There is provided a highly accurate velocity sensor having a reduced detection error. As the porosity influences not only the friction between two workpieces, but also the rigidity of a material, a careful selection of pore diameter, number of pores per unit volume of material and the distribution of the pores in the material is necessary. One of the circuit parts has an RS flip-flop with two complementary signal outputs and the other one has a signal input and a first and a second switching transistor. At least one of the traces is shaped like a helix and is applied onto the substrate by direct printing.