Patent GB-823151-A (GB)
Abstract: 823,151. Automatic burglar and fire alarms. PAAR, J. Dec. 8, 1955 [Dec. 9, 1954], No. 35312/55. Class 118(1). [Also in Group XXXV]. An impedance bridge comprises plural bridge arms including a transistor with base, emitter, and collector electrodes, a frequency-determining element being connected in at least one of the bridge arms, which is interconnected with the transistor to form a self-excited frequency generator which is energized from a D.C. source to operate as an A.C. supply for the impedance bridge while forming an integral part thereof. Fig. 1 shows an impedance bridge comprising a centre-tapped magnetically cored inductance 32 in arm 10-20, a variable capacitance 26 in series with a D.C. battery 27 and a switch 28 in arm 20-30, a fixed capacitance 36 in arm 30- 40, and a blocking capacitance 33 in series with the capacitance 35 to earth of a valuable object to be protected in arm 10-40. The base electrode of a transistor 50 is connected to point 20, the emitter electrode is connected to the centre point of inductance 32, and the collector electrode is connected to the junction of capacitance 26 and battery 27. The stray capacitances of the transistor co-operate with inductance 32 to provide a tuned circuit which on closure of switch 28 to energize the transistor is maintained in oscillation to excite the bridge, which is balanced when the stray capacitance between the protected object and ground is at a predetermined constant value. The introduction of any foreign body between the object and ground introduces further stray capacitance to unbalance the bridge and develop a signal across diagonal 10-30 which energizes relay 60 to break contacts 39, 42, make contacts 39, 44 and operate an alarm; the transistor current increasing on unbalance. Capacitance 26 is variable to adjust the sensitivity of the apparatus. In a modification (Fig. 2) the transistor 50<SP>1</SP> base electrode is connected to the junction of one end of the centre-tapped inductance, fixed capacitance 51 and the blocking capacitance 33<SP>1</SP> in series with the stray capacitance 35<SP>1</SP> of the protected object, while the emitter electrode is connected to the inductance centre point, and the collector electrode 23<SP>1</SP> to the other side of capacitance 51 and to sensitivity-adjusting variable capacitance 52, which is returned to the free end of the inductance 32<SP>1</SP>. Capacitance 52 is shunted by relay 54 in series with switch 58 and battery 59, and the battery and relay junction is grounded through fixed capacitance 57. As before, on closure of the switch the transistor generates oscillations to energize the bridge, which is unbalanced by increase of stray capacitance 35<SP>1</SP> to energize alarm relay 54; the transistor current decreasing on unbalance. The relay circuit may comprise transistor and/or a thyratron operating a measuring instrument and/or a telephone receiver, while plural transistors in parallel may be employed to operate the oscillating circuit; the bridge may be responsive to changes in capacitance, inductance or resistance; the component capacitances and resistances may be made of materials independent of temperature variations, and the frequency determining circuit may comprise a piezoelectric frequency stabilizing crystal. The device may be attached magnetically to a ferrous object to be protected.