A low power magnetic-field detector, of the kind for detecting an ambient magnetic field that is greater than a predetermined field strength, is comprised of a Hail element, a transducer-voltage amplifier, and a zero-crossing comparator, all connected in tandem. A clock and switch are used to chop the energizing current to the Hall element. Alternatively, the amplifier and comparator are also chopped to further reduce power consumption. A clockable flip flop is synchronously enabled for an instant at the end of each period of energizing the Hall element. The comparator output signal is transferred to the flip flop Q output and held there during each period of not energizing the Hall element. A Positive-feedback hysteresis circuit adds a bias voltage to the amplified Hall voltage and is applied to the comparator input to effect comparator hysteresis with memory covering clock-period portions when the Hall element is not energized. The comparator is thus transformed into a Schmitt trigger circuit having hysteresis memory, making it possible to chop the Hall element to substantially reduce the detector power consumption without loosing the hysteresis feature.