ADAPTIVE PASSIVE PING

Systems, methods and apparatus for wireless charging are disclosed. A charging device has a charging circuit that includes a charging coil located proximate to a surface of the charging device, a pulse generating circuit, and a controller. The pulse generating circuit may be configured to provide a pulsed signal to the charging circuit, where each pulse in the pulsed signal includes a plurality of cycles of a clock signal that has a frequency greater or less than a nominal resonant frequency of the charging circuit. The controller may be configured to detect a change in resonance of the charging circuit based on a difference in response of the charging circuit to first and second pulses transmitted in the pulsed signal. The controller may be further configured to determine that a chargeable device has been placed in proximity to the charging coil based on the difference in responses.