A passive infrared sensor system comprising: a direct current (DC) voltage source supplying a DC voltage; a passive infrared sensor supplied with the DC voltage, the passive infrared sensor comprising at least one sensor element; an alternating current (AC) voltage source supplying an AC voltage, the AC voltage source arranged to induce an alternating current to flow through said at least one sensor element; an amplifier arranged to amplify an output signal that is output from the passive infrared sensor to generate an amplified output signal; and a filter arranged to filter the amplified output signal to provide an output of the passive infrared sensor system, wherein the filter is configured to filter a frequency of the AC voltage.