PHOTODETECTION CIRCUIT OF LIGHT BEAM TYPE DETECTOR

PROBLEM TO BE SOLVED: To provide a light beam type detector that can solve the problem that the desired characteristic of an LC parallel resonance circuit cannot be kept and thus a normal signal cannot be extracted because a DC voltage value is increased by current occurring when strong light such as sun light or the like is incident to a photodetection element or a variable coil is magnetically saturated.SOLUTION: A capacitor of an LC parallel resonance circuit is configured so that plural capacitors are selectable by a switching elements in order to use a fixed coil having no narrow allowable tolerance in place of a conventional variable coil, a signal whose frequency is subjected to sweep control by CPU is input to the LC parallel resonance circuit, a frequency fo at the output signal of the LC parallel resonance circuit being maximum is determined, an inductance of L is calculated from fo and the electrostatic capacity of C is calculated, and capacitors constituting C to an electrostatic capacitance matched with the calculated inductance of L are combined and selected so that the LC parallel resonance circuit has the resonance frequency which is equal to the frequency of modulation light emitted from a projector, whereby a photodetection circuit is capable of adjusting the frequency of a signal to be extracted. 【課題】光線式検知器において、受光素子に太陽光などの強力な光が入射された場合に発生する電流で直流電圧値が上がったり可変コイルが磁気飽和したりと、求めるＬＣ並列共振回路の特性を維持できず正常な信号抽出ができなくなる問題があった。【解決手段】従来の可変コイルの代わりに狭許容差でない固定コイルを使用するためにＬＣ並列共振回路のコンデンサは複数のコンデンサをスイッチ素子で選択できる構成とし、ＬＣ並列共振回路に対してＣＰＵで周波数をスイープ制御される信号を入力し、ＬＣ並列共振回路の出力信号が最大となる周波数foを求め、このfoとＣの静電容量からＬのインダクタンスを算出し、投光器から発せられる変調光と同じ周波数の共振周波数を有するＬＣ並列共振回路になるように、算出したＬのインダクタンスに合う静電容量にＣを構成するコンデンサを組み合わせて選択し、抽出すべき信号の周波数を調整することができる受光回路とする。【選択図】図２