Ultrasonic signals are like audible sound waves, except the frequencies are much higher.Ultrasonic transducers have piezoelectric crystals which resonate to a desired frequency and convert electric energy into acoustic energy and vice versa.Diagram A shows how sound waves transmitted in the shape of a cone are reflected back to the transducer. At this stage, an output signal is produced to perform some kind of indicating or control function.A minimum distance from the sensor is required to provide a time delay so that the “echoes” can be interpreted. Variables which can affect the operation of an ultra- sonic sensor include: target surface angle, reflective surface roughness, change in temperature or humidity. The targets can have any kind of reflective form and even round objects are an acceptable target.