Detection of portable recorder

Lorenzo Busetto ( info@istitutomercurio.it )
Mon, 12 Apr 1999 07:58:10 +0200

I'm looking for a device that can detect a portable recorder (in function)
at the distance of about 1 or 2 meters.
This device could work by detecting the following things:

- the magnet inside the speaker
- the rotating magnet inside the dc-motor
- the magnetic fiels generated by the recording/erasing head.

Can anybody help me for this problem ?
Thank You in adwance.

the frequency bias of a clening head of a tape recorder, but I don't know
what is the frequency bias.
Can You help me ?