Scattered-light smoke detector with a shielding structure of detector circuits

Claim 1

A scattered-light smoke detector comprising: a smoke detecting section including a planar base and a surrounding wall extending downwardly from a lower surface of said base and having at least one opening allowing smoke to enter therethrough, said surrounding wall defining a smoke detecting space therein; a light emitting element and a photodetector element disposed at positions where they are not opposite each other and optical axes thereof intersect each other at a predetermined angle and said photodetector element can recieve light from the light emitting element scattered by smoke entering said smoke detecting space; a printed circuit board attached on an upper surface of said base; a shield layer interposed between said printed circuit board and said base, said shield layer comprising an electrically conductive material on a lower surface of said printed circuit board; electrical parts formed in chips and packaged on an upper surface of the printed circuit board; said light emitting element and said photodetector element as well as said electrical parts passing through said base and said printed circuit board and being connected on an upper surface of said printed circuit board and a shield case disposed so that it covers said electrical parts; said electrically conductive material being flat on the lower surface of said printed circuit board; said shield layer being an electrically-conductive metal plate; said shield case comprising an electrically-conductive material coating on the inner surface of said case; a casing for accommodating therein a capacitor operating as a detecting circuit together with said electrical parts, said casing being located within said smoke detecting space at a side of said light emitting element and a side of said photodetector element, said casing being spaced from the optical axis of said light emitting element and from the optical axis of said photodetector element; said surrounding wall being formed by a plurality of substantially L-shaped wall elements and said casing, said L-shaped wall elements being arranged circumferentially at predetermined intervals.