of nesting by my local screech owls, as of this evening, I've now evicted various numbers of squirrels from the box three times, have opened-up and cleaned the camera compartments, and have repaired a broken wire from the entryway sensor. So, the box is being kept available to, and ready for, the owls, should a pair choose to claim it this year.

Unfortunately, I still haven't setup the A-to-D converter board I bought to replace the one that burned-out a few years ago, so I have no data on whether, or not, the box is being visited. Hopefully, I'll work-up the energy to do that soon. And this time, I really must use an optoisolator in the circuit.

Otherwise, the only remaining problem with the setup is a semi-persistent humming noise on the audio circuit. It has bedeviled me ever since I put a microphone in the box. I assume that it's a grounding issue, but I've grounded everything I can think of to the common household ground and the problem still occurs. The intermittency of the problem makes trial-and-error resolution especially difficult, although I've pretty much run out of ideas, anyway. Needless to say, I'm open to suggestions from anyone with relevant experience.

6 comments:

  1. We were so disappointed about last year, that we are really looking forward to watching owls in the nest box.
    Good luck! from
    Roy & St Alban's Primary School, Warrington, UK.

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  2. Alison in Austria6:52 AM CST

    Wonderful to read about your preparations for the coming season and we will hope with all our might that owls will take your offer again this year.

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  3. Susan Gatlin4:33 PM CST

    So happy to hear you are getting ready for vistors of the feathery kind. Will send good thoughts that you get the bugs worked out before the first arrival! Thanks

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  4. Chris... This is AWESOME news! So glad you are in prep again... Ours didn't come back to our tree last year. We had sewer work done in our ditch next to the house the summer before last and I think it freaked them out!

    I am looking at preparing a box for them this year in hopes we get our family back!

    Have a VERY blessed weekend Chris!
    Cheryl in Sachse

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  5. Anonymous1:41 PM CST

    Regarding the audio hum - make sure you're not creating a "ground loop" - that COULD be what you're dealing with.

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  6. Assuming that the current hardware is not too different than what you show at
    http://gargravarr.cc.utexas.edu/owl/2006/box/ , here are some ideas on fixing the hum:

    The +12 V DC apparently comes from your house, over the co-ax. I'm guessing it is provided by a big "wall wart" or similar power supply. These are typically not very well filtered, and have some 60 or 120 Hz ripple in the output. The LEDs definitely don't care and the camera probably doesn't care, but this can induce a buzz into your audio, either via the video modem on either end, or simply over the long run of cable to your house. One thing to do is to power both the nest box and the video modem inside your house from the same +12 V DC power supply. A quick way to see if the power supply is causing the problem is to temporarily power the rig from a battery, like a car or garden tractor starting battery, or a 12 V gel-cell from a UPS, or even from six 1.5 V alkalines or ten 1.2 V NiCd/NiMH cells in series. If battery power kills the noise, you need a cleaner +12 V power supply. You can hang some capacitors across the supply you have now (use at least a couple of hundred microfarads, with a voltage rating of at least 25 volts), or you can add a linear regulator chip, heatsink, and capacitors to your unregulated supply. Or, buy two batteries, each large enough to run the nest box for a while. Put one on a charger and let the other one run the nest box; when the nest box one gets discharged, swap them.

    At the nest box, make sure the cable from the microphone to the video modem is shielded, with as few breaks or gaps in the shield as possible. If it has to be plain wire, twisting the wires (just like twisted pair Ethernet cable) may help a little. Also, are you running the microphone straight into the video modem? Microphone-level audio is typically not very strong, and if there is no audio amp in the video modem, will have trouble making it to your house without any noise. Something like an LM386 chip is fairly easy to use, provides reasonable gain, and is even available at Radio Shack. If you don't want to mess around at chip level, a Radio Shack "Mini Audio Amplifier" (277-1008) is the same thing all packaged up. You would run the mic into the input jack, and run the output jack into your video modem. You do have to power it; it wants a 9 V battery (which might not be completely insane, as long as you're driving a sound card input instead of a speaker) or 9 V DC supply, which you could derive from your 12 V supply at the nest box. If you add an amp at the nest box, you may have to turn down the gain on your sound card input (or whatever) at the house.

    You might look at the routing of the co-ax from the nest box to your house, and inside your house. If it runs parallel to any AC power lines (especially the main power line to your house), it can pick up hum. Co-ax is better at this than other kinds of cable but it can still happen. The best way is to have the co-ax cross the AC power lines only at right angles, but this is not always practical; if the co-ax has to be sort of parallel to the AC power lines, moving the co-ax even a few feet away can help. Same thing inside; try to run the co-ax at right angles to any computer power cords, extension cords, etc that are nearby.

    If you get the audio working out really well, and you are feeling particularly sneaky, don't worry about the discrete wires out to the nest box for the door sensor. Just have the phototransistor in the door turn on a small audio oscillator in the nest box, whose output is mixed with the microphone and fed to the video modem. You'll get a "beep" in the audio at your house, which maybe you could run through some DSP to have your code do something interesting when the beep happens. Some disadvantages are that you will probably have to set the phototransistor trip level at the nest box (it will be hard to compensate for things like changing sunlight angles during the day) and that whenever an owl is in the doorway, you won't have much microphone audio to listen to; such are the perils of in-band signaling. Depending on how good the video modems are, you may be able to get away with using a very low frequency (below a few hundred Hz) or relatively high (above 15 KHz or so) for the "door chime".

    Standard disclaimers apply; I don't get money or other consideration from any companies mentioned.

    I hope this helps!

    Matt R.

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