Abstract: The infrared system employs an optical focusing arrangement for collecting radiation from a moving object such as an intruder and directing the energy to a sensing element which may be a thermistor whose resistance changes with energy level change. The detection circuit comprises a biasing means comprising a voltage divider network in series with the sensing element and a modulator or oscillator circuit coupled to the sensing element. The modulated output signal from the sensing element couples to an impedance matching circuit which in turn couples to an amplifier. The output from the amplifier couples to a low frequency blocking filter for removing very low frequency components of the signal. From there the signal couples to an integrator and an output trigger alarm circuit. When sufficient energy is received by the sensing element the amplifier saturates and the modulating frequency disappears or is shunted. Once this occurs the trigger circuit is activated and an alarm condition prevails.