Alarm signalling over lines - switches zener diodes to discriminate between line faults and genuine alarms and includes transistor for triggering

摘要

The remote station (M) is connected to the monitoring station (U) by a line pair (a, b). The remote station contains a resistor (R11) and a Zener diode (Z11) which is normally short circuited by the alarm switch (K11). The monitoring station contains a batter, a Zener diode (Z2) and a controller (K) which monitors the line voltage and current. The two Zener diodes each operate at half the battery voltage and are both connected in opposition to the battery. The alarm is given by opening the alarm switch (K11), whereupon the line current falls to zero. The controller responds by triggering a transistor (T) to short circuit its Zener diode (Z2), which should restore the normal line current. This distinguishes a genuine alarm from a line fault and the alarm (A) is then energised. A higher voltage Zener diode (Z12) and a different resistor (R12) can be switched (k12) for signalling over the same line pair without risk of triggering the alarm.