Idea adapted from http://my.execpc.com/~rhoadley/magmeter.htm.
■ FIGURE 10. If you would like to amplify your signals, you can
use this diagram.
■ FIGURE 9. The finished product. Note that the
two small Neodymium-Iron-Boron (NdFeB) magnets
shown are enough to drive the chips to their limit of
approximately five volts. If you flip them over, you
will get approximately negative five volts.
Once you've built the magnetometer, you can
use it to measure a variety of things. You'll find
that ferrous materials that you haven't magnetized
will still provide a signal when held near the
probe. It's certainly not a very precise compass,
but you will probably be able to find the general
direction of magnetic North.
This only requires a cheap LM358 op-amp
and half a dozen resistors. The two green wires
on the left side of the diagram are to be
connected to a voltmeter. The two white wires on
the right side are to be connected to your
magnetometer outputs. For some cases, the gain
will be too large. You can reduce the size of the
larger resistors (shown as 100K here) and/or
increase the size of the smaller ones (1K here).
This can be powered by a separate 9V battery.
No voltage regulator is necessary in this case.
If your voltmeter is set for AC, you can pick
up the magnetic fields produced by electrical
appliances. The key here is to split an extension
cord so that the two wires are not side by side. If
they are, the opposing currents will produce
magnetic fields that tend to cancel each other
and it will be hard to detect anything.
Once you've built this magnetometer, you
may be surprised at the uses for it. Anything that
produces a magnetic field is a fair target, and the
Hall chips respond to changing fields far faster
than mechanical detectors such as reed switches.
This means they can also be used as rotation
sensors when placed near the edge of a gear. The
field when a tooth is directly under the Hall chip
will differ from the field when a gap between
teeth is directly under it. In this way, you can
produce a signal that corresponds to the rotation
of a wheel.
You might find that a surprising number of
your sensor projects can be completed with some
magnets and this detector. If you build one and
come up with a neat application for it, be sure to
let me know! NV
■ FIGURE 11. Schematic of magnetometer
if constructed on a breadboard.