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I'm designing a low frequency board with some analog signals inputs and a few discrete logic gates which drive warning LEDs. I want to save the status of the LED even if the board is powered off. No microcontroller on the board is allowed.

The most common option to save the LED's status when cycling power is to use a backup battery. Is there any other solution?

The best choice would be a sort of 1 bit EEPROM flip-flop, but I haven't found anything on the market.

Using a parallel EEPROM and driving it through a discrete gate only network could be an option, but this would result in a quite complex hardware.

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You can use a non-volatile voltage reference in a way similar to this circuit. https://www.maximintegrated.com/en/design/technical-documents/app-notes/4/4624.html It uses the DS4305 which is a voltage refence that stores and outputs the measured value on Vin at the toggle of a pin. It can do 50,000 write cycles.

For an analog warning system that might just be the thing you're looking for. Of course you can also do this with an micro and some eeprom or fram, depending on number of write cycles you require.

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You might look into bistable relays : instead of having a single coil, they have 2 : one to "set" the relay, and one to "reset" it. That means that you don't need to power the relay permanently, but just for long enough to change state. So if you loose power, no coil is powered, and you keep your state.

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  • \$\begingroup\$ Neat non-standard use for that sort of relay. Though most of the ones I've seen are designed for 28V operation. And that approach can get out of hand if your need to store more than a bit or two. \$\endgroup\$
    – SteveSh
    Commented Apr 12, 2022 at 15:07

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