Electronic blackboard system marker and eraser therefor

Abstract

A determination is made as to which one of plural objects is proximate a region having a coil therein. Each object includes a tuned circuit having a different resonant frequency. AC energy at the different resonant frequencies is supplied to the coil. The tuned circuit on the particular object in the region having substantially the same resonant frequency as the AC energy supplied to the coil causes a change in the current flowing in the coil at the same resonant frequency. The change in the current flowing in the coil at the same resonant frequency is sensed. In response to the current change at the same resonant frequency the particular object having the tuned circuit with the same resonant frequency as the frequency of the applied AC energy is indicated with a phase detector arrangement responsive to the changed current flowing in the coil at the same resonant frequency and a reference wave at the same resonant frequency. The phase detector arrangement derives indications of the polarities of the phase of quadrature components of the changed current relative to the reference wave and indicates the durations of the polarities relative to predetermined values therefor.