| 摘要 |
A loop detector arrangement for detecting metal objects (e.g. motor vehicles) comprises an oscillator (1) and programmable frequency divider (2) operative on energisation of the arrangement to progressively produce a range of different frequencies in turn and to supply them to a parallel resonant loop circuit (9) via a resistor (8). A phase comparator (11) is connected across the resistor (8) such that its inputs will go out of phase when the loop detects a metal object, the output of the comparator thus operating a relay (15) via an amplifier (13) and driver (14). A state of tune sensor (12) connected to the output of the phase comparator (11) effects automatic tuning in that it stops the above-mentioned frequency progression when it detects that correct tuning has been effected. <IMAGE> |
