In case anybody in motivated, I developed a simple magnetometer (based on an Allegro chip) for use in measuring the field strength of guitar pickup magnets. Note that this circuit is intended to drive a high-impedance load like a digital voltmeter or the like.
Thanks. The predecessor circuit, never published, was far more complex.
It occurs to me that a gradiometer circuit might be useful for detecting nearby field generators at the expense of distant field generators.
If one uses two hall-effect chips separated by a known distance and oriented the same way, if one feeds the outputs into opamp plus and minus inputs, the output will be the field difference.
In my simple magnetometer circuit, if one replaces the 15K-500-15K divider string with a second hall chip oriented the same as the first, the difference in outputs will be reported by the voltmeter.
One can implement a differential sensor with two hall chips glued to one another back-to-back. This may be useful because the Micronas HAL401 may be hard to buy in small quantities.
Analog Devices offers one linear hall chip, the AD22151, which appears suitable as well, and should work in my simple circuit in place of the Allegro chips (although the pin numbers will likely change). The AD22151 is a little SOT-23 surface-mount chip, but I bet one can easily solder magnet-wire leads to it, and make a nice little probe with it.