Re: [EE] measuring the magnetic field from a small trace carrying a < 50uA load?

Chuck Olson <[email protected]> writes:
> Do you have an old busted cassette recorder ?  Maybe the playback head
> could be used to sense the field ?

	That's an interesting thought but you only get voltage
from an inductor sensing a magnetic field if the inductor is
physically moving or the magnetic field is constantly changing.

	A Hall-Effect device is a solid-state component whose
resistance changes when exposed to a magnetic field.  Think of a
transistor's response to a current change on it's base or a
voltage change to the gate of a FET.

	The only Hall-Effect device I own is a chip that looks
just like a flat-pack transistor.  The Hall device is in there
along with a level detector .  There are 3 leads coming out.  One
is Negative/Common.  Another is 5-volt VCC and the third lead is
the sense lead and switches between 0 and 5 volts depending on
proximity to the correct polarity of a magnetic field.  When it
receives that field, the logic changes suddenly so there is no
linearity to read the strength of the field.  One uses this chip
anywhere one might use a magnetic reed relay such as in a door
interlock or maybe a timing indicator.  I haven't played with
that device in a while but it starts generating pulses at AC
power line frequency if you get near a transformer or motor with
it.  It also doesn't have the mechanical issues one sometimes
runs in to with reed switches such as false trips or opening when
jarred.

	Hitachi once even sold a tape recorder with a Hall device in
the playback head which would show output all the way down to no
movement of tape at all to aid in editing.  I don't know how well
it worked but a sensitive Hall device might do the job.

	I once lost a really nice magnetic pickup tool in our
yard and wished I had a linear Hall device that would detect a
magnet which, I think got accidentally buried in a flower bed
when it fell out of a tool box.  It may still be there for all I
know.

Martin	WB5AGZ