07-25-2019, 12:36 PM
(This post was last modified: 07-25-2019, 12:38 PM by heavyhorse.)
I have tried powering a load as tiny as a voltmeter from an electric fence. All the "safe" ag dept approved fence machines use a pulse of a few thousand volts m/l (the sheep ones up to 12,000v to arc through the wool), but the pulse is literally only a millionth of a second long. So powering a longer duration load of only 1 ma (1/1000 amp) would require a draw of 1000 amps from the fencer for 1 millionth of a second. (Physics will bite you in the ass every time...) The typical mains-powered fencer starts to load down pretty badly at a 30 amp load. So the power just isn't there for continuous loads. All of the fence power indicators either only draw power for a millionth of a second (neon bulb flasher) or have their own power source to run the electronics (volt meter).
I have also run cable to power PIR sensors. Total fail. Every rodent for a mile in all directions comes to dig up and chew through your cable. Lightning anywhere within a similar radius will induce a voltage on the cable back to your house (nice fireball and bang).
The only thing that seems even slightly operable is battery-powered PIR detectors. But they are extremely prone to false alarms; birds closer to the detector sets it off, squirrels, sun flashing on leaves, etc. And this: car bodies are generally the same temp as ambient if they have been driving through the wind. Glass blocks IR so a warm interior is not detected. You need a body temperature to set the detector off. And a fairly unobstructed line-of-sight back to your house to get anywhere near the "up to" 300 or 400 feet range advertised. The good news is they are fairly cheap, $20 to $100 or so, so doesn't hurt to try. But mine sit unused; after the first hundred or so times of jumping out of bed it gets a bit old.
Barking dogs are probably the best detectors, though they are prone to false alarms too, and can be high maintenance sometimes. But still, they are just so nice to have around.....
