Abstract: A parking meter contains both optical detection elements, to detect the presence and the diameter of inserted coins, and conventional coils which are inductively coupled to a passing coin and which load a RF oscillator. The optical detection information is used to control the sampling of the frequency of the oscillators which are loaded by the metal in the falling coins. This provides a synergistic way to combine optical detection with electromagnetic detection to achieve compact recognition templates which readily perform difficult discrimination. Addition of a temperature compensation algorithm makes the resulting system extraordinarily insensitive to temperature-dependent variation in the permeability of a falling coin.