I've got my Raspberry Pi, and I've installed the DHT 11 temperature and humidity sensor and the PIR sensor.  And I've put the Raspberry Pi into an acryllic case; that's just going to protect the circuits on top of the Raspberry Pi board so that they don't short on anything when it's in the enclosure.

For the enclosure, I'm using a small plastic food container.  So, this is the base of the container, and I've cut a small hole into the corner of the container using a pocket knife, so that the cables can run into the container.  So I've already threaded through my microUSB cord which is going to provide power to the Raspberry Pi, and my LED strip, as well as the power cable for the LED strip, so that goes to a 12 V wall adapter. So, the LED strip wires have been threaded through and attached to a relay module. So you can see on the relay module I've got these screw terminals on the end here, and I've attached the wires that come from the LED strip on to those screw terminals.  If you look closely, you can see marked as normally open?  

I've also got my pins on the relay, so this is what I'm going to use to attach the relay to the Raspberry Pi. So, let's get that hooked up. I'm going to need some jumper wires.  Starting with a black jumper wire, for ground, to the negative pin on the relay module, just plug that in to the relay module, and then that attaches on to a ground pin on the Raspberry Pi GPIO headers.  Like so. 

And then next I'm going to need to attach a 5 V wire on to my relay module. But you'll notice that I've already used both of my 5 V pins on the Raspberry Pi. So what I'll do, I'll use a very small bread board to join those wires together.  I could solder, but I'm just going to use this tiny bread board, and I'm going to use some additional jumper wires to do that.  So, I've got 3 extra jumper wires, we've got pins on one end.  And I'll just plug those 3 jumper wires into the rail down the side of the bread board.  I'll take the first of those jumper wires and plug it into the Raspberry Pi;  so all I need to do is  just swap out one of the existing 5 V pins on the Raspberry Pi.  Plug in the jumper wire from the bread board on to the Raspberry Pi, and then that jumper wire which is attached to the PIR sensor needs to go to the bread board. So, the middle jumper wire that I've got on the bread board there, has a pin on the end that I can plug straight into that jumper wire...or I could have just replaced the jumper wire on to the PIR sensor and just used a single jumper wire there. So, finally, this last jumper wire, is going to go to the 5 V pin on the relay module, that's the one in the center.  And then the relay module also has a signal or a data pin, so I'll take one more jumper wire, and attach that on to pin 13 on the GPIO headers of the Raspberry Pi and then that attaches on to the signal pin on the relay.

So now that I've wired up the relay to the Raspberry Pi, it's simply a matter of squashing all of the components into my box.  You'll see that I've already removed the plastic cover from the PIR sensor.  So what I've done is I've taken the lid of the box, and cut a small hole in the box, and inserted that plastic cover with a rubber grommit into the lid.  So that's where I'm going to mount my PIR sensor, on the back of the lid, like this, and I'll attach it there with some double-sided tape.  I'll just get this small piece of tape, on to the back of the lid, and then attach the sensor directly on to the tape, so that the PIR sensor is behind that plastic cover.  

Now, all of the other components need to fit into the box, and it's a bit of a tight fit.  But the Raspberry Pi goes in first, so that goes in the base of the box.  The micro USB power needs to plug in to the Raspberry Pi.  So plug that in to the top micro USB port on the Raspberry Pi. Like so. And then all of the other parts squeeze ito the box.  So once I have the Raspberry Pi and relay module squashed in to the box, I just need to make sure the DHT 11 temperature and humidity sensor isn't (is NOT) installed inside the box because it won't give an accurate reading.  So what I'll do is I'll pull those jumper wires off of the sensor, and feed them through the hole in the side of the box, so that I can plug them back on and have the sensor sticking out the side of the box.  So I've got those wires hooked up again, making sure that the green wire goes to the signal pin, the red wire goes to 5 V, and the black wire goes to negative or ground pin on the sensor. 

So, then it's just a matter of squashing all those wires into the box; it is a bit of a tight squeeze, but they do eventually fit and then popping the lid back on and closing the box up.  To keep the box together and to keep the sensor out to the side, I'll use a cable tie. I'll just pop it around the sensor, and around the lead, so that all of my components are enclosed in the box, like so.  

So there's my very simple home automation system....ready to install at my front door using some double-sided tape.  So now when I trigger the sensor, my lights turn on and I'll also get a notification to my phone.