The principle is relatively easy. A zener diode, two rectifier diodes and a capacitor as series impedance.

Look at this example which is for 15V @20 mA. For 12V, 50mA you should use a larger series capacitor (680nF or perhaps 1uF). 330nF is probably sufficient for 5V.


Note: You are dealing with high voltages which are potentially lethal and extremely unpleasant at least! Components must be chosen with care. The series capacitor must be rated to withstand the full AC line voltage. Use a Class X suppression capacitor for this. The 'bleeder' resistor (R8 in the schematic) is another must, to ensure that the capacitor is quickly discharged when power is removed. This resistor should also be able to withstand the line voltage. Standard 1/4W resistors do not qualify for this! Use a higher rated resistor or two 1/4W resistor in series.

Best regards,
Rob.