I wonder if this is one of the reasons why the phone system standardized on 48 volts. With DC, you need to get above about 60 volts to have any chance of lethality. If someone were to grab the barbed wire, they'd get a jolt, but it wouldn't kill them.
When I was in kindergarten in the late '50s, I pranked my class with electricity. I had a big fat electrolytic capacitor that I charged up before bringing it to school, and a screwdriver.
I don't remember the exact voltage I charged the cap up to, but it was definitely only a few volts.
So for show and tell, I explained how electricity worked, and then I grabbed one of the cap's terminals in each hand and started twitching and shaking like I was being electrocuted! Finally I somehow managed to break free, took a deep breath, and told my classmates "don't be as foolish as me."
To protect them from the danger, I then took the screwdriver blade and shorted it across the capacitor's terminals, with a big spark and a loud bang! Clearly, this cap had enough power to kill you.
I told my classmates, "Now it's safe. I discharged it. You can touch it now."
And they did. Nobody was harmed.
School was great back in those days. When I got to third grade and I wanted to etch a printed circuit board, I told my teacher I needed a tank of nitric acid, and she got it for me! But that is a story for another day.
Oh, I loved those Radio Shack all-in-one kits too - the spring connectors made it so easy to wire stuff up. Also did a bunch of Heathkits a few years later - color TV for the family, ham radio gear for myself, etc.
I was trying to remember my first experience with electricity and electronics, and it finally came back to me. Whenever our TV or radio went on the fritz (which happened quite regularly), I got to pull out all the tubes, and my dad would take me to the local convenience store where they had a tube tester. I would put each tube in the tester and learn what to set the dials to and what to look for on the meters and figure out which tube was bad. It was a great adventure!
Yes, it is technically true that it is the current that disrupts the heart rhythm. Theoretically, a 9v battery can supply enough current to cause a potentially fatal heart arrhythmia, if it's shorted through a person's blood. But it's also being pedantic.
Consider Ohm's Law: `I = V/R`. Why is it so frequently printed in this way, rather than the simpler `IR = V`? If we don't care about simplicity, why not `V = I/R`? What is special about `I = V/R`?
Resistance--be it your work load or your human fleshy bits--is usually roughly considered a constant--or at least a known--for a given application. And most power sources that you'll encounter in the wild are voltage-controlled and able to supply practically as much current as you want (aka "more than enough to kill you"). So Voltage is the independent variable, leaving Current to be the dependent variable.
While it is true that "it's the current that gets ya", it's the voltage over which we have control, so we tend to focus on that instead.
I always learned Ohm's law as, "V = I*R". You know, over the handful of classes that included it at some point. When you say 'Ohm's law,' my brain says 'vee-equals-aye-arr.' So...I did check wikipedia first to make sure I wasn't having a stroke, but I think 'V=I/R' is a typo.
Anyways, that does seem fairly self-explanatory. More volts means more current with the same resistance. More resistance means less current with the same voltage.
FWIW, I was always taught Ohm's law as V=IR (GCSE and A-level Physics/A-Level Electronics/BA Computer Science). I don't recall ever seeing it printed as I=V/R except where a calculation called for that form.
Yup, the resistance of your skin drops in the rain or heavy perspiration. While it's a nice mitigating factor it's better to just not mess around with anything that can deliver high voltage or high current.
There's a 1999 Darwin Award for this, even. A 9v multimeter killed someone because he broke his skin with the probes and lost the benefit of that resistance. That's pretty much a worst-case scenario, but it's still a vivid reminder that "low voltage" is not "safe".
People might find it intersting to know that in the car industry Gigabit Ethernet over unshielded twisted pair is common [1] since it allows to shave off some cents on extra copper.
Why would not having shielding save copper? Shielding is usually made out of a metal foil or mesh, often aluminium. I've never seen shielding out of copper.
These are 2 fundamentally different things. In twisted pairs, shielding is there to prevent interference from changing a signal. In TEMPEST protection systems, shielding is used to prevent radiance of transmissions that could be intercepted. While there is some overlap in conductive materials that could do both (prevent interference AND radiance), the methods employed and benchmarks used to measure performance differ significantly.
