AC Power Flasher

AC Power Flasher Design

The design presented here is an AC power flasher that provides flexible control of AC load circuits. Optical isolation between the control input voltage and output load voltage permits control input to be different (and well-isolated) from the driver output. When the recommended control input voltage is applied, the power flasher begins a flashing cycle, and the output load will continue to flash for as long as the trigger input remains applied. The design provides “silent” output switching of 240-VAC loads up to 2 amperes (2 A) and offers a smooth flash rate period control option. The design is realized with the help of a few easy-to-use economic standard components (I got mine on eBay).


The working principle of the circuit is very simple: An adjustable pulse generator with the long-familiar timer chip LM555 (IC2) controls a general-purpose solid-state relay (SSR) G3MB-202P from Omron ( Because the SSR is connected in series with the output load, when the SSR is enabled, the output load will wake up. A standard potentiometer (P1) is deliberately used in the circuit to alter the flashing cycle, and an NPN transistor S8050 (T1) is used to drive the SSR. There’s also a red LED (LED1) to indicate the pulse activity. Furthermore, a 5-V stabilized power supply, based on the popular three-pin fixed regulator LM7805 (IC1), is used to power the front end of the circuit from an external control input voltage in 9- to 12-VDC range.


5-VDC Version of the G3MB-202P SSR

The schematic of the AC power flasher is:



Note that the LM555 (IC2) produces a continuous square wave of period T (set by P1, R1, R2, and C5) with an adjustable mark: space ratio/duty cycle. For example, if the wiper of P1 is at its extreme lower position (0R), then the approximation is:


  • Period (T) = 1.777 seconds
  • Mark = 1.053 seconds
  • Space = 723.8 milliseconds
  • Duty Cycle = 59%
  • Frequency = 0.563 Hz


For an easy walk around the math, just refer to the timing diagram shown below. The voltage across the output load would look something like the instance at the bottom part of the figure.

And here’s a model electric wiring diagram for quick reference:



Because SSRs use optical semiconductor switching elements, they have a variety of features that the conventional electromagnetic/mechanical relays do not incorporate. The greatest feature of SSRs is that they do not use switching contacts that will physically wear out. Looking for a technical explanation for SSRs? Here it is:


Other useful links:




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  • T.K.Hareendran

    Postscript: This flasher circuit can be used for switching common electrical loads
    such as lamps,heaters,etc safely, if the connected load is within its limits. Note that, G3MB-202P is an easy-to-use sub-miniature PCB mounting single in-line package (SIP) type with inbuilt input resistor, zero-cross, and snubber circuit.

    Since output section of the circuit at live potential, it should only be constructed/tested/installed by experienced and suitably qualified personnel!

  • pm

    In the power flasher circuit, I believe a current limiting resistor is required in the collector circuit of the driver transistor, 500 ohm would limit the current thru the opto isolator input to 10ma.