“Track the 230 VAC power line with your favorite HW using a small, efficient, and inexpensive circuit.
In my wireless doorbell tutorial I gave the advice about how to get a digital signal from a 12 Vac source so it could be read with any desired HW. That solution is not valid for high voltage, since the dissipated power is not acceptable. Hereby I describe another circuit that solves the issue.
If you want to track when a blackout occurs, there might be several solutions:
Use a transformer to reduce the voltage, rectify it and adapt the signal to your system.
Get whatever phone charger you have in your drawer, disassemble it, remove the output capacitor or change it for a lower value to get a quicker response, and there you have your 5 Vdc signal.
Build the following explained circuit.
All these three solutions are perfectly valid, and it is easy to understand the pros and cons of each one. But in my opinion, the proposed solution is the cheapest, most efficient and smallest.”