Alarm trigger for key actuated cylinder lock - uses divergence of two half-discs to detect key and move rocking lever which makes contact

摘要

Designed to fit existing key-actuated locks, the mechanism detects the presence of a key (28) or a lock-picking tool and energises an electric alarm circuit. It utilises the radial expansion of a split disc to displace a return sprung rocking lever (18) and make an electric contact. A slender housing (8) contains the mechanism, retained by bolts which are accessible from the inner door face, and fits over the outstanding barrel end of the existing lock. A stepped bore through the major face of the housing gives key access, and locates two demi-discs (15a, 15b) which abut on their diametric edges. One demi-disc (15a) is fixed and the other (15b) is able to move laterally to displace the rocking lever (18). The lever may move a spring steel arm (24) which controls a micro-circuit breaker (23), and may be returned by a spring (21) loaded plunger (20). An inserted key (28) or other separates the demi-discs to actuate the mechanism, and time controls may delay the alarm for a definite interval pending recoil.