Sleep mode conserves laptop battery when the device is not in active use. A quick mechanism to put a laptop in sleep mode is simply to close the lid. Most laptops include a Hall Effect magnetic sensor to detect whether the laptop lid is closed. This imposes additional costs and can increase thickness. This disclosure describes the use of existing speakers and microphones of a laptop to detect lid state. An inaudible ultrasonic ping is emitted via built-in speakers and reflection of the ping is captured using the built-in microphone(s). The sound signature is analyzed to determine lid state. Distinct Doppler spectrogram signatures can be associated with the state of the laptop lid, as closing an open lid results in a blueshift with a dip toward higher frequencies while opening the lid causes a redshift with a rise toward lower frequencies. The Doppler spectrograms are input to a suitably trained binary classifier that generates an indication of whether the laptop lid is closed. The indication can be used by the device operating system to activate sleep mode.
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Shin, D, "Sleep Mode Activation Based on Detecting Laptop Lid State via Ultrasonic Ping Signal", Technical Disclosure Commons, (June 16, 2022)