The small circuit shown here could act as a power indicator for the 230 V mains supply and in terms of efficiency is the equal of classical neon bulbs. Note first that the LED in this circuit flashes rather than lighting continuously, and is therefore also suitable for applications where a flashing light is wanted for decorative purposes or as a gimmick. Diode D1 rectifies the input voltage, and C1 is charged by the rectified mains voltage via R1. When, after a number of halfcycles of the mains, the voltage on C1 exceeds the breakover voltage of the diac D2, the diac conducts and C1 discharges via R2 and light-emitting diode D3. This discharge results in a brief flash of light. A 470 μF/40 V capacitor is suitable for C1.
For the diode a 1N4004 can be used, and R1 should have a value of 33 kΩ, be rated at 0.6 W and be suitable for use at 350 V. As an alternative, the value of 33 kΩ can be made up from two (or more) resistors wired in series: forexample, 15 kΩ + 18 kΩ or 2 x 10 kΩ and 1 x 12 kΩ. R2 should be 390 Ω. The firing voltage of the diac should be 30 V. Using these values the LED flashes for 0.3 s every second.