Current Level Detection

Just measure the voltage drop from the battery to some point down from the main cable. If necessary, at a small resistor. But the cable itself might be enough to be detectable.

I can't see that working with a current source of such low internal resistance - and *thick* cabling.

Yse that method to measure the current and one of the Arduino processors to measure that drop and activate a buzzer, or just use a speaker and let the Arduino generate the audio tone.

There is still some voltage drop. You just need a very sensitive detection circuit.

wind some 15A magnet wire round a reed switch until you get the right threshold.

A 10 mohm resistor, or equivalent wire, will drop 2 mV at 200 mA. You can pick that off with a comparator or a rrio opamp used as a comparator.

It drops 150 mV at 15A and dissipates about 2 watts.

Something like that.

A 15 amp schottky diode would give a better voltage range but get hotter.

FYI, gauge 1 wire, 1/3" diameter is around 100 mohm per foot. Small but measurable.

100 mohms per 1000 feet.

#10 is 1 mohm/foot.

Plus the AWG numbers correspond to amplitude in decibels--20 AWG is 10 mohm/foot.

Cheers

Phil Hobbs

Probably a Hall effect sensor.

It may pay you to form the lead into a single loop, and wrap it around a chunk of ferrite so that you have a well-defined magnetic circuit with a place where you can stick the Hall effect sensor and detect a predictable magnetic field. This is much the same idea as using a reed-relay, but the hysteresis will be a lot smaller.

Getting something like this to work under the hood of a car (where temperatures can get quite high) could be difficult. but this doesn't seem to be what Curistor Doom has in mind. Sounds more like something criminal (like brewing up illegal drugs).

Alternately, a C-clip of silicon steel sheet material, with a hall sensor located between the butt ends will give an indication. Threshold adjustible by varying distance between clip ends (plastic screw).

. . . . although I've only ever used this to trigger on high current levels. It's surprisingly fast, if needs be.

RL

It's called a sensitive relay. A few wraps of wire around a bar, and a weak-spring set of points, with a fine adjust screw, can be set to trip at the current of your choice.

This presumes that you can wire it in series with the battery, of course. Possibly, you can adjust a permanent magnet and use a reed switch (like in home security systems) to accomplish the same effect.

Sounds similar to Jason's idea, which had an elegant simplicity about it. This will just be for a one-off quick and dirty work-around, so doesn't warrant the investment of much development.

Which is basically Jason Bett's idea re-badged as your own.

That earns you another .sig append.

Cursitor Doom seems to be in there competing with John Doe and Flyguy to post the most idiotic misapprehension.

An Allegro hall effect sensor is a rather different kind of magnetic field sensor from a reed relay, and a much less cranky one.

So who cares why a psychotic idiot ventilates his idiocies? All we know is that you like to be obnoxious, and it does seem to be only area where you demonstrate any competence at all.

The trick is to get a reasonably accurate trip point that is about 1% of the maximum current.

Hall sensors are not very temperature stable and have tiny outputs. 15 amps will heat it up. The magnetic circuit could easily be magnetized by the big current and shift the threshold.

Wrapping 15 amps worth of wire around a reed, and tripping at 200 mA, would be interesting too. Reed relays typically get warm at operating current, much less at 100x operating current. The 15 amps could magnetize the reeds so that they never open.

The series schottky diode takes care of the dynamic range problem, at the cost of maybe half a volt at 15 amps. Or just use a shunt resistor and some good electronics.

Ideas are fine, but eventually you have to make the numbers work.

True, but for a one-off at least I don't need to worry about device parameter spread or monte carlo analysis. I can taylor it to work with the devices I pluck from my junk box and toss it away once it's done it's job. Selecting the best suggestion here is the tricky bit AFAIC. I can see some merit in all of them - except the one Bill Sloman put forward (as usual).

I doubt that a reed relay that can tolerate 15 amps will trip around

200 mA.

Or that a Hall sensor would be stable enough to work over that current range.

Even a one-off has to work.