Abstract: A small, compact motion detector alarm is sensitive to low frequency vibration (20 to 250 Hz) and emits an alarm sound when this vibration is detected. A piezoelectric transducer element serves as a low frequency sensor and also produces an alarm sound near its resonance, nominally 5 KHz. When low frequency vibrations occur, a detector formed of a amplifier and a current comparator actuates a one-shot which turns on an oscillator circuit to drive the transducer. A time out circuit disables the one-shot for a few seconds after the alarm has sounded, so that the device can return to its standby state. The alarm can be made to emit in the human audible range or in the ultrasonic range, which will be effective with pets but will not disturb humans.