tailieunhanh - Apress - Smart Home Automation with Linux (2010)- P32

Apress - Smart Home Automation with Linux (2010)- P32:Linux users can now control their homes remotely! Are you a Linux user who has ever wanted to turn on the lights in your house, or open and close the curtains, while away on holiday? Want to be able to play the same music in every room, controlled from your laptop or mobile phone? Do you want to do these things without an expensive off-the-shelf kit | CHAPTER 4 HOME IS HOME the output waveform varies too and is usually governed by the cost of the device. The output of the cheaper devices is usually a square wave while more expensive ones have a sinusoidal wave form. This doesn t matter much for computers but can provide a difference when powering an audiophile record player as mentioned in Chapter 3. Whenever a power drop is noticed an alarm will sound and repeat the audible warning periodically. When the battery reaches a critically low level the shutdown procedure will be initiated via the USB or serial cable so that the machine s connected to the UPS can close down safely. Each unit comes rated for a different VAs indicating how much you can draw from it when it is disconnected from the mains. You will usually need a higher VA than the wattage. The required VA is the watts divided by the power factor of the connected device s . The temporal duration of protection ranges from a few minutes to quarter of an hour depending on what machines are connected to it and the tasks running on those machines. You can refer to Table 4-1 for a rough guide. For a buying always get as high a power rating as possible. Note With most UPS units the power sockets will be divided between those that are powered in the event of an outage and those that aren t. All sockets are generally protected against surges. Given say four powered sockets you have to decide what devices will use it. Naturally your server should be a given. That s followed by the home s internal router or switch so that a shutting down message can be sent and processed by the other machines on a UPS. This is for the computers benefit only since any human will have noticed the lights going out and will instantly panic knowing they haven t hit the Save button on their application. You may also want to keep the broadband router on the UPS also so that a warning message via e-mail for example can be sent. This is usually a minor consideration but if you work remotely