A wireless power transmitting device transmits wireless power signals to a wireless power receiving device. The wireless power receiving device has a rectifier and a wireless power receiving coil that receives wireless power signals. The wireless power transmitting device uses a layer of coils to transmit the wireless power signals. A dielectric layer in the wireless power transmitting device defines a charging surface that receives the wireless power receiving device. A layer of temperature sensors is interposed between the layer of coils and the dielectric layer. Control circuitry in the wireless power transmitting device uses temperature information from the temperature sensors to determine whether a foreign object such as a coin is present on the charging surface.