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Electronic blackboard system marker and eraser therefor

  • US 5,134,388 A
  • Filed: 04/25/1991
  • Issued: 07/28/1992
  • Est. Priority Date: 09/14/1987
  • Status: Expired due to Fees
First Claim
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1. In combination, a coil, plural objects adapted to be selectively in proximity to the coil, each of the objects including a tuned circuit having a different resonant frequency, means for supplying AC energy at the different resonant frequencies to the coil, the tuned circuit on the particular object in proximity to the coil having substantially the same resonant frequency as the frequency of the AC energy supplied to the coil causing a change in the current flowing in the coil at said same resonant frequency, means for sensing the change in the current flowing in the coil at said same resonant frequency, and means responsive to the current change at said same resonant frequency for indicating the particular object having the tuned circuit with the same resonant frequency as the frequency of the applied AC energy, said indicating means including phase detector means responsive to the changed current flowing in the coil at said same resonant frequency and a reference wave at said same resonant frequency for:

  • (a) deriving indications of the polarities of the phase of quadrature components of the change current relative to the reference wave, and (b) indicating the durations of the polarity indications relative to predetermined values therefor.

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