The Walden Effect: Farming, simple living, permaculture, and invention.

Solar powered deer deterrent upgrade

updating the previous deer deterrent model from a D cell to two AA rechareables

Still no no signs of deer damage to the garden this year.

I finally got around to upgrading the solar powered deer deterrent from the Alkaline D cell to a pair of rechargeable AA batteries just in case we need a mechanical deer deterrent on short notice.

The batteries were a little undercharged at only 1.2 volts, but I'm trying a series circuit which will double that to 2.4 volts DC. My thinking is that the motor stops turning just under 1.3 volts, and even though it turns a bit faster with 2 batteries it's not too fast and the increase should help boost the longevity factor.

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About us: Anna Hess and Mark Hamilton spent over a decade living self-sufficiently in the mountains of Virginia before moving north to start over from scratch in the foothills of Ohio. They've experimented with permaculture, no-till gardening, trailersteading, home-based microbusinesses and much more, writing about their adventures in both blogs and books.

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Mark, what was the voltage of the original battery in the solar-powered light? If you deviate a lot from that it is likely that the power output from the panel will decrease sharply. Above a certain voltage, the power production of a solar panel drops like a rock.

If you don't know it, you can measure the power curve your the panel with a multimeter and some resistors. The linked webpage also does a good job of explaining what the maximum power is. BTW, with the small panel you're using for this, I don't think you need special high-power resistors to do the measurements.

There is also a webpage about building a solar battery charger that might be useful. Observe that you need a diode to prevent the batteries from discharging through the panel!

And this PDF tells you why and how to build an MPP tracker.

Comment by Roland_Smith Mon Jun 18 01:50:12 2012
Roland-The voltage for the original battery was 1.5 volts DC. I hear what you're saying about the values being too far off. The new series circuit brings that up to 3 volts if the rechargeable batteries are fully juiced up which might be too much deviation. The original circuit was set up to charge the 1.5 volt AA battery, which seems to have a diode for that value, but I guess time will tell if the diode can handle the extra power. I noticed last night that the LED light was not glowing, yet the motor was still turning. Have not checked yet, but I suspect the extra volts may have been too much for the LED which might create problems.
Comment by mark Mon Jun 18 16:29:09 2012

You really need to rip out that LED, and anything else you don't need. If there is a diode, it should have a model number on it. With that you should be able to find the datasheet, which will tell you what it can handle. Who knows, if you open the unit up, there might even be a model number or a spec on the solar cell?

But the best you can do is take two measurements of the solar cell with your multimeter. Disconnect the panel from everything, and then only connect the multimeter. First put it in DC voltage mode. What you measure in that case is the open circuit voltage ("Voc"). Next, switch the multimeter to DC current mode. What you then get is the short circuit current ("Isc"). Note that both Voc and Isc will vary according to both temperature and sunlight! At least measure them at different amounts of sunlight (e.g. full sunlight vs. overcast day). The maximum power point is usually just below Voc, with a current that is just below Isc. (Google for "solar panel IV curve" to get an impression of how these curves look.)

If the measured Voc is much less than your battery voltage, you won't get much charging done unless you add a boost convertor. If it is much higher, you might damage the batteries, unless you add at least a voltage regulator (78xx) in the charging system or add more batteries.

Comment by Roland_Smith Mon Jun 18 17:24:15 2012
As a new reader I find this post really interesting - have you had much luck with other solar powered pieces of kit? I live in the UK, so sun isn't on tap, but buildings are starting to get some of their energy from solar panels, which is a step in the right direction.
Comment by Peter Wed Jun 20 11:33:40 2012
Peter --- You might click on the "energy" tag below to see all of our related posts. We haven't done much, but are slowly working on a small setup to power our laptops and lights. My favorite use of solar power, though, is my flashlight --- no changed batteries for four or five years now!
Comment by anna Wed Jun 20 14:11:12 2012

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