detecting a magnet

could

An ordinary inductive coil requires the magnet to be in motion and the voltage induced is a function of the number of turns and the rate of change of the flux, dphi)/dt. Therefore the speed of a moving magnet is very important. Practically you might be able to sense the magnet moving a foot away or so depending on the number of turns in the coil and the speed of the magnet. It is a signal to noise issue and power line interference (hum) would limit what you can detect.

However it is possible to detect small stationary magnetic fields (DC) by arranging and energizing coils in a device called a flux gate. These things can be designed to measure fields as low as 1/1000 of the earths magnetic field or even lower, less than a milligauss and could easily measure your magnet many feet from the fluxgate. Being DC, it's easy to get rid of the power line interference. There is also hall effect devices which has been mentioned but they are not particularly sensitive. Beyond that, there is a quantum mechanical device called a "squid" that can measure extremely small fields like the field from the iron in a single blood cell. These are probably way beyond what you need or could pay for but it gives you an idea of the technology available. What are you trying to do? Bob

say it crosses the plane in 2 seconds.

Are there ways of detect>

Roughly proportional to how fast the magnet is moving - give us a clue.

Pearce Consulting

Ah, flat statements are (almost ;-)) always wrong. What about a fluxgate senor? Or a reed? Or magnetoresistive (eg. GMR) sensor? There are probably others I've forgotton

But the first would require core material, and the others don't use the coil that the OP posited.

Best regards, Spehro Pefhany

Semi-seriously, you could use a compass and an led-phototransistor sensor. You might resolve milligauss that way.

John

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I'm guessing, maybe wrongly that the OP is interested in those vehicle detection loops burried in the ground at red lights. These sometimes fail to respond to motorcycles and some people sell magnets that you can attach to your motorcycle that alegedly allow the coil to see your motorcycle.

I believe the principal of these coils is a tuned resonant tank that is detuned by a large hunk of metal. I don't believe that the magnet helps in this case but there are people out there that will swear that they do work.

Mark

it

size of an

the

change

foot

of the

(hum)

(DC) by

things

magnetic

your

the

been

is a

extremely small

an idea

I'm guessing, maybe wrongly that the OP is interested in those vehicle detection loops burried in the ground at red lights. These sometimes fail to respond to motorcycles and some people sell magnets that you can attach to your motorcycle that alegedly allow the coil to see your motorcycle.

I believe the principal of these coils is a tuned resonant tank that is detuned by a large hunk of metal. I don't believe that the magnet helps in this case but there are people out there that will swear that they do work.

Mark

I am tryning to accomplish "mental exercise". Sometimes I see things and it just makes me ponder how to do it.

This "application" could be many, but is similar to RFID I suppose. Tagging something, but without identification and a completely passive tag. Could be to see if a box is on a pallet, or a dog is in his doghouse. Basically thinking of ways to have a flat "mat" and detect the presence of something within 2-4 feet directly above it.

In the end, I do it cuz I am a geek :)

A Hall Effect sensor is the only option for a stationary magnet. It is also the best choice for such a slow moving magnet.

The magnitude of signal you get depends on stuff like how strong the magnet is.

Pearce Consulting

System used by 3M was to basically use it to tune an oscillator. When metal entered the loop, the frequency changes. Traditionally, you had a filter for the frequency, and detected when it changed enough to no longer pass signal. 3M just used a digital counter to measure the frequency. When count changed by about 5, a car was there!

Much smaller scale, much smaller target, proportionally much longer range.

A fluxgate magnetometer (Google search terms) can probably be built easily enough by the hobbyist. You need a high permeability material like mumetal, a few solenoids, and electronics.

E.g.

A SQUID will work, as well. They work best when they are very cold.

Leon

Hi Spehro,

Yep. Reed is really easy, just take a reed relay and use only the inner glass body with the contacts. That's how I usually did it. It cost next to nothing but that might have changed. Of course, it'll age after umpteen thousand cycles and once I had a reed contact break off and float about in the little glass tube.

Such contacts can also be purchased separately with proper mounting holes, matching screws and all. They are used in alarm systems to detect when an intruder starts to move a sliding window (the magnet slides away). Possibly a store like Radio Shack has these.

Regards, Joerg

it

Tagging

be

Easy - just run a cat past the dog house to see if he is home ... (also counts as a "cat scan" )

For those of you without "hick" experience.... Lotta metal aroud for cows to pick up and eat. Ya stuff a big magent down their gullet, catches the debris before it perforates something :)

I have played with the capacitance detection in the past. Range stinks.

They're called metal detectors when you put them on the end of a stick. Beat it against a ref and listen to it click.

Maybe as the cycle pulls up to the light, the coil senses the flux that wasn't there a split second ago (dphi/dt) and it gives the detector enough bump to make some noise. I've seen dumb detectors switch the lights after a right turn on red. Annoying little bastrds, they are.

Dang! Now I have to throw away my chunk of iron glued to a switch!