The schematic shows a one-wire controlled mains switch. One-wire is a simple protocol which is typically used for networks of temperature sensors. (The diagram is a simplified version of a circuit by Simon Atkins.)
Features of this circuit are:
- The diodes on the left provide surge protection, for those occasions when your network is hit by lightning.
- The DS2406 is active-low, the LED is entirely optional.
- The RC pair at the far right is called a snubber. It is used for inductive loads (e.g., electric motors) when the current is out of phase with the voltage. The resistor should be rated at 1W and the capacitor 400v.
- The resistor R3 is required to limit the current through the opto-isolator during the instant of time when it is conductive but before the triac has switched on.