In the Freetronics customer forum, member Dan has published details of his interesting new project that's used to monitor variations in the frequency of the mains power supply. His reason for doing so was to monitor the power and predict brownouts or blackouts. Dan explains:
A brownout is usually caused by a load being too high for a given circuit, or short circuits in neighbouring sub-circuits, while blackouts are usually caused by shorts on your own sub-circuit, or in some extreme cases, the entire distribution grid. In many cases, the transformer and protection circuitry for your suburb will detect a brownout condition, and kill the power entirely until it is restored to normal voltage to protect equipment.
He monitors the frequency by using a mains-rated AC transformer to bring the voltage down to a safe level, then runs through a bandpass filter and the Arduino frequency measurement library. It's an interesting solutoin to what can be a large problem, especially for those with expensive equipment relying on motors or concerned about the accuracy of mains-driven clocks.