Patent GB-1483782-A (GB)
Title: Ionization smoke detector and alarm system
IPC: G01N27/64
Filed: 1974-10-21
Abstract: 1483782 Fire alarms EMHART INDUSTRIES Inc 21 Oct 1974 [25 Oct 1973] 19420/76 Divided out of 1483781 Heading G4N A non-latching, pulsating smoke alarm system comprises a smoke sensor 20 in the form of an ionization chamber, a switching device 120 responsive to the sensor 20 for turning an alarm device 40 on and off, coupling means 108 interposed between sensor 20 and device 120 for triggering device 120 to energize the alarm in the presence of smoke, deactivating means 130, 140 for resetting the device 120 and shutting the alarm off a predetermined time after the alarm is energized, and recycling means 116 including the coupling means 108 for again triggering the device 120 to energize the alarm an interval after the alarm is turned off in response to the sensor continuing to detect smoke. As shown in Fig. 7, the coupling means is provided by a programmable uni-junction transistor 108 having its gate connected to the source of a MOSFET 102 whose gate is connected to the junction of the chamber 20 and a resistor 100 (which may be a closed ionization chamber) connected across a battery B. The cathode of transistor 108 is connected to the gate of a SCR 120 forming the switching device, the SCR 120 being connected in series with the alarm device 40, so that when smoke enters chamber 20, the transistor 108 is turned on to trigger the SCR and thereby energize the alarm 40. The alarm remains energized for a pre determined period, determined by a capacitor 136, a resistor 142 and programming resistors 132 and 134, at the end of which a programmable uni-junction transistor 140 is turned on to discharge a commutating capacitor 130 which reverse biases SCR 120 and turns it off by blocking the holding current passing through alarm 40 and resistor 122. Recycling of the alarm in the event of a continuing smoke sensing condition is accomplished by a capacitor 116 which is completely discharged through a diode 148 and SCR 120 when the alarm is turned on, thus causing transistor 108 to be turned off shortly after the alarm is energized, and which is recharged through a resistor 110 and a potentiometer 114 when the alarm is turned off. Recharging of capacitor 116 eventually causes transistor 108 to conduct again thereby triggering SCR 120 to energize the alarm again. The repetition rate of the cycling alarm signal increases with increased smoke level because the voltage to which capacitors 116 must recharge is lower for higher smoke levels. Low battery voltage detections circuitry is also provided, and includes a programmable uni-junction transistor 150, for triggering the SCR 120 to energize the alarm 40 when the battery voltage falls to a predetermined low level. The alarm is again re-cycled but with a fixed interval between energization of the alarm determined by a capacitor 162 and a resistor 160. This fixed interval is relatively long so that a ow battery alarm can be distinguished from a fire alarm. The chamber 20 is described with reference to Figs. 5 and 6 (not shown), and forms the subject matter of the parent application (see Division H1). As shown in Figs. 2 and 3 the chamber 20 is mounted on a printed circuit board 44 which mounts the components of the alarm circuit and this arrangement is mounted in a two part housing 14, 16 together with the battery 13 and the alarm device 40 which gives an audible warning (a visual warning may be additionally given).