Apparatus for testing the authenticity of paper currency

Claim 1

Apparatus for testing the authenticity of paper currency comprising, in combination, an oscillator emitting a radiation oscillating at a predetermined frequency, conveyor means conveying paper currency to be tested across the path of said radiation, said conveyor means generating a synchronizing signal for indicating the location of paper currency conveyed by said conveyor means, a receiver receiving radiation transmitted from said paper currency sensing the received radiation and producing an output signal determined only by the radiation received oscillating at substantially the same frequency as said predetermined frequency, said receiver comprising photoelectric means generating an AC output signal corresponding to the oscillating frequency of said radiation, an optical filter for transmitting radiation of essentially the same wave length as radiation emitted by said oscillator located between said conveyor means and said photoelectric means, sensing means sensing said output signal selectively generating an accept or reject signal as determined by said output signal, said sensing means including an amplifier amplifying selectively only AC voltages which lie within the range of the oscillating frequency of said radiation, an evaluating channel comprising a second amplifier, a band filter tuned to a preselected frequency and an amplitude discriminator in series with said first mentioned amplifier for receiving the output signal of said amplifier, a collecting stage in series with said evaluating channel for receiving the output signal from said evaluating channel, and means at said collecting stage for switching through said output signal from said evaluating channel at a preselected value of said synchronizing signal produced by said conveyor means.