Hi, I am working on a project where I would like to measure mains frequency, ideally once per second, and send the measurements to a raspberry pi. There are plenty of cheap plug-in energy monitors which will display the mains frequency on a screen, but I haven’t found one that will talk to a raspberry pi yet. Does anyone know of one which would fit the bill? Many thanks!
Where are you? Makes a difference. 
I’ve never seen such a thing, but you may be able to use the ESPHome Pulse Meter Sensor to accomplish your project.
If you are in the US and go down the ESPHome route, you can reflash these with ESPHome:
using these instructions:
One note on the above instructions. I had to keep holding down the reset button on the Sonoffs until the reflash was done to get it to work.
Hi, apologies - I’m just trying to get up to speed with home automation. I’m in the UK