Blinking LED for Generator Power

Thread Starter


Joined Jun 10, 2021
I need one small circuit which can be placed at the back of a switch board. The functionalities will be as follows.

1. There are 2 supply lines (220v), (A) and (B), where (A) is active in case when power supply is there OR diesel generator (DG) is running. B) is active in case when ONLY power supply is there.

2. One LED indicator (LED attached with resistance) will be there which will BLINK if generator is running, i.e. (A) is active and (B) is inactive, and will glow STEADY if diesel generator is not running, i.e. both (A) and (B) is active.

It would be great if you can help me out.


Joined Nov 6, 2012
Designing Circuitry to cause the LED to Flash, ( when you are supplying 220V ),
adds a ton of complexity, and/or, expense, to this project.

Simply using 2- LED Panel-Indicator-Lamps, that are factory designed to
operate on ~220V greatly simplifies everything.

I would simply use a Green-LED to Indicate good Mains-Power, and
a Red-LED Indicator for Generator-Power.

Of course these Indicators need to be connected to their respective
Power-Sources BEFORE your Change-over Switch-Gear.

Here is a selection of Red & Green, 220V Rated, LED Indicators from DigiKey ............


Joined Mar 26, 2017
Hi My Solution to this problem is to realize that what can be used to differentiate between Gen supply and mains supply and I found that there is always a frequency difference. Mains being very stable and Gen always being a bit off. Generate a stable 60 hz signal with a watch crystal and compare . If it is on frequency it is mains and off frequency it is Generator. Simple to implement with a small arduino.


Joined Dec 4, 2015
You could go down the CMOS logic route or a small microcontroller.
This might not be for you right now but worth keeping in mind.
A relatively simple approach might be to use relays on both sources.
If you used some low power relays with coils at whatever your supply voltage is (240-VAC @ 50Hz UK for example), you could safely get away from handling nasty mains stuff.
A relay with say 2-poles and changeover contacts gives you mains isolation, contacts to use at a safer low voltage like 5 or 12 volts DC.
You would need to get a suitable power supply (mains to low voltage) and LEDs could be run with just a resistor (2K for example at 5-volts)
Power supplies are sometimes called wallwarts for some strange reason or plug-in mains to low voltage adaptors.
The advantage of this route is that discrete LEDs are very cheap, and in ultrabright form, are very visible and use next to no current.
The spare contacts on the relays gives you scope for remote (cable) indication or even a cheap wireless connection.

I forgot you wanted them blinking.
Buy an LED flasher module.
Use a cheap chip solution like 755 timer module or one of many CMOS logic chips (4011) to flash your LEDs
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