| 摘要 |
1,184,043. Bridge circuits. TELEFONAKTIEBOLAGET L M ERICSSON. Feb. 21, 1967 [Feb.22, 1966], No.8316/67. Heading G1U. In a circuit arrangement, for use in burglar or other alarms, a self-locking relay 18 is connected in the output of a transistor amplifier and has a disconnection time less than half the period of the A. C. fed to a bridge network whose output under balanced conditions is a series of unidirectional pulses Fig. 3 (not shown) controlling the amplifier, when the bridge balance is upset, alternate pulses at the bridge output 32 have different amplitudes Fig. 4 (not shown) and the relay disengages, thus operating an alarm. Either (or both) of the impedances 2, 3 in the bridge network 31, 32, 33, 34 comprises a series of coils whose impedance is charged when approached by an intruder so that the bridge balance is thereby upset. A varistor 4 converts the sinusoidal input to a trapezoidal or rectangular waveform, but may be replaced by a linear resistance, in which case the input to the bridge is preferably trapezoidal or rectangular. After operation of the alarm the circuit is reset by a switch 20. |
