a device for detecting the likelihood of the presence of at least one human finger on a surface , and a method of using the device in the user interface of a machine , device or system . the device comprises at least one element having piezoelectric characteristics , located under a surface , the element arranged in the device in such a manner that a force when exerted on the surface , such as when the surface is pressed by finger , is transferred to mechanical stress of the element , the element then producing a voltage output in response to the mechanical stress . also included are means for measuring the voltage output in a predefined time period and means for determining the degree of stability of the measured voltage output , and configured to indicate whether or not the degree of stability shows that the measured voltage output is likely to be a human finger or not .