Cindy said:
Knowing someone enters with a powerful magnet can give them
a heads up so they can keep an eye on the person.
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
http://www.electronicsurplus.com/cc...ic-linear-hall-effect-sen-ugn3503u-135600.htm
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
[email protected] http://amasci.com
EE/programmer/sci-exhibits amateur science, hobby projects, sci fair
Seattle, WA 206-789-0775 unusual phenomena, tesla coils, weird sci