GB1310169A - Alarm systems - Google Patents

Abstract

1310169 Smote detectors; SIEMENS A G 24 March 1970 [25 March 1969] 14154/701 Heading G1N [Also in Division G4] A fire alarm system comprises a number of smoke detectors connected to a central station, each detector being adapted to produce a different frequency a. c. signal. Each smoke detector K2, in the form of an ionisation cell with two radio-active chambers, is connected in series with a reference detector K1, the detector K1 being sealed and the detector K2 communicating with the air. The presence of smoke causes the conductivity of the detector K2 to be reduced so that the potential at a point 3 changes, terminals 1, 2 being connected to a source of potential in the control station. A field effect transistor T1 is therefore triggered and an audio frequency signal, having a frequency dependent on the values of a capacitor C and a winding L1 of a transformer L1, L2, is fed to the central station. Filter circuits at the central station channel the signal to a respective indicator device to identify the position of the detector that has sensed smoke. In a modification the circuit is maintained in oscillation when the detector ceases to sense smoke. The detector circuits may be triggered for test purposes by increasing the voltage applied to the terminals 1, 2. A lamp and further transistors may be included in each detector circuit so that the lamp is energized when the detector senses smoke, thereby indentifying the operated detector.