Patent ID: 7129487

Claim:
An infrared detecting circuit, comprising: a current-to-voltage converting circuit that is connectable to a pyroelectric element operable to generate a current signal in accordance with a received infrared ray, wherein the current-to-voltage converting circuit converts the current signal output from the pyroelectric element into a voltage signal, the current-to-voltage converting circuit includes an operational amplifier, a capacitor, and a feedback circuit for feeding back a direct current component, the capacitor and the feedback circuit being connected between an output terminal and an inverting input terminal of the operational amplifier in parallel with each other; an amplifying circuit that amplifies the voltage signal output from the current-to-voltage converting circuit; a band-pass filter circuit that is configured to pass components of a voltage signal from the amplifying circuit in a specified frequency band, wherein the band-pass filter circuit includes a switched capacitor; a clock generating circuit which generates a reference clock signal for controlling the switched capacitor; and an output circuit that outputs a voltage signal output from the band-pass filter circuit as a detection signal when the voltage signal is at a threshold level or higher, wherein a high-pass filter having a specified gain is connected between the band-pass filter circuit and the output circuit.