This is the schematic diagram of LED indicator for 220V AC Mains which can be used to indicate that the cable lines is connected to 220v electrical installations.

The current through the LED is not limited to the resistor, but the capacitance of the capacitor. The capacitance can be selected so that the current passing through it could regulate directly after power LED. Since the current leads the voltage by 90°, does not arise (ideally) no power loss of the capacitor and the capacitor is not heated. Reactance capacitor can be easily calculated with the following formula:

where Xc is capacitive reactance in Ohm, pi (3.14), f frequency in Hertz and C the capacitance in Farad.

A capacitor with a capacity of 100nF will have for grid frequency reactance:

`1 / (2x3.14x50x10-7) = 31831 Ohm`

If you connect this capacitor voltage network, it will be the power flow 230/31831 = 0.0072 A. If we connect in series with the capacitor bridge rectifier with LED, the current has substantially altered.

Resistor R1 has only one task – during off indicator light capacitor discharge and prevent unpleasant “kick” while handling the device off.

**Warning: The entire circuit is connected to the high voltage electrical network. Therefore, it is necessary to work at maintaining the necessary caution.**

source:http://www.belza.cz/ac-led/kontrol.htm

I want to use this circuit on a 12v system

Am impressed with those cct diagrams.I”ll start building some of them.