Step 1: The Parts
Everything linked here is the exact same part I used but you can always substitute comparable parts.
I used a 12vDC/125vAC relay from RadioShack. (I'm sure you can find a similar one somewhere else for cheaper) You can use what ever you like as long as the reed switch is rated for current up to 120vAC. Although this one says the coil requires 12vdc you should be able to get away with using a 9v battery.
http://www.newark.com/nte-electronics/r73-5d10-12/power-relay-spdt-12vdc-10a-pc-board/dp/02H7519 (This one should work exactly the same)
http://www.newark.com/jsp/search/productdetail.jsp?sku=94C2790 (Your 9v batteries probably wont last long in the 12v relay so you may want to try a 6v one)
*A Distance Sensor
This is the HC-SR04 Distance Sensor. We will use it as a digital tripwire. This one is cheaper than the "Ping" sensor and just as easy to use. I will provide the appropriate libraries later on so you don't unnecessarily rip out your hair.
You are going to need something to control the 9v power supply. If you decide to use a 5v relay you should still use a transistor to protect your Arduino from the relay.
The diode is to keep you from ruining your transistor. Admittedly I didn't use a diode for my first version of this and it worked fine but after reading more it started looking like a better idea.
The resistor is for a little added protection for your Arduino from the 9v battery and the relay. I used a 220OHM resistor.
*A Battery Connector
($2.99 for 5)
You need to connect to your battery some how dont you?
*A Project Box
I used a 2x3'' project box from radio shack but you should probably look elsewhere to get a better price.
You want your wires to stay neat and tidy dont you?
*Copper Clad PerfBoard
To put your project on. I cut mine to about 1 1/2'' x 1 3/4''
*Stranded Hookup wire
You need stranded wire to go from the remote to the project box. Use stranded wire for this because it can handle the constant movement this wire will endure. It also must be able to handle the AC current.
*Solid Core Hookup Wire
This wire is used for hooking up your circuitry and for connecting to the Arduino.
*A Fog Machine WITH A REMOTE!
(I paid $30 for mine but you may find them at better prices depending what time of year it is)
You need to make sure you get a fog machine with a corded remote with a switch or else trying to follow this intractable could get tricky. (if you like tricky then go ahead and get one with a different style remote) We are going to modify the remote so the whole fog machine will not be turned off completely and it will not need to heat up again every time you want fog.
(This is the one I bought but they are sold out for now so I have another one listed aswell) http://www.partycity.com/product/fog+machine+400+with+remote.do?sortby=ourPicks&size=all&from=Search&navSet=fog
Do I really need to go into why you need this?
To Check continuity and for shorts.
You are going to use it to cut holes in your project box
And I suppose thats it.
Oh and an Arduino!
Step 2: Schematic
Step 3: The Relay
Check the data sheet for your specific relay to make sure you are connecting to the right pins.
Tack down one pin of the relay then you can continue to solder on the other components.
Step 4: Solder the Components
Step 5: Wire the Components
Step 6: Connect the Remote Wires
Step 7: Arduino Connectors
You should mark your ground wire some how. I put a piece of tape around mine.
Step 8: The Remote
Its time to connect to the remote. First we need to open it up with the two screws and make a small hole in it.
Next we need to find out which wires are controlled by the switch. Cut away a small amount of the heat shrink tubing and get out your multimeter. There should be two wires that have continuity when the switch is on and loose it when the switch is off. Those are what we need. In my case it was the white and green wires. Solder the long wires from the relay to these two spots. It doesn't matter which one goes where. You may want to use some hot glue for more insulation if you are worried about shorts.
You can close up the remote now, you are done with this part.
Step 9: Box It Up!
Step 10: The Distance Sensor
You can either plug it into a bread board and connect that to the Arduino. Or solder wires directly onto the sensor. I chose to use a breadboard.
For this sketch the Trigger pin is connected to pin 12 on the Arduino, and Echo is connected to pin 13.
Once you download the Ultrasonic Library drag it into your library folder.
Step 11: The Sketch
Now you are ready to spookify your walk way even more.
Below is a sketch I made to get you started.
Simply position the remote and sensor near a walkway or door and when the sensor's path is obstructed closer than 6ft the fog machine will turn on for 20 seconds.
Note: The switch on the remote must be in the OFF position for this mod to work.