How do you Build a Magnet Detector

Try a battery connected to a resistor and a LED. Place a reed switch on one of the supply lines. If a magnet comes by, the reed switch will close and the LED will light. This is simple and effective if you can get the circuit reasonably close to the magnets. If you build this, make sure it's in a plastic case-- not metal.

Ron

I'm looking for a circuit that can detect a magnet passing through a field about 1 foot away from the detector. The magnets are the small powerful type as found in harddrives. We need to detect if a person is carrying a magnet in his or her pocket or purse.

Is this possible?

Thanks, Cindy

Scientific American had an article in their Amateur Scientist some time ago. Get it at the library or on their CD. The device detected very minute variations in the earths magnetic field. As I remember it was like an ultra-sensitive galvanometer. It used a tiny mirror on a torsion thread in a magnetic field. The mirror had a laser pointer bounced off it. I'm sure this thing could detect a magnet within 10 feet. A wire loop around a door frame and a sensitive enough detector would also work. This also gets into the area of violating someone's rights to privacy and freedom from illegal search. Anyone scanned would need to be warned. Richard

you could try a compass with wires either side of the needle, the magnet deflects the needle and it contacts the wire... it worked for submerged sea mines, it's not suitable for a handheld detector (and less than ideal for portable) but may work for your aplication.

Bye. Jasen

Not sure what you mean by a magnet "passing through a field". Do you already have some sort of magnetic field present? To simply detect a moving magnet, you just need it to pass by a coil of wire. The moving magnetic field of the magnet will induce a curent in the coil, which you can detect. The trick is making the setup sensitive enough. Having a soft iron core in the coil will help. You may need to have an unusual core shape, such that the face of the coil is much taller than wide, in order to allow for people having the magnet at different heights on their bodies. Or you may want to use a column of some readily available relay coils, appropriately wired in series.

Note that the magnet should pass perpendicular to the axis of the coil.

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

Home of DaqGen, the FREEWARE signal generator

A magnetic compass might work?

Other extreme - a gauss meter.

Jordan

C> I'm looking for a circuit that can detect a magnet passing through a

Very possible.

Look for info on "fluxgate magnetometers." There's a brief intro at

lots of links at and some relatively inexpensive (and easy to interface) gadgets at Also possible to roll (wind) yer own.

It's probably even possible to mount the sensors in such a way as to derive real-time X-Y positioning of the magnet.

--
Rich Webb   Norfolk, VA

Add to the coil an op-amp with a gain of say ten, and it's output connected to a transistor to act as a driver for a LED or a small relay. While the magnet(s) moving through the coil will only produce a short burst of current, the relay can be used to turn on a simple 555 oscillator to produce a tone of your choice.

Ron

I got sold one with the device still attached, it took me a while to figure out how to unlatch it. he hard drive magnet was at-hand fimly attached to the refrigerator and did the trick easily, I was wondering if your application was related.

magnets are easy to shield from sensors.

Bye. Jasen

Very possible. Look at magnetometers. A fluxgate magnetometer would certainly detect the field from such a magnet but you might be able to do it much simpler technology. Look at metal detector circuits and other circuits which detect currents induced in a loop or coil.

Someone might care to comment on the possibility of feeding the two ends of a search coil into the + and- inputs of an op amp and detecting the output.

R

A coil constructed in or on the door frame will generate a current when a magnet passes through it. Try an op-amp to detect the current in the coil.

R

Have all entering customers roll in a pile of iron filings before they are allowed to browse the selection.

Or, why not focus on the actual problem: one or more people who have figured out your system, and are morally OK with stealing. It's probably no more than a couple people. Buy or rent security cameras, taking note of the criminal's video preferences. Once you have a tape of the criminal in the act, alert the authorities to be on standby when you spot that person again. Have them wait outside so the criminal can be caught with the stolen items...problem is now solved.

Have you been building your own electronics stuff in the past? If not,

then this is probably too complicated for a total newbie. But a newbie with lots of ambition might pull it off.

If I was doing this, I'd put a tiny chip of mirror on a compass, then bounce the light from an LED off the mirror chip to a photodiode. Use a cheap lens to focus the LED to cast a small spot on the photocell, then

move things so the spot of light is only halfway on the photocell. If the compass moves, the photocell output changes. Put it all in a light-tight box. To make this more sensitive, just move the photocell farther away from the compass so that a small compass deflection moves the spot a larger amount (a larger box might be needed.)

For the mirror chip, I'd smash a cheap front-surface mirror bought from a surplus optics dealer. That, or try to put a silver or mercury plating on a microscope slide's cover-slip. Maybe put the whole compass in a cup of kerosene or light mineral oil to keep it from wobbling from floor vibrations.

Then rig up some kind of alarm circuit with a photodiode amplifier and a "window comparator" that fires a piezo beeper whenever the light gets

brighter or dimmer.

A totally different method would be to amplify the output of a linear Hall chip, then filter out the noise. Hall chips aren't too sensitive,

but they can just about detect a signal as strong as the Earth's field (like a few Gauss.) This might detect a supermagnet from a couple of feet away. Or it might not be sensitive enough. The main problem is that the output voltage of a linear Hall chip will drift a bit, and the drift could mimic the magnet signal.

Linear Hall Chip $2.99

Another method: If you have a cheap oscilloscope, shut off the H deflection and turn down the brightness. Use a tiny dot of opaque tape or foil to block the little green glowing spot. Temporarily glue a photodiode or photocell over the location of the dot. Now if a magnetic field should cause the electron beam to deflect, the green spot will come out from behind the opaque dot and shine upon the photocell.

(((((((((((((((((( ( ( ( ( (O) ) ) ) ) ))))))))))))))))))) William J. Beaty SCIENCE HOBBYIST website snipped-for-privacy@eskimo.com

amateur science, hobby projects, sci fair Seattle, WA 206-789-0775 unusual phenomena, tesla coils, weird sci

or use a sequin, or a fragment of a blank part of a surplus cd.

Bye. Jasen

That's what I just said.

Ron

Do you suppose that a multi turn coil, about to metres in diameter, shallowly buried under a driveway or path, if connected thus to an op-amp would be sufficient to detect a firearm or crowbar carried over it? I presume that such a coil would always detect a car?

R >

As sold in the US most DVDs have a security device inside the retail case, installed by the maker. Detector loops at the doors will alarm if the strip is not cancelled by being magnetised.

Yes, if you have enough gain. You can't get that kind of gain from a single op-amp stage but two (or more) chained together will provide sufficient gain without too much noise. You would also have to pick the op-amps carefully to get good gain without excessive noise that could drown out your incoming signal. Of course, you could probably do the same thing with several transistor amplifier stages which needn't be too complicated. If you look through some of the free circuitry websites that have all kinds of circuits for alarms, games, etc., you might find a circuit that you could easily adapt to your needs.

Ron