Help: brownout detection for Arduino Uno

I want to detect brownout and use it arduino uno which is powered by Secure UPS.

Maybe I use adaptor connected directly on 220v, and detect its voltage in arduino (to detect brownout?)

or maybe I hack this Secure UPS, finding the circuit pin to detect brownout?

"dry contact" with your UPS model and manufacturer is a good key-phrase for a web search.

"serial connection" is also a possibility.

A 5V wall wart works well for power failure detection (but not for a brown-out).

to undestand this comment think about what is in your wall wart. They are either transformer with rectifier and decoupling/filter or switching with rectifier and filter. Either case they are decoupled using a capacitor. Due to the capacitor filtering out fluctuations in the power supply, it is not possible to sense a brownout unless it was for a significant amount of time on a power supply that was drawing significant amount of current (which this one does not sound like).

Jack Christensen made a power trouble logger. I believe he used a chip specifically designed for the purpose (that includes a real-time clock). This should get you started...

This looks more directed without eliminating the positives...