Patent GB-796227-A (GB)
Title: Testing apparatus for an intruder and fire detection system
IPC: G08B29/14
Filed: 1955-08-29
Abstract: 796,227. Intruder and fire alarm systems and testing means therefor. KIDDE & CO., Inc., W. Aug. 29, 1955 [Sept. 27, 1954], No. 24704/55. Class 118 (1). The detector.-A standing wave intruder and fire detection system comprises a transmitter 2 energized by a six-volt output oscillator 4 to transmit at about 19 kilocycles/sec., a receiver 6 which normally transforms the radiations into electrical signals of the same frequency which are amplified in the circuit 8, passed through a high-pass filter 68, 70, 72, 74, and are then caused to beat with the frequency from the transmitter 2 so that if the frequency difference or a harmonic thereof is between 3-18 c.p.s. an alarm is sounded. The low frequency difference is amplified by the circuit 8, passed through a low-pass filter comprising resistors 82, 84 and condensers 86, 46, and is then fed to an amplitude limiting circuit 11 which fixes an upper limit only to the amplitude of the output signals. The output is then fed to an integrator circuit 12 and then to a control circuit 14 biased by the circuit 12 and connected to a relay 16 operating the alarm 18. The circuits 11 and 12 produce a bias to operate the control circuit 14 only when a predetermined number of impulses are received from the low-frequency signal within a given time and when those signals have a predetermined minimal magnitude. The lowfrequency signal caused by an intruder or fire is converted by rectifier 124 into a series of negative pulses, one pulse for each cycle, which are fed into the integrating circuit. Only if the magnitude of the low-frequency signal is great enough will any rectification take place so that minor signals caused by air turbulence or circuit noise will not be applied to the integrating circuit at all. The power supply is taken from a plug 152 and passed through a rectifier 179, the cathode of which is connected to a filter circuit having resistors 180 and condensers 182, the resistors 184, 186 forming a voltage drop arrangement. The transformer 160 supplies current to a full wave rectifier 170 which supplies the heaters of the tubes 36, 48, 92, and 133. For safety purposes a switch 232 is normally closed to energize coil 228 and move the relays 210, 216 upwardly to ground the networks 196, 198 and isolate the network 222, 224, 225, 227, to set the system. Testing the system.-When a test is to be made the normally continuous radiation into the space to be protected is converted into a cyclical pulsing transmission. The switch 232 is opened and relays 210, 216 assume the positions shown. The network 196, 198 now becomes operative and causes the oscillator 4 to operate intermittently. The oscillator will then generate for approximately one rnillisecond and cease for forty-nine milliseconds to give a cyclic pulsing of high-frequency oscillations of approximately 20 c.p.s. which fall within the frequency sensitivity alarm range, the pulses themselves, however, having a duration insufficient to produce standing waves in the space to be protected. The connection between the relay 210 and relay contact 204 grounds the line between the oscillator 4 and the detector per se so that the output never reaches the lowpass filter in the detection circuit. Engagement between the relay 216 and the contact 214 connects the germanium diode 222 and associated circuitry to the detecting circuit which now functions as follows. The individual pulses of high frequency oscillations pass through the amplifier 8 and the high-pass filter 68, 70, 72, 74 without appreciable rectification by the selenium rectifier 78 because of their low voltage, and travel along lead 218 to the germanium diode 222 where they are rectified. A predetermined proportion of these low-frequency voltage pulses is picked off the potentiometer 225 and passed back via slider 227 along the lead 218 to the lowpass filter 62, 64 through which the pulses can pass. Thereafter these signals now in sawtooth form pass through and are detected by the intruder circuit which actuates the alarm 18. The limiter-integrator circuit necessitates that the signals be continued for a sufficiently long time to cause that circuit to function as described. In practice the sensitivity adjustment 52, 54 is set to detect signals only of pre-determined minimal amplitude and then the potentiometer 225 is adjusted to this setting with the switch 232 open.