Abstract: A light detector comprising a photo sensing chip having a light detecting surface, a cylindrical converging lens to collect incident light from different directions to a light detecting surface, at least one electrode connected to the photo sensing chip, a light detecting surface, and in a cross-sectional view along the axis Z of the light detector, one end of the lens is comprised of a central spherically curved surface having a first radius of curvature, and a peripheral spherically curved surface having a second radius of curvature. In another aspect of the invention, a flat surface is further arranged between the central spherically curved surface and the peripheral spherically curved surface. In the invention, the lens is comprised of just one lens element, and is formed by resin molding.

Abstract: In the revolution frequency indication apparatus in which the revolution frequency of a rotating body is measured and indicated according to a sensor pulse obtained from a revolution sensor mounted on said rotating body or its surrounding, a revolution frequency indication apparatus comprising an indication updating time counter circuit which counts predetermined indication updating time intervals, and a revolution frequency measurement portion which measures the revolution frequency of said rotating body by means of a sensor pulse entered within a measurement time interval of said counter circuit.

Abstract: A circuit system for preventing a measuring device such as an engine speed measuring device or the like from being erroneously operated includes as essential components a filter circuit, a Schmitt trigger circuit, an oscillating circuit, a counting circuit, a pulse monitoring circuit and an adding circuit. A combination of the filter circuit and the Schmitt trigger circuit serve to properly shape a wave form of each of a series of pulse-shaped sensor detection signals. The oscillating circuit generates a series of pulses each having a reference frequency. The pulse monitoring circuit monitors a wave-form shaping output pulse by counting the number of a series of output pulses each having a reference frequency for a predetermined period of time with the edge of the wave-form shaping output pulse as a trigger.