Detect H field from magnet?

[mvdswaluw]

Hello,

I would like to know if any of you have tried to see if variations in the H-field could be detected if you move a magnet in front of a ferrite antenna.
It would probably need to be moved/waved a bit fast because of the low pass filtering in the preamps to see anything.

The reason why I ask is because I want to see if I can make a system to detect magnets being brought into a fitting room.
Shoplifters use strong magnets to take of security labels. If you can detect their magnets, you can catch them in the act.

I already have a system that can detect a magnet at about 30cm, but I want to extend this to about 80-100cm.
The (leaked) pictures of the system blue preamp look a lot like what I had in mind to make, but in my case with low pass filtering and probably a lot of gain.

Milan.

[RichoAnd]

You need a big coil with many turns and a DC-amplifier (nearly, say 0,2-2Hz)
Best if people go through the coil.
Ferrite give no meaning here.

[Steph]

How about a magnetometer? The folks from the SAM project use very sensitive ones that can detect changes in the nanotesla scale:
http://www.speakesensors.com/products.htm

[mvdswaluw]

How about a magnetometer? The folks from the SAM project use very sensitive ones that can detect changes in the nanotesla scale:
[/url][url=http://www.speakesensors.com/products.htm]http://www.speakesensors.com/products.htm

My first experiments I did with an HMC5883L IC hooked up to an Arduino. The sensitivity is a bit poor. I can get a proper detection from about 30cm when I do oversampling and take moving averages over a long and short term. At that distance I get about 3-10 milligauss (1000nTesla) deltas . At about 10cm one of the three axis already saturates, so 12bit is not enough dynamic range for me.
 
I've already looked into fluxgate magnetometers, but have not tried it yet. I've seen graphs where people can detected 0.5 milligauss variations at a level of around 500 milligauss.
My next experiments? I think I'll try to build a small magnetometer (smaller version of this) and hook up a 16bit A/D-converter to that and feed that into an Arduino.