Science Buddies: "Ask an Expert"

We built this and it seems to work, voltage reads correctly before magnet is tested. The problem is we can't seem to register anything other than a negative reading. When we flip to the opposite magnet pole, the needle might barely budge off baseline in a pos direction, but only just. Should be a symmetrical change in voltage reading to the negative per description, right? What are we missing here?

Thanks!

mwferris - According to the procedure, if everything is wired correctly and you are observing 5.0 volts at the output of the voltage regulator, you should be reading something like 2.5 volts on the voltmeter as your baseline. This baseline is observed without the magnet near the sensor. Is the voltmeter set to an appropriate range? Usually on digital multimeter's that would be the 20 VDC scale. Is the output of the 7805 voltage regulator (pin 3) connected to pin 1 of the Hall effect device? As you position one of the poles of the magnet toward the face of the Hall effect device you should get a reading above or below 2.5 volts (or whatever your baseline voltage is). This depends on which pole of the magnet is being sensed by the sensor. If you measure lower than baseline voltage implies you are reading the 'N' pole. If the voltage is above baseline, you are measuring the 'S' pole.

Rick Marz