This is a magnetic field meter or Gaussmeter which uses a UGN3503 Hall effect device to drive a meter. It shows the magnetic field sensor reading the strength (in Tesla’s) of a small horseshoe magnet. The meter is not yet calibrated so the reading is not correct.
“Continue reading” for circuit diagrams and specifications. 2 photos and links…
The circuit is simple and has provision for zeroing the meter and adjusting the gain for calibration. A North pole will read negative on the meter and South will be positive. Note that the Hall device has a sensitivity of 1.3 mV per Gauss so only reads up to about 1000 Gauss (= 0.1 Tesla) so will over-range in the field close to a typical NIB magnet. It also only reads from 1 Gauss so it does not read the earth’s magnetic field well which is only about 0.5 Gauss.
The underneath is untidy but fast point to point wiring.
The circuit diagram has provision for a switched external sensor. Not sure if this is a real smart circuit. It should really have a proper voltage reference rather than a simple voltage divider.
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Photo Date: 2008