We have cats, three of 'em. That means we have a litter box (thankfully, they are all content to share) With a litterbox comes kitty litter - and with kitty litter comes dust. Between the three of them in and out all day stirring it up, and it getting stirred up a couple times a day during clean out, the dust gets airborne and was covering every surface of my house. I love my cats but hate the dust - and when I dust, vacuum, wash the windows...it smelled like kitty litter. (Note: this was not meant to be an odor filter. I added a bit of a smell catcher after we got the kitten, but that was not my main concern. I just needed to stop the dust, dust, dust - ugh)

I know a covered box would help, but due to the 'particular' needs of the two oldest cats, we have to use a large, deep box without a cover. I decided something to reduce the dust during use would go a long way toward making my life easier - so through trial and error, this is what I came up with. (Sorry for the grainy pics, my camera is getting old. How great would it be to win one ;-) ) I finished the project without taking pics, so I am recreating the bits I can.

Here it is - my first 'ible

Supplies:
- CPU cooling fans (2) $6.00
- Wall wart with 12v output (or whatever your fans run on) $8.00
- TIP31 transistor $0.47
- Motion activated light $8.00 (mine is battery powered)
- Filter material (I used the foam stuff for a window A/C unit) $1.00
- Container for your fans (I used the Ziploc large rectangle food storage box) $2.00

Step 1: The Fans

When you snip the connector from the fan wires, you will have 3 wires to choose from. Yellow is useless for our purposes; red & black are standard power (red = positive, black = ground)

Attach the wires from the 2 fans together, red to red and black to black (solder or wire nuts, noting they will have an additional wire attached in step 3). Leave as much extra wire as possible to allow positioning later (or cut it close if you have already measured your box)

Step 2: The Sensor

When the motion sensor is triggered, it sends power to the LED inside - provided there is no other light source.

You will need to open up the light and disable the photoresistor so the motion sensor is triggered any time of day. I simply put a piece of electrical tape over it - perpetual nighttime. You don't want to LED going on, because it might scare your cats (especially if they are shy ;-) ) and it is usually pretty bright, making it ideal to use in another project (I made a travel light with mine) When you remove the LED, pay attention to positive & negative - or use your multimeter - because we are using the LED connections to trigger our transistor and start the fans. Solder additional wires to the wires that had lead to your LED, making sure they will be long enough to reach your transistor.

Step 3: The Connections

I was having trouble finding the right way to trigger my fans, having tried several things unsuccessfully. Fortunately I happened on motadacruz 's Music LED Light Box . His method of triggering LEDs via sound input easily translated to triggering fans via the tiny voltage sent to the LED by my motion sensor. I have modified his schematic here - hopefully it is clear enough.

You will need to clip the end from your wall wart so you can get to the bare wires -please be sure it is not plugged in when you do this. Slice down the middle to separate the two wires then strip a bit of the insulation of each, you'll need enough to solder to - or to attach via wire nuts. I was told early on that if you cut the wires to different lengths, you have less chance they will accidentally touch and short out. You will need to use your multimeter to find the positive & negative wires. Attach the positive wire to the two red wires from your CPU fans, attach the negative to the ground pin of the TIP31. (For the one I purchased at Radio Shack, the schematic above is exact when looking at the front of the transistor)

Attach the wires you soldered to the motion sensor to the leads on either side of the TIP31, attach your black (ground) wires from the fans to the center lead of the TIP31. At this point, you should have a working circuit, best to test it now. For some motion sensors, the one I used included, it takes a few minutes for the sensor to 'warm up' and become functional so you may think it isn't working when it just needs a bit of a lead. If your sensor has a switch like mine, you can turn it to "on" for the testing phase. Once you have the fnas blowing it is time to put it all in a box.

Step 4: The Box

Trace the dimensions of your fans onto the lid of your box, making sure to note the location of the screw holes - you'll need those later. Cut out enough of the lid to allow for airflow, but leaving enough to attach your fan and to maintain a solid close when you put the lid back on. Cut a hole in the side of your box where the wiring will come out, on mine that was the top as it sits by the litter box, and holes for the air to flow out of, I put mine on the bottom.

Screw your fans to the inside of your lid, being sure the airflow goes into the box and that your wiring is conveniently placed near the exit hole. Attach elastic across the top of your lid on the outside, this will hold your filter in place but allow for easy removal for cleaning or replacement. Cut a piece of your filter to fit on the lid and slide it under the elastic.

Position everything as conveniently as you can. At first I had the sensor down by the side of the litterbox so they set it off when they came in. Because this particular model turns off after 30 seconds with no motion, that wasn't giving me much fan time. I moved it up top and now it stays on as long as any critter is in the vicinity - including a human taking pictures.

Step 5: Variations & Options

In order to help a bit with the smell, I added an extra filter. I simply took a used drier sheet (the used ones are softer and more porous for better airflow), folded it in half to create a pocket which I filled with activated charcoal, and sealed it up. This size was perfect for the inside of my box, and sat nicely over the vent holes I cut in the bottom of my box.

So far, a LOT less dust around the house.

This could easily be adapted to a covered litterbox just by cutting a hole in the cover and attaching the fans.

If I were to do this again I might:
- Solder the transistor and wiring together via a perf board rather than directly to each other.
- Find an enclosure that would be an all-in-one, rather than having to perch it on a cardboard box. I was looking for some sort of sweater storage box, but those that would be tall enough were too deep, those that were slim enough were too short.