Apparatus for passive
infrared sensing of an intruder that discriminates between signals corresponding to a human target and much larger signals may include first and second IR sensors, at least a first
amplifier having a first
voltage operating range. The first
amplifier may be connected to the first IR sensor and the apparatus may further include at least a second
amplifier having a second
voltage operating range. The second amplifier may also be connected to the first IR sensor and the apparatus may further include a
comparator for comparing the respective outputs of the first and second amplifiers to predetermined values. The invention also includes the method for differentiating between a
signal caused by human target and a
signal caused by other much larger by IR sources which includes providing at least a first IR sensor, providing at least a first amplifier connected to the first IR sensor, providing a second amplifier connected to the first IR sensor having a second operating range, comparing the outputs of said first and second amplifiers to predetermined values to differentiate between the output from a sensor corresponding to the maximum
signal for human target and a signal that is larger than the maximum signal from a human target.