GB1160760A - Automatic Fire Alarm System - Google Patents

Abstract

1,160,760. Fire alarms. HOLGER NIELSEN & CO. A/S. 19 Aug., 1966 [21 Aug., 1965], No. 37366/66. Heading G4N. A fire alarm system comprises a plurality of temperature-responsive relaxation oscillators coupled to a central station and a further relaxation oscillator arranged to feed pulses of different amplitude to the central station for line testing purposes. All the temperaturesensitive oscillators and the line-testing oscillator are coupled in parallel to the central station by means of one signal conductor 6 and a pair of power supply conductors 4, 5. Each temperature-responsive oscillator may comprise a capacitor C1 (Fig. 4) chargeable at a rate depending on the relative values of an adjustable resistor R1 and a temperature- or radiation-responsive resistor T, unijunction transistor T1 breaking down when C1 is charged sufficiently, the frequency of pulses thus generated being a function of temperature around resistor T. The pulses produced by T1 are amplified by T2 and fed to the central station on a common signal lead 6. The breakdown voltage of transistor T1 is made independent of the voltage on supply lines 4, 5 by means of a Zener diode Ze1 Fig. 3 (not shown) depicts a line-testing oscillator generally similar to that shown in Fig. 4 but omitting the temperature dependent resistor T and including a Zener diode in the collector circuit of T2 so as to provide pulses of different amplitude from those of the temperature sensors. Central station (Fig. 6).-Alarm signal pulses on line 6 are arranged to swing from + 12 V. to 0 so that they can pass diode D2 to an integrator comprising capacitors C1, C2 and transistor T1 If the frequency is sufficiently high (indicative of a high-temperature condition), unijunction transistor T2 will break down, triggering SCR, energizing a lamp AL and a relay RE2. Line testing pulses are arranged to swing from + 12 V. to + 9 V. and thus cannot pass diode D2 but are fed only into the integrator C3, C4, T3 which is coupled via a super emitter follower T4, T5 to a relay RE1. Under normal operation this relay is energized, so as to light a lamp KL, but under fault conditions the relay becomes released, energizing a fault lamp FL. The supply to the local sensors on line 4 is also disconnected under these conditions. The alarm circuit may be reset by operating push button TR. The sensitivity of all sensors is adjustable by potentiometer 21.