It is actually. I've seen hazy images recovered from your monitor by collecting the radio emissions of the cable. The signal traveled through windows, walls and an airgap of about 15 meters. The antenna used in that attack was only about half a meter long I think. I imagine you could put a much bigger antenna (or a dish) inside a van in the parking lot and spy on some analysts or something...
A good-sized antenna at range works, and so does a tiny device at close range! There are Van Eck listeners small enough to conceal completely that can still eavesdrop table-to-table at a cafe. The conference debut involved hiding one in a piece of pita bread, where it could sit right out in the open.
Jeez, at this point we might as well assume you can collect information from anything. I bet someone has made something that can restore the image on your screen by the vibrations on the windows that the sound of the power supply makes as the data flows in.
Has this become standardized yet? I've got a bunch of multi-drop CAN stuff that I would LOVE to move over to single pair ethernet with multi-drop a la 10BASE2. Unfortunately I haven't see a 10BASE2 compatible PHY in 20+ years.
IEEE had a working group on this but I hadn't seen any actually shipping chips that mortals could get their hands on.
(Edit: Apparently still vapor. Still can't find a source for the Marvell 88Q1010 & 88Q2112.)
My biggest problem is software stack performance--not physical layer performance. Most protocol stacks in the embedded space other than TCP/IP are mind-blowingly bad.
Being able to obliterate USB, CAN, and RS-232 (to a lesser degree--RS-232 is really reliable but the problem is generally in frequency drift if you don't have a crystal anywhere) in all of my designs and replacing it with ethernet will dramatically improve the reliability of my hardware. The only places I don't currently have ethernet in my designs are where I'm limited to a single pair of wires.
As for RS485, it doesn't really offer me anything that I don't already get with CAN.
Lots of industrial stuff related to various types of testing. Performance matters some, but I'm not hard real-time.
However, everything has to work, reliably, in less than ideal environments--mostly high-voltage spikes caused by differences in ground potentials.
USB is a PITA because while data is differential, connection detection is not. So, spikes cause USB disconnects and drivers never cope with that.
CAN is a PITA because it is limited to 8-bytes per message. Good for transducers and 8 bit microcontrollers--not so good for complex control and 32-bit microcontrollers.
Ethernet hits a really great sweet spot of fully-differential and nice single packet size for a 32-bit microcontroller--program like it's 1989, baby.
It would be neat to have a mesh topology that uses the bandwidth and redundancy of multiple fence paths in order to prevent an outage from damage to any one particular fence. Solar-powered routers anyone?
Otherwise: Dabnabbit Maude, the cows gone loose again because we lost the interwebs. (I’m half-Texan so it’s not -ist.)
Seriously, this reminds me about the stories I read on HN about how early telephone networks were established by farmers (ab)using their electric fencing for communication.
> In 1998, Wide-Band Systems demonstrates Gigabit Ethernet running on four pairs of old, rusty barbed wire.
Yet in 2017 we still have only 1Gbit Ethernet in LANs :(
1Gbps = 125 MB/s !!
I am running 1Gbit since 2003 at home. And all f..king home network devices still support only 1Gbit Ethernet. It's not fast enough. When can some network company finally release 10Gbit or 100Gbit Ethernet (copper or fiber) for cheap home usage.
I feel for you, but I think if you need a 10GBit or 100GBit network at home, your needs are a little extreme.
My home network still runs on 100MBit, because of a stubborn old switch that refuses to die. By now, my curiosity to see how long this thing will last (it is about 13.5 years old) outweighs my impatience when pushing large files around. ;-)
Definitely on the high end, but having centralised most of my storage (backups, media etc.) to a NAS, 10Gbps would definitely be welcome. Pushing large backups to my NAS is time-consuming, and e.g. video editing basically requires pulling it down to a local drive first (not to say it'd be ideal to use a network store for that at all, but it'd be more viable).
Used enterprise hardware that supports 10Gbit isn’t that expensive. For the average home user even gigabit is overkill, which is why 90%+ of people just use whatever WiFi router their ISP provides.
