A daemon running on an Orange Pi or RPi, exposing a web API for switching the audio equipment on or off, using a relay connected with one of the GPIO pins.
This software reads a configuration file (/etc/hifipowerd.conf) and gets the pin numbers for shutdown and reboot buttons, automatic mode sense and relay drive output. Then the web API is started, exposing endpoints accessible with GET method:
address:port - main page,
address:port/json - get state data as JSON,
address:port/power - get the current state,
address:port/power/on - turns the power on, returning the current state of outputs,
address:port/power/off - turns the power off, returning the current state of outputs,
address:port/power/toggle - turns on if power was off, turns off if power was on (like the on/off button does)
address:port/power/1/on, address:port/power/1/off, address:port/power/2/on, address:port/power/2/off - individual channel control.
Use PulseAudio or ALSA to detect lack of audio signal on the input (configurable), then if no audio is present for a preset time (e.g. 10 minutes), turn the equipment off automatically. Prevents idle power draw by e.g. vacuum tube amplifiers.