[TriEmbed] Need help with a beginner circuit design

Thu Oct 29 19:00:40 CDT 2015

Wow... cool math.  I like that.  (Seriously.)

I'll share a bit more since you brought it up.

The purpose of the device is to be an electronic shark deterrent for scuba
divers, and I've already filed patents for this.  Therefore, at the risk of
boring the folks on this thread, I'll provide some details.  (I'm happy to
take this off-line if it's getting too heavy.)

Sharks have sensors in their snouts called Ampullae of Lorenzini that are
sensitive to electric potentials as small as 5 nV/cm.  (Yes..that's 5
nano-Volts over a centimeter...amazing)  Scientists theorize that they use
this as their final "hunting sense" as they close in for a kill by sensing
the nerve activity in muscles of their victims.  Electronic shark
deterrents use this fact to "overload" these sensors, which disorients the
shark in the final phase of the attack.  In fact, commercial electronic
shark deterrents have been proven to be very effective, but they are way
too expensive.  ($700 and up for a "Sharkshield" device).  I want to build
a better, cheaper version, and also include notification (flashing LED and
beeps) for other divers.

Therefore, my goal is to build a simple (i.e. cheap) device that generates
and electric field in salt water that will generate a sufficient field.  In
my (uninformed) opinion, I want as strong a field as I can generate and
still not harm the human.  The effective distance will depend on the
voltage.  The electrodes will be shrouded from direct contact, but probably
on the outside of the PVC pipe (opposite sides) in the prototype.

More data:
I had someone make a device with a variable voltage output powered by a
9-volt battery using that max1771 chip.  It worked.  I was able to vary the
voltage from 1 to about 90 VDC and I measured it.  Then I put together a
salt water tank at home (using saltwater aquarium chemicals from the pet
store).  I stuck my hand in the water and gradually turned up the voltage.
With my hand 2 inches away from the electrodes, I could feel a tingling
when I got up to about 80 VDC.  Note that the electrodes were only about an
inch apart, and I never put my hand between them.  I was just feeling the
football shaped field around them.  I think the reason the voltage was so
high but didn't hurt was that the resistivity of human skin is much higher
than seawater.  The electricity flowed around my hand- never through it.  I
ran a series of a bunch of tests, using the device powered my a 9 volt
battery.  It was never exhausted.  However, I'm only requiring the device
to operate for 30 minutes.  That's double the time for a diver a
recreational depth (< 100 feet) to ascend safely.

Brian-  Thanks for your help so far.  If you'd like to get together to
discuss this some more, I'm happy to do it and buy you coffee or lunch or
something.  That offer is open to others as well.  Just ping me.

By the way... bitcoins are back.  They are over $300 this week. With an
ASIC miner and a Raspberry pi (which I already own), I can just make it
cash-flow positive because I have time-of-use rates from Duke power.  After
9pm and weekends, I'm below 7 cents per KWh.  I'll find a mining pool that
let's me come and go at night.  I'm not gonna get rich, but it's an
interesting education.  (If I'm lucky, I'll net about $5 per month.)

Dave

On Thu, Oct 29, 2015 at 10:46 AM, Brian <triembed at undecidedgames.net> wrote:

> The flashing LED and beeping piezo speaker are pretty easy to do with a
>> 555 timer. I also need around 80Vdc, pulsed about twice per second.  The
>> current can be extremely low- just 1 mA or less.   The goal is a simple
>> electric field in salt water to annoy fishes
>>
>
> Watch out, lots of math and science coming to make sure your fish stay
> safe (if annoyed).  Short version: 80VDC is probably way too much. Where
> did you get that figure?  Anyway:
>
> All materials have a resistivity property, which expresses the ratio
> between the magnitude of a uniform electric field and the current density
> that develops because of it.  This is going to get a little ugly in plain
> text, but let's work through it.
>
> According to Wikipedia (an inscrutable source!), the average resistivity
> of seawater is 0.2 Ohm-meter.  For some sense of scale, copper has a
> resistivity of 0.000000017 Ohm-meter, and glass has a resistivity between
> 1x10^11 and 1x10^15 Ohm-meter.
>
> Let's look at the electric field.  Though in reality, the field is
> mathematically infinite, it is strongest in a straight line between the two
> electrodes.  If your electrodes are 1 meter apart (on opposite ends of a
> largish fish tank, perhaps?) and have a potential difference of 80V, the
> maximum electric field magnitude will be 80 V/m.  Simple.
>
> Now we can take the resistivity factor and figure out current density
> (current density is current normalized by cross-sectional area).  Given 80
> V/m and 0.2 Ohm-meter, we find that current density is about 400 A per
> square meter.
>
> Now we can look at your electrodes.  Let's say you decide to fashion them
> from two 1-cm-square pieces of some rust-resistant metal (platinum!).  One
> square centimeter is 10^-4 square meters.  Multiply by our 400A/m^2 current
> density and we get...(drumroll)... 40mA.
>
> That's probably going to kill your fish.
>
> So let's assume that your intent is to have the electrodes close together
> and shrouded in some way, so that the fish are only exposed to the outer
> fringes of the field.  Inverse square law and all that.  Well that's fine,
> but now your electrodes are not nearly so far apart. Inside a 1-inch PVC
> pipe, for example, would put them about 2 cm apart.  Now your field
> magnitude is 80V/0.02m, which leads to a current density of 20 kA/m^2.
> Even if we used the same 1-cm^2 electrodes from our previous example,
> that's two amps.
>
> That's probably going to kill your fish, and will definitely suck down a
> battery in no time.
>
> One more thing to consider: Water cracks (undergoes electrolysis to split
> and recombine into O2 and H2) around 1.2 volts.  Stick your 80V electrodes
> in salt water and you're going to generate some highly flammable gas.
>
> That may or may not kill your fish, but it could burn your house down.
>
> (Okay, before anybody jumps on me, yeah, it'll actually be a miniscule
> amount of H2 and 02, such that in a decently-ventilated area there will be
> no hazard whatsoever.  But that didn't fit the story as well!)
>
> So...why do you want to do this, again?...  But really, it sounds
> interesting, but 80V is probably too much.
>
>
>
> stun guns that use a 555 timer output into an audio transformer to
>>
> > step up the voltage to 10s of thousands of volts.  That
>
>> would work, but since I only need 80 Vdc
>>
>
> Yeah, you'd use a resistor divider so that the transformer only saw a
> fraction of the supply voltage.  Lower winding current => lower voltage.
> But as we've seen, 80VDC may be way too much.  Even 5VDC may be enough,
> depending on salinity and electrode placement.
>
>
> Meanwhile, my Bitcoin miner arrives tomorrow...  so you KNOW I'm crazy...
>>
>
> No kidding.  BC mining stopped being profitable a long time ago...
>
>
>
>

-- 
Dave Buster

KK4ELT on 2 meters

Never look at the trombones, it only encourages them.
*Ri
<http://t.sidekickopen23.com/e1t/c/5/f18dQhb0S7lC8dDMPbW2n0x6l2B9nMJW7t5XX45vMtjYVQZgwH64R1y4W8r4TLx56dPn8f5RTxgR02?t=http%3A%2F%2Fwww.brainyquote.com%2Fquotes%2Fauthors%2Fr%2Frichard_wagner.html&si=4964982235594752&pi=698aaf6a-ee74-41ed-d533-d252d628138e>chard
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