| 摘要 |
<p>A transmitter is actuated by a control signal (17). Oscillator (12), coil (10) and core (11) generate an oriented magnetic field received by a tunable, rotatable coil (14) and amplifier (140) feeding a threshold circuit (141). A metallic mass (M) near the receiver increases the received signal above pre-set threshold and operates a circuit (142) to indicate presence of (M). The transmitter may be triggered by a coded signal. The receiver core orientation may be adjusted to detect any local field and associated information; it may also be triggered by an applied control signal. The units may be fixed on a vehicle so as to detect the passage of further vehicles. Information may be transmitted between fixed and vehicle units.</p> |
