Simple flasher operates off ac mains

-June 23, 2011

Looking for a mains switch in the dark is easier if the switch contains a built-in neon or filament miniature lamp. Adding a small indicator to any mains switch is helpful. It is even better if the indicator flashes. This circuit makes a simple flasher using only four discrete components (Figure 1).

Simple flasher operates off ac mains figure 1

The neon lamp flashes at a frequency of 1 to 5 Hz, according to experimental values for resistance and capacitance. When the switch terminals are open, diode D1 acts as a halfwave rectifier. Capacitor C1 charges through resistor R1 until the voltage on the capacitor exceeds the breakdown voltage of a miniature neon lamp. C1 then discharges rapidly through the lamp, which flashes. You can access the switch terminals by prying off the front panel with a small screwdriver. This circuit can be installed inside the switch block. You assemble it on a 5×10-mm PCB (printed-circuit board).

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Design Ideas
This flasher circuit works properly only with 220V tungsten lamps. To use it with 220V fluorescent lamps, you must make a small modification (Figure 2). Disconnect resistor R1 from the flasher circuit and place it in parallel with the starter of the fluorescent lamp, which is connected in series with the mains switch.

Simple flasher operates off ac mains figure 2

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