GB1522001A - Monitoring circuit arrangements - Google Patents

Abstract

1522001 Monitoring voltage levels SIEMENS AG 31 July 1975 [21 Aug 1974] 32018/75 Heading G1U A monitoring system e.g. for signal levels in telecommunication systems, includes a two-transistor bi-stable trigger, which is operated by the conduction of a diode when the monitored level falls below a first reference, or by the conduction of a transistor when the monitored level exceeds a second reference. The bi-stable formed by transistors 195, 196 rests with 195 conducting when the levels applied to the inputs 6-8, are correct i.e. around -7À5 volts. If the levels at 6, 7 rise towards zero, the input diodes 157 or 155 begin to conduct at a level defined by the reference Zener 163, via resistors 73-78 and transistor 199. At this point, transistor 195 turns off and 196 comes on. If the level at input 7 falls negatively, eventually the voltage drop across diode 156 and the base-emitter junction of transistor 191 from the reference set by Zener 163 via resistors 73-76 and transistor 194 is sufficient to turn transistor 191 on, raising the voltage at the base of transistor 195 via resistor 49 and diode 158, with the results as described above. The state of the bi-stable 195, 198 is passed to an output stage providing momentarily grounded pulses on outputs 12, 13 and constant ground on outputs 15, 17. Two of the circuits shown are used to provide redundant protection to a telecommunication arrangement. To avoid signalling an alarm condition at the switching on of the apparatus when -12 volts is applied to terminals 9, 11, capacitor 130 charges up to this voltage and turns on transistor 193. This causes the base-emitter junction of transistor 192 to be forward-biased turning this on and making the base of transistor 195 negative, ensuring that this turns on and remains so until the capacitor 130 is fully charged and the apparatus is stable and set to monitor. Transistor 192 may also be turned on by applying earth on terminal 21 or operating push-button 215. Operation of an alarm signal by spurious signal level changes, is prevented by transistors 196, 197 and capacitor 133 as described in Specification 1522002.