The high durability nature of a light emitting diode (LED) make it suitable for ON/OFF indicator application. Because the limitation on low operating voltage, LEDs are normally applied on low voltage application. Fortunately, there is a simple method to apply LED indicator for high powerline voltage: reactance of a capacitor can be used to limit the current without dissipating any power, this because the voltage at the capacitor differs 90 degrees from the current. Here is the schematic diagram of the circuit:
The zener diode is used to limit the voltage and to allow the current to flow in the reverse direction at the cycle where the the current flow cannot flow through the LED. Without this zener, the current will completely stop flowing since the the capacitor block the current whe it flows in one direction only. Capacitor selection determine the current setting, about 4 mA for 100nF and 20 mA for 470nF.