This is thought to be used with an Arduino powered trough its VIN pin.
The circuit on the right emulates Arduino's voltage regulator (Resistor + Zener) and some load (Resistor + LED) with an electrolytic capacitor.
VIN pin can accept safely voltages between 7V and 12V, but to ensure power loss detection with a reasonable time window it's better to use at least 9V.
Since 5v is given by a voltage regulator, as long as Vin will stay above 7V, 5v will remain stable.
On the left there's a simple adjustable voltage divider which should be adjusted to give a little more than 5V.
On power loss, 9v starts to drop and with it also its voltage divided reference that will quickly trigger the OP AMP.
The OP AMP is powered with 0v and 5v, and its output should be connected to an Arduino's interrupt pin.
You can adjust the reference voltage to set the sensitivity and the capacitance to increase the time window according to your circuit's power consumption.
