A power tool (200) includes a moving implement (222) and an ultrasonic transducer (226) that emits ultrasonic signals and receives echo return signals from an object (224, 229) that reflects the ultrasonic signals. A controller (240) identifies a distance between the implement and the object and identifies a type of the object based on the echo return signals from the object. The controller activates an implement arrest mechanism (236) in response to the distance between the object and the ultrasonic transducer being less than a predetermined threshold and the type of the object corresponding to a portion of a human body.