detecting a magnet

In article , Gregory L. Hansen wrote: [...]

I think 2 toroids inside two solenoids will work better.

You place a toroid inside a solenoid with its axis at right angle to the bore of the solenoid. You drive the toroid with an AC current that just saturates it in each direction. The solenoid coil will develop a voltage proportional to the external field at twice the drive frequency.

Such a design will have trouble with picking up stray AC fields. If you place 2 side by side, drive the toriods at 90 degrees to each other and wire the coils to subtract, you get more than twice the signal and a lot less pick up.

it

Tagging

be

Somebody below mentioned a car detector that used the metal locator principle of changing an oscillators frequency when a metal object (car) alters the field of a coil. This wouldn't work very well on a dog or a nonmetalic box but changing frequency by capacitance would. There is a musical instrument called a Theramin where the frequency of an oscillator is varied by waving ones hand over an antenna. This is beat with another oscillator producing an audio tone played through an amp and speaker. You can hear these things in space movies and other places where weird etherial music is used. In any case you could make a dog detector that worked on this principle; i.e., varying the frequency of an RF oscillator by changing the capacitance in a tank circuit. An oscillator running at several hundred kilohertz having an insulated metal plate connected to its tank could easily detect a dog or other nearby object by change in frequency. No magnets or metalic objects required. Do a search on Theramin and see if that idea might meet your needs. Bob

Put a solenoid on a crank and spin it around.

_Good Vibrations_ by the Beach Boys.

A cow magnet? This would be a magnetized cow, possibly one that has a large uptake of iron from feeding on ferrous soils? Or a magnet used to stick a (small) cow to a (large) refrigerator?

....clickety... google ...clickety...

Well, by the Wrinkled Sack of Zeus! I have lead a sheltered life.

If you don't want to mess with coils, think about solid state sensors.

They go, in order of increase of sensitivity: Hall Effect sensor - GMR sensor - Magnetoresistive sensor - Magnetometer.

Solid State Magnetometer (like the one made by Honeywell) is made around Magnetoresistive sensor(s) but with well thought signal conditioning which increases it's range. It will give you 2 - 3 feet range for the magnet you describe.

You should think in advance what you gonna do about stray targets and false triggering.

Andrey

could

Or a GMR sensor - more sensitive. NVE and Philips make them. For ultimate sensitivity, try a fluxgate magnetometer.

At audio frequencies, you might find a difference, due to the differing incremental permeabilities of the magnet when demagnetized and magnetized. But the difference could well be quite small.

No, for the ultimate try a low temperature SQUID. The have a noise of something like 10fT / sqrt(Hz).

In article , Mike Fields wrote: [...]

The reaction depends on the dog. I think feeding the dog radioactive food and using automatic radiation detector would work better: That would be Radio-Acitve Dog Automatic Ranging (RADAR)

Those who have tried this have posted glowing reports.