There is an interesting site in the UK called Dynamic Demand. The idea is to let the 50 Hz power-line frequency vary slightly to indicate the current load of the entire grid. Simple line-monitors can then track this locally and respond by voluntarily shutting down some power consumers (i.e. turning off freezers, A/C units, or electric heaters for a little while). With a bit of collaboration, the grid would then presumably recover as the load eases off – and overcome these demand peaks. I love the idea, it’s so beautifully simple.

The Dynamic Demand site also has a page showing the current frequency online, here’s a snapshot:

I don’t think this mechanism is being used – or even considered? – in the Netherlands but I couldn’t resist setting up a JeeNode to monitor the frequency myself. The sketch:

What it does is measure elapsed microseconds between 100 transitions of its input signal against the 1.1V bandgap reference voltage. That’s 50 ups and 50 downs, which should take 1 second.

The AIO pin of port 1 is connected to a rectified but unregulated AC voltage, coming from a 17 VAC power brick through a diode plus 1:10 voltage divider. The signal looks as follows:

And here’s some sample output:

Note that for stable and repeatable results, the JeeNode will need to be fitted with a crystal as clock.

The accumulation over 100 transitions is a simple way to average all the pulse durations. Most of the code then tries to present a nice result, with sufficient accuracy to display minor variations.

Nice and tidy.