A method for detecting the presence or absence of a target molecule(s) in a binding assay with potentially single molecule sensitivity. The quasi-digital magnetic field sensor switches between distinctive states in response to a change in the local magnetic field due to the presence of a nearby magnetic nanoparticle reporter. The signal produced by a single magnetic reporter is so strong that it can be detected with a digital voltmeter. The detection of a reporter in a binding assay thereby corresponds to the presence of a target molecule.