Patent GB-1347306-A (GB)
Title: Fire detector
IPC: G08B17/113
Filed: 1971-09-28
Abstract: 1347306 Semi-conductor oscillator and switching circuits GENERAL SIGNAL CORP 28 Sept 1971 [28 Sept 1970] 45099/71 Heading H3T [Also in Divisions H1 G1 G3 and G4] A fire detector includes a plurality of ionization chamber detector circuits 11 connected in parallel with a control unit 15 for indicating an alarm condition, each detector circuit 11 including an arrangement for sounding an alarm when smoke is detected and when there is a fault in the apparatus. Detector circuit 11, Fig. 1.-Each detector circuit includes series connected reference 20 and measuring 21 chambers the junction point of which is connected to dual gate MOSFET source follower 23. In the absence of combustion particles capacitor 28 charges through resistor 27 until its voltage exceeds the reference level on gate G3 of PUT transistor 26 when transistor 26 conducts to discharge capacitor 28 through the base emitter of transistor 32. When the voltage of capacitor 28 falls below the reference level PUT 26 ceases to conduct and the capacitor 28 is recharged. Each time transistor 32 is rendered conducting by the discharge of capacitor 28, a capacitor 40, charged through resistor 41, discharges through rectifier 42 and transistor 32. Thus the voltage that appears across capacitor 40 depending upon the relation of the repetition rate of the discharge of capacitor 28 and the time constant of 41-40 is such that PUT 43 is non-conducting and no alarm signal is generated. If smoke particles enter chamber 21 the voltage at gate G3 of PUT 26 rises and the voltage across capacitor 28 is insufficient to cause PUT 26 to conduct and hence transistor 32 remains non-conducting. Capacitor 40 charges to a higher voltage than average and when this voltage is greater than the gate G4 voltage of PUT 43 this latter conducts and discharges the capacitor 40 causing SCR 47 to be triggered and lighting FIRE indicating lamp 50 associated with the detector unit 11. Fire sensing, Fig. 2.-The line L to which the detectors 11 are connected in parallel is connected to terminals 113, 114 of a central control unit 15 which includes relay 61 and resistors 96, 97 in series with the detectors. When a fire is detected the increase in current through the relay coil 61, due to the added load of the lamp(s) 50 and resistors 56, 60 activates relay 61 and closes contacts 101 to trigger SCR 104. The current through the SCR 104 energizes relay 105, contacts 109 of which are available for fire indication and contacts 110 used to short out relay 61 to protect it from overcurrent. A resetting switch is provided to reset SCR 104 to restore the system to its inactive condition. Fault detecting circuit 17.-Pulses supplied to the line L by an end-of-line sender, Fig. 3 (not shown), located at the last detector 11 of the line are applied via transformer 112 to circuit 17. If the pulses are received by circuit 17 then transistors 129, 130 are conducting and relay 131 energized, its contacts remaining closed. If the pulses fail to reach transformer 112 transistor 129 is cut off causing relay 131 to de-energize and its contacts to open indicating a fault. Transistor 130 still remains conducting by the current through an "External fire lamp" (not shown) connected to terminal 133 and resistors 134, 135, 136, which current is insufficient to light the lamp. If the lamp is removed or fails or the positive supply voltage is lost transistor 130 will cut off and fault relay 131 will be de-energized. The control unit 15 includes a voltage regulated power supply (see Division G3). A construction of the ionization chambers and unit 11 is described, Fig. 1 (not shown), see Division H1.