Vehicle presence loop detector

Claim 1

A metallic object presence detector capable of detecting multiple entry and exiting, of maintaining true presence detection for an indefinite period of time, and of operating in either one or both of two modes namely, one mode in which the detector is capable of detecting the entry of said object and a second mode in which the detector is capable of detecting the removal of said object comprising: A. electrical signal generating means comprising a tuned circuit means and tuning indicator means, said tuned circuit having at least one magnetic field-producing induction loop for receiving said metallic object in proximity thereto to change the field configuration and corresponding signal resulting therefrom, B. detector means which receives a signal from said loop, C. signal amplifier means to amplify the signal from said detection means, D. bi-directional amplifier means which responds only to positive and negative polarity rapidly changing signals from said signal amplifier means which signals are caused by movement of a metallic object into or out of the area of said loop, said bi-directional amplifier means further comprising: (1) dual amplifier stage means each having output voltages of opposite polarity in a non-detection state and the same polarity, whether positive or negative, in a detection state, and co-acting therewith (2) dual resistor-capacitor means which momentarily pass said rapidly changing signals from said signal amplifier means to said dual amplifier stage means so as to change the output voltage polarity of one of said dual amplifier stage means, E. means for sensing the signal polarity of the output signal from said bi-directional amplifier means to produce a pulsed, gated output signal, and F. means for receiving said pulsed, gated signal to produce an indication of presence or absence of said object in the vicinity of said loop.