Electrical – Wiring diagram of aller retour switch with lamp

The modification will work but the sense of the indication changes entirely:

• In original schematic, the lamp would turn on when the switch was off (open), and would turn off when the switch was on (closed).
• In the modified version, the lamp will turn on when the switch is on, and off when the switch is off.

The voltage across the lamp when it is lit would be the same in either case, so the lamp should last as long as in the earlier mode.

You can do this with direct conversion. Simply use a rectifier, followed by a mains-rated capacitor, then connect this to your LED in series with a very large resistor. You don't need to generate 5 or 12 volts - the series resistor will limit current through the LED. Something like this:

^{simulate this circuit – Schematic created using CircuitLab}

You could go even lighter weight and use a single resistor and capacitor but for two issues: it would flicker at 60Hz, and most LEDs aren't rated to withstand mains voltage in reverse bias.

Things to look out for:

• Remember that rectified mains is 1.41 times the RMS voltage, so spec the DC part of your circuit for 338 (169) volts, not 240 (120).
• Size R1 appropriately to the amount of current you want through the diode. In the schematic above, I've sized it to allow 2mA through for 240V mains.
• Make sure R1 can handle the power dissipation; in this case it'd be 0.67 watts, so make sure you use a power resistor.
• Mains voltages everywhere! Take care, and make sure it's well insulated.