[TriEmbed] Need help with a beginner circuit design

[Brian]

> 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...