Patent GB-1248763-A (GB)
Title: Security container
Inventor: MUNTON EDWARD AMBROSE
IPC: E05G1/14
Filed: 1968-07-18
Abstract: 1,248,763. Preventing theft. VIGIL SECURITY Ltd. 15 July, 1969 [18 July, 1968], No. 34283/68. Heading E2X. A portable security container comprises a lock 22 which if opened before a correct sequence of voltage pulses is applied to a cascaded series of gating circuits 34, 36, 38, 40 causes the detonation of an explosive fuse 26 which ruptures an adjacent container 28 containing an indelible dye or a corrosive substance. A 10-pole switch 70 routes a voltage pulse, generated by connecting a switch terminal to the battery 32 by the push button 80, to either one of the four gate circuits 34, 36. 38, 40 or directly through a diode 72 to a silicon controlled rectifier 50 in series with the fuse 26. The pulses to gates 34, 36, 38 are routed through diodes 74, 76, 78 to the control terminals of silicon controlled rectifiers 35, 37, 39 respectively in the gate circuits. Initially the four rectifiers 35, 37, 39, 41 are non-  conducting with their cathodes at the battery positive voltage and operation is as follows: a positive voltage pulse is first routed through terminal Tl of switch 70 to the control terminal of rectifier 41 of gate 40 causing the rectifier to conduct thereby reducing its anode potential and consequently the potential on the cathode of diode 78 connected to gate 38; a subsequent pulse applied to the anode of diode 78 by routing a second positive pulse through terminal ,T9 of the switch 70 can then be transmitted by the diode to open gate 38. simultaneously lowering the cathode potential of diode 76. After four pulses in the correct sequence, gate 34 is open and the anode of rectifier 35 is at such a low potential that on connection by means of the key operated switch 30 through c to the control terminal of rectifier 50, the rectifier is not switched on and thus the fuse 26 is not blown. If a pulse is routed to a gate such as 34 out of the correct sequence then the potential at the cathode of the diode 74 will be high, thus the voltage pulse applied to the anode sees a high impedance and passes through diode 75 to switch on the rectifier 50. Thermal contacts 109, 110 are provided which connect the control terminal of rectifier 50 to the positive battery line A- when the container temperature is respectively raised above or lowered below predetermined values. A Zener diode 52 provides a reference voltage at the base of transistor 88 such that when the battery voltage falls the consequent voltage drop of the transistor emitter connected to the battery by the potential divider 84, 86 turns on the transistor at a predetermined value to produce a positive pulse at the collector of the transistor 90 to turn on the rectifier 50. A further guard circuit in a wall of the container consists of one or more continuous convoluted electrical conductors 96, 96a together with convoluted or branched open-circuit conductors 98, 100. Attempted entry into the container through the wall causing the conductor 96 or 96a to be open-circuited or short-circuiting 98 or 100 to 96 or 96a would result in a positive voltage applied to the control terminal of rectifier 50 thus blowing fuse 26. In a circuit modification the capacitance between the continuous conductor 96 and an open-circuited conductor 98 can be balanced in a Wien Bridge 138 of Fig. 4 (not shown) such that a change in capacitance due to pressure on the container wall produces a bridge output which after amplification is rectified and used to trigger a Schmitt Trigger circuit to produce a positive voltage at the control terminal of rectifier 50.