(19)
(11) EP 0 206 664 A3

(12) EUROPEAN PATENT APPLICATION

(88) Date of publication A3:
01.03.1989 Bulletin 1989/09

(43) Date of publication A2:
30.12.1986 Bulletin 1986/52

(21) Application number: 86304518.3

(22) Date of filing: 12.06.1986
(51) International Patent Classification (IPC)4: G08B 13/18
(84) Designated Contracting States:
AT BE CH DE FR IT LI LU NL SE

(30) Priority: 12.06.1985 GB 8514862

(71) Applicants:
  • Hughes, John Peter
    Little Neston Wirral (GB)
  • Fletcher, Brian Dows
    Neston Wirral (GB)

(72) Inventors:
  • Hughes, John Peter
    Little Neston Wirral (GB)
  • Fletcher, Brian Dows
    Neston Wirral (GB)

(74) Representative: Dodd, David Michael et al
ROYSTONS 531 Tower Building Water Street
Liverpool L3 1BA
Liverpool L3 1BA (GB)


(56) References cited: : 
   
       


    (54) Guard system


    (57) Guard system for protecting some space against undetected intrusion uses a plurality of wave beam paths (from transmitter T1-Tn to receivers R1-Rn) selectively energised in cycles (multiplexer Tx-Mx) so that each beam (transmitters T1-Tn) is energised for part only of each cycle. Pressure or absence of beam energy indicative of intrusion is monitored (receivers R1-Rn and multiplexer Rx-Mx). Signal level discrimination (46) of received signals operates both logic circuitry (flipflop 55) to give effectively continuous output (56, 58) except for beam interruption (52) even through a quiescent state of each cycle, and fault indication logic circuitry (100 via 70, 72) that is shown duplicated.






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