I’ve always been a big fan of home automation. My first project, back in the days of X10 controllers, let me turn off the lights in the living room from the bedroom. As handy as that was, it required a centralized computer that cost more in electricity than it could ever possibly save. That’s true for all too many home automation projects.
A Web-enabled four-outlet power strip would go a long way to solve that problem—you could turn appliances on and off remotely or on a schedule. And while we’re at it, shouldn’t it show how much power those appliances are using?
This month’s project is a smart power strip that does all that. One big caveat: The total build exceeds US $500, which is an insane sum for an outlet strip, no matter how smart it is.
Here’s another caveat: safety. Obviously, making a power strip is going to mean working with alternating current. You’ll need wire of the appropriate gauge. Make sure none is exposed, and be careful not to confuse signal lines—probably 20 gauge—with AC lines, which should be 14 gauge for a 10-ampere project. And of course, check your local electrical code. I got a very powerful reminder when an upstairs lighting fixture in my house caught fire because of improper grounding. Remember, AC power can kill within the blink of an eye.
The two main components of the build were a SheevaPlug Development module and a Phidgets input/output board with four current-sensing modules and two 10-A dual-relay modules. The Phidgets I/O board is a sweet little guy. Mine has eight digital inputs and outputs and eight analog inputs, which use a standard connector type that also supplies +5 volts and ground. The dual relays need a digital input for each relay (on or off), but they also need a certain amount of ”wetting” current—the current needed to break through any thin film of oxide that may have formed on the contact surfaces of the relay switches. That current is supplied by the analog input ports, so I ended up using six analog inputs (four current sensors and power for the two relay modules), four digital inputs (switches), and four digital outputs (LEDs and relays). One of the nice features of the board is that the digital inputs have built-in pull-up resistors, so you can just hook a switch up between ground and the input.
I first attached the relays and the LEDs to the Phidgets board to see if everything worked. The SheevaPlug was going to take a while to arrive, so I used my Mac as a stand-in. Once I had determined that the Phidgets board was alive, I took a standard square electrical box and drilled a number of holes around the base for four switches and four LEDs—I wanted to be able to see whether each receptacle was live by checking an LED next to it. In other words, the LED would be on if—and only if—the relay was active, even if the computer went nuts and started sending garbage to the board. I consider this a basic safety feature, and it’s easy to do by running parallel wires to the relay input and the positive side of the LED.
For me, every do-it-yourself project has a milestone, and this one will be remembered as The Build That Made Me Finally Buy a Drill Press. Using a hand drill, putting the first hole through the electrical box took three drill bits and 20 minutes of the most awful noise. A nice 12-inch (30-centimeter) drill press was on sale at my local Home Depot. With it, the remaining seven holes took about 20 seconds apiece.