| 摘要 |
In an inductive loop (36) detector used for detecting the presence of an object within an active area of the loop (36) a first signal dependent on the loop frequency is obtained at terminal (1) and compared in comparator (18) to a first reference signal which is dependent on the frequency of the loop without the object being within the active area of the loop. The signal produced by the comparison is compared at (22) to a threshold signal and an output signal is generated when the comparator (18) output crosses the threshold level. The reference signal is derived (using comparator (24), control (26) and memory capacitor (28)) from the first signal and is used to maintain an operating mode of the detector in the event of a power failure. <IMAGE> |
