Sometimes the smart home simply needs to detect the presence of ac voltage. Our mains power sensor is a good example of this but we may also want to be able to track appliances being turned on and off.
This is a feature of most 'smart plugs' but we have also developed simple circuits to achieve this that can be connected to an Arduino. These tend to be much more reliable and responsive and have much lower latency. Another reason for doing it this way is that it is so much cheaper.
