Abstract: A security system (10) suitable for use in monitoring property at an outdoor site that includes a portable housing (12) that supports a motion detector (14) for sensing motion within a detection zone. The security system further includes an ultrasonic transducer (74) and several alarm generators capable of repelling a human intruder, including a high intensity illumination source (18), high decibel siren (82), strobe light (19), and a speaker (78) for transmitting a prerecorded message. When an intrusion is initially detected by the motion detector, the ultrasonic transducer is activated to emit an ultrasonic signal that is irritating to nonhuman animals, thus clearing the detection zone of nonhuman intruders. If after sounding of the ultrasonic transducer motion is still detected, one or more of the alarms is activated. Additionally, an automatic telephone dialer (86) may be included in the system and activated by the system as another type of alarm, in response to detected intrusion.