A triggerable power transmitter for power transmission from a primary coil to an inductively coupled secondary coil in a power receiver has a primary coil； a driver for electrically driving the primary coil； a front end receiving analog signal indicative of resonance properties of the primary coil and generating digital information in response to the analog signal； and a processor capable of: determining if said primary coil is coupled to a secondary coil based on the digital information, and triggering power from the primary coil to said secondary coil when said primary coil is inductively coupled to said secondary coil. The effective inductance of the primary coil increases reducing the resonance frequency when a secondary coil is inductively coupled to the primary coil. However, foreign material such as a metal sheet placed on the primary coil increases the effective resistance thus shortening the decay time of the resonance.
