# Computing stuff tied to the physical world

## Measuring the AC line frequency

In AVR, Hardware on May 28, 2009 at 00:01

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.

1. I seem to remember that there are electric clocks which derive their time normal from the power line frequency. Such clocks would start to run faster or slower when connected to a net modulating the power-frequency to transmit information.

2. Off-topic question: I see you’re using DSO-2090 USB oscilloscope.

What’s your experience with it? Is it any good? I am looking to get some DSO for myself, however there seem to be not so many USB DSOs available on the market with Mac OS X support. Any advice, thanks?

3. Atmel has an application note that shows a zero cross detector where the AVR connects to the mains line over two 1M resistors without a transformer or opto coupler: http://www.atmel.com/dyn/resources/prod_documents/doc2508.pdf . This of course requires sufficient electrical insulation of the device for safety reasons.

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