Abstract: Apparatus for detecting the presence of a single banknote or a double banknote (5) comprises a cylindrical roller (1) biased by way of a pivot arm (2) against a moving surface (6) which conveys banknotes under the roller (1). The other end of the pivot arm is connected to the core of a linear variable differential transformer (4) which is activated by an oscillator (7). The output of the L.V.D.T. (4) is proportional to the displacement of the roller (1) normal to the surface (6) and is thus indicative of the thickness of any banknotes passing under the roller. In order to compensate of mechanical and electrical drift in the thickness level corresponding to the absence of a banknote, the auto-reference circuit (9) derives first and second threshold values for single and double notes respectively by adding predetermined potentials to the reference potentials. These predetermined differentials may be controlled manually by a variable resistor (11) within a thickness control circuit (10).