| 摘要 |
An emergency vehicle proximity warning system is disclosed which includes a low power transmitter subsystem carried on board an emergency vehicle and a special purpose receiver subsystem carried on board a second vehicle. The transmitter issues a continuous stream of equally timed-spaced pulses. The receiver subsystem detects the reception or non-reception of the pulses and includes a retriggerable multivibrator having an output which assumes a first logic state when the pulses are not present and a second logic state when the pulses are present. A relay coil, which is energized only when the retriggerable multivibrator is in its second logic state, actuates a set of normally closed relay contacts which are connected into the speaker leads of the vehicle's conventional radio to interrupt the connection between the conventional vehicle radio and its speaker system when the pulses from an emergency vehicle are sensed. To provide a very clear indication to the vehicle operator that the emergency vehicle is nearby, one or more lamps and one or more emergency-sound-issuing devices are alternately energized and deenergized so long as the retriggerable multivibrator is in its second logic state to provide a visual and sonic alarm indication. Preferably, an astable multivibrator is coupled to the retriggerable multivibrator such that it is enabled only when the retriggerable multivibrator is in the second logic state to obtain the information for driving the alarm indicators.
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