A contactless energy transfer system to supply computer peripherals is under development. It consists of an array of primary coils that are integrated in a table and secondary coils placed under the peripherals. For an everyday use, it is unthinkable to supply continuously all the primaries. In this paper, the electronic implementation of the primary coils is presented, including the circuit used to detect peripherals and store their position above the table. Therefore, each primary coil has to integrate the possibility to be individually supplied. A strategy to locate the peripherals and then supply only the concerned primary coils has been developped and applied to a prototype. The process is controlled by a DSP and does not require additional sensors.