| 摘要 |
An intruder alarm system comprises a first housing 2a containing a first sensor 4a, and a second housing 2b containing a second sensor 4b. A control circuit 10 receives signals from the first and second sensors 4a,4b and determines an alarm condition only when both sensors detect an intrusion within a predetermined time interval. The respective sensors 4a,4b can be installed in locations that are spaced apart, while monitoring a single zone 6. Thereby both the sensors 4a,4b will respond to an intruder entering the zone but they will respond differently to environmental factors such as flying birds that are local to just one of the sensors so that false alarm conditions can be distinguished and ignored. The sensors may comprise at least one passive infra-red sensor and at least one Doppler microwave sensor. |
