SE330659B - - Google Patents

Abstract

1,200,120. Fire alarms. CERBERUS A.G. 29 Dec., 1967 [29 Dec., 1966], No. 59190/67. Heading G4N. [Also in Division G1] A fire alarm comprises a smoke, temperature or radiation detector 1 feeding an amplifier (such as 4, 8) having feedback arranged so that a change in the condition of detector 1 is maintained until the system is reset. Detector 1 may be an ionization chamber or photocell; field-effect transistor 4 and junction transistor 8 being normally non-conductive. The occurrence of a fire causes the resistance of detector 1 to increase, driving transistors 4, 8 into conduction and providing an indication on a meter 9. This condition is maintained by feedback across resistor 3 which reduces the energizing voltage of the ionization chamber 1. The threshold at which the circuit operates is adjustable by resistor 6. A plurality of such circuits may be connected across terminals 10, 12, 15 to feed a common indicator 9; the circuits being tested by momentary operation of a switch 17 which provides an indication on a meter 19 should any one detector fail. In Fig. 4 (not shown) a further transistor 21 is shunted across resistor 6 so as to reduce the threshold in the event of a fire. Transistors 4, 8 also become inherently conductive in the event of an increase in ambient temperature. If a photo-cell is used as detector 1, the circuit may operate a light source, optical feedback being provided between the lamp and the photo-cell. Fig. 1 (not shown) relates to a known alarm comprising an ionization chamber coupled via a field-effect transistor and Zener diode to an indicator controlled by a thyristor.