GB1329828A - Infra-red fire alarm apparatus - Google Patents

Abstract

1329828 Fire alarm EMI Ltd 19 April 1971 [21 Nov 1969] 57007/69 Heading G4N [Also in Division Gl] A fire alarm comprises a detector responsive to infra-red radiation viewing the area to be protected via a chopper, an amplifier being provided, in the detector circuit, which is responsive primarily to the chopping frequency. The alarm means used is a multilevel magnitude and/or rate of signal increase detector. In the arrangement shown, filter 2 reduces the bandwidth of radiation received by photo-cell 2 to between e.g. 2À8 to 3À5 microns, to avoid spurious radiation, e.g. sunlight producing an alarm signal. An apertured diaphragm may also be provided to reduce extraneous radiation. The chopper 4 is either a device having a plurality of radiation opaque blades positioned as shown, or one having blades opaque only to the restricted bandwidth e.g. of glass transparent up to 2À6 microns, positioned in front of the filter 2. Rotation of chopper 4 is sensed magnetically or photo-electrically at 6. The detector circuit includes amplifier 11 passing only signals at the chopping frequency, which are rectified at 12, filtered at 13 to remove mains interference and passed to the alarm circuit 8. Three threshold circuits 15-17 are provided for different increasing magnitude levels of the signal, causing lamps to be energized as a result of these thresholds being exceeded. Level 1 may indicate "normal" operation. The rate of change of the signal is indicated, via a differentiating circuit 23, by alarms 20-22. System faults e.g. motor or supply faults are monitored by device 18. Overheating of electrical installation. e.g. the tank coils of a high power radio-transmitter may be detected and initiate flashing coloured lamps, bell and switching off of supply. Fire detector.-The device could be mounted with an extinguisher at the end of a ladder to locate the most dangerous part of a conflagration and direct extinguisher towards it.