Embodiments of low cost prosthetic feet include a footplate with a connection mechanism embedded within a first foam element having a first stiffness. After a capacitance value of a variable capacitor in a resonance coil of the power receiving device is once set to 0, an S11 parameter is detected while a frequency of an electromagnetic wave is changed, and the resonance frequency of the power transmission device is estimated on the basis of the S11 parameter. Finally, pairing is performed without pressing a button on the wireless door sensor. While the device is typically worn with a helmet and attached thereto, the device also may be used alone, or in combination with a soft-style cap.