RelAC is a HomeKit 2-channel Relay module with mains Switch Input based on the HomeSpan library. It is mainly intended to be used with mains voltage appliances controlled in parallel with the Home app and physical switch. It could be hidden e.g. behind a wall light switch.

HOW DOES IT WORK?
RelAC is connected to a regular circuit where the Load (light bulb etc.) is connected to the mains voltage source with a simple switch. However, now RelAC acts as a switch, and the physical switch is connected to the input on RelAC to detect its state change. This allows switching the load with either a physical switch, Home app or Siri in parallel (eg. the Home app can turn it on, and the wall switch can turn it off, next you can ask Siri to turn it on again)). What's better, RelAC has two channels so the single module can do all the above for two separate circuits (e.q different lights, light and fan etc.)