| 摘要 |
The security system involved in the arming and disarming of a car alarm, includes a transmitter (2) and a receiver (4) each including an oscillator (6, 8) incorporating a highly accurate and stable quartz time base. Each oscillator (6, 8) drives a respective encoder (14, 20) to develop a pseudo random succession of codes. Reset switches (10, 12) are provided for both transmitter and receiver (2, 4) to synchronise the two oscillators and therefore to synchronise the succession of codes. The receiver (4) includes a decoder (24) for decoding the received signals and a comparator (26) for comparing the decoded signal with the signal generated by the encoder (20). When equality is detected an arm/disarm circuit (28) is triggered. Because the codes of the transmitter and receiver are continually changing, it makes it very difficult for a thief to produce a coded signal which will trigger the arm/disarm circuit. |
