Patent GB-1476736-A (GB)
Title: Smoke detector
IPC: G08B17/107
Filed: 1974-07-17
Abstract: 1476736 Photo-electric smoke detectors MATSUSHITA ELECTRIC WORKS Ltd 17 July 1974 31665/74 Heading G1A Light-scattering smoke detection apparatus comprises a light source and a photo-detector  mounted on a base-plate; a plurality of light shields between the source and the detector; a cylindrical upper lid having a space for arranging the baseplate diametrically across the lid and having a pair of integrally-formed chambers symmetrically disposed; and a lower lid associated with the upper lid defining a smoke lead-in path and having at least one aperture in a lower surface. A supporting frame 29a-29d Fig. 11, attached to a base-plate 26, carries a GaAs light source 24 gripped in a split hole 29b, 29c. Attached near the opposite end of the base-plate 26 is a photodetector 25 at a height different from that of the light source 24. A light barrier 27 having flanges 27b partly surrounding the source 24 has in a front portion 27a a square hole 27c at the height of the source 24. A further light barrier 28 has blade parts 28b, 28c extending down to different heights to shield the detector 25 from all direct light emitted by the source 24 through the hole 27c. The base-plate 26 is attached to the underside of an upper lid X, Fig. 7, having integrally formed chambers 13 housing respectively a rectifier and oscillator arrangement 18 for supplying pulsating energy to the source 24, and an amplifier (19) to process the signal from the detector 25, the circuits in both chambers 13 being encapsulated in silicone rubber 20. Pairs of resilient tongues 8a, 8b Fig. 9 in the top of the upper lid X, which by rotation of the lid X engage rigid tongues 7 Fig. 7 on the underside of a mounting base X a , provide electrical connections and mechanical support. The mounting base X a  has mounting holes 3 and an entry hole 4 for leads which may be connected to the tongues 7 by terminal screws 6. The upper side of the lid X, Fig. 9 has drainage channels 12, and the tongues 8 are mounted on bosses 9 for protection against accumulated moisture. A lower lid Y Fig. 8 is provided with an apertured plate 43 from which is spaced a further apertured plate 49 for admitting smoke but not light, the plate 49 having a U-shaped bracket 55 for securing the lower lid Y to the upper lid X with a central screw 57 Fig. 9. The upper lid X Fig. 7 has inner and outer peripheral walls 17, 15 and pillars 40; the lower lid Y Fig. 8 has corresponding inner and outer peripheral walls 37, 35 and pillars 39. When the lids are engaged the pillars 39 abut the pillars 40, the peripheral walls 17, 15, 37, 35 defining a tortuous path for admitting smoke but not light. All surfaces shown dot-shaded are matt black. The electrical circuit is such that when the apparatus terminals are connected to a power supply (E) Fig. 14 (not shown) through a relay coil (Ry) the detection of internal light scattered by smoke lowers the impedance across the terminals sufficiently to energize the relay (Ry) to switch on an alarm bell (B) or a lamp (L). The lids X, Y and the mounting base X a  may be non-metallic, electrostatic screening being provided by earthing the plate 49, Fig. 8.