A measuring device was developed to capture the grid frequency, which digitizes, filters, and analyzes the voltage supplied by an AC adapter using a microcontroller. The measurement data is transferred via USB to a Raspberry Pi, which handles storage, graphic generation, and upload to the web server. The following image shows the setup:

The measuring device consists of a simple circuit that brings the AC voltage supplied by the power adapter into the voltage range necessary for digitization and a STM32F103 module:

The circuit consists of only a few components since the actual signal processing takes place in the microcontroller:

The frequency is determined with an accuracy of 1mHz, which cannot be reliably achieved by simply counting zero crossings, as any small disturbance can shift the zero crossings enough to cause sporadic significant errors. In particular, a digital bandpass filter avoids this problem.

As a byproduct, unusual voltage patterns can also be recorded. Here are some examples that can be seen in the local grid almost daily, at least once or twice: