Introduction: Low-power, Low-cost, Cat-sensing, Litter Box Lighting and Ventilation

This Instructable will demonstrate how to build a system for detecting cat presence and providing light/ventilation for their litter box experience. The circuit includes a timer to prevent wasted electricity and uses no microcontrollers.

This system consists of two distinct components: a cat-sensing pressure mat and a circuit to power the lights and fans. I'll show you how to build both.

Note: The mat can be used as a simple switch to activate any circuit. Likewise, the circuit can be activated by other means. Today's your lucky day - this is like 3 Instructables in one!

    Cat Mat:
        Aluminum Foil
        2x Posterboard/foamboard*
        Spray adhesive
        Craft foam
        Hot glue
        Wire and connectors (male | female)

    Power/control Circuit:
        Breadboard or protoboard or PCB
        1x 12v AC/DC Adapter
        2x 12v PC fans
        12v Lights**
        1x 12v relay
        1x 555 Timer IC
        1x 10nF Capacitor
        1x 0.47μF Capacitor
        1x 470μF Capacitor***
        1x 10kΩ Resistor
        1x 350kΩ Resistor***
        1x Diode
        Assorted wires

Of course, the most important ingredient is love.

*I originally selected posterboard because of it's small profile. Unfortunately, the top layer turned out to be a little too flimsy/unreliable/inconsistent, so it was replaced with foamboard.
**For the lighting, I used 12 LED's from a reel I purchased online. I bought these specifically because they only required a 12v power supply, which matched the PC fans I was using. You may find something more suitable for your needs.
***The values of these components affects the timing of the circuit. For more information, see step 7.

Step 1: Cat Mat: Functional Overview

The idea behind the mat is simple, although there are some caveats to it's construction.

Basically, two pieces of aluminum foil are physically separated by foam. When weight is applied, the foil layers come together to make contact. When weight is removed, the foam lifts and separates the two foil layers.

Step 2: Cat Mat: Preparing the Aluminum Foil

Measure the area you want the mat to cover. Keep in mind that a portion of the mat's border will not detect weight.
Measure the aluminum foil to be glued to each posterboard. The foil will need to be about smaller than the posterboard from all 4 edges (5cm should be fine).
Before you cut a perfect rectangle, you'll want to leave some excess foil on one end (see diagram). This is where we'll attach a wire, and we need the connection to be strong. You can't solder wire to aluminum using regular (lead) solder, so we'll have to make sure we have a strong physical connection instead.

Fold the excess foil in half 3 or 4 times.
Fold the excess foil over the main foil portion, leaving a rectangle.
Cut a small slit in the foil adjacent to the folded excess.
Strip enough wire to wrap around the folded, excess foil at least once. Stranded wire probably works best here.
Feed the stripped wire through the slit and wrap it around the excess foil tightly.
Pinch/squeeze the wire and foil tightly.

Repeat this for both sheets of foil.

Step 3: Cat Mat: Building the Layers

Cut both posterboards to the desired size, a little larger than the foil - approximately 5cm extra around each edge should be fine.
Mark the posterboard where the corners of the aluminum foil should fall into place. This will aid with placement, because the spray adhesive is very sticky and the foil tears easily!
Cut strips of craft foam to run along the entire border of the posterboards. 2-3cm width should be fine.

Prepare an area for your spray-adhesive work. Lay down old newspaper or magazines so you don't spray glue onto the floor. I like to do this sort of work in the garage because I can keep it closed off and (hopefully) free of debris.

Lay your posterboards flat on your work area, make sure the side you marked for the aluminum foil is face-up.
Read the directions for your spray adhesive, then cover one posterboard with it.
Carefully lay the aluminum foil onto the posterboard, using your marks as guides. Try to avoid wrinkles and creases, but we will work these out soon (I don't think shiny-side matters, but I did mine shiny-side up).
Lay the craft foam around the inside border of the posterboard. Press firmly so it sticks!
Using a credit card or something similar, work wrinkles out of the aluminum foil - we want it as flat as possible with no air pockets!
Move that posterboard somewhere safe and flat where it can dry.

Spray the second posterboard with adhesive, and lay more aluminum foil on it.
*The top layer does not need a foam border.
Again, use a credit card to work out any wrinkles and bubbles.
Move the posterboard somewhere safe and flat where it can dry.

While you wait for glue to dry, you can skip to the circuit board (Step 7) and continue with the mat later.

Step 4: Cat Mat: Adding Spacers

Before you start more work on your mat layers, you may want to glue down the wires. The more they move, the more likely it is to tear the foil and ruin all your hard work. I used hot glue to help keep my wires in place.

At this time, I found that they spray-adhesive didn't adhere the posterboard to the craft foam very well (I may have neglected the borders while spraying). You may not need to do this, but I ran some hotglue around the inside and outside of the foam border to help keep it in place. It kind of felt like caulking, which probably isn't a bad thing.

The size and shape of your spaces, as well as they layout of them, will depend on your specific requirements. So before we go crazy and glue anything down, we'll want to test and adjust accordingly!

I started with very small squares (approximately 3-5mm) spaced in a uniform grid. Remember, no glue yet! We need to test first!

Step 5: Cat Mat: Testing

Before you glue down your spacers, you'll need to test the mat to make sure it performs as desired.

To test, I put my multimeter into continuity mode. In this mode, it will make an audible tone if the two leads are electrically connected. You can use resistance testing mode if your multimeter doesn't have this function.
I also attached a female power connector plug to the mat's wires to make this a little easier.

For testing, place your mat on the same floor type it will be used on (hardwood/laminate/tile may yield different results from carpet).
If your mat is already indicating an electrical connection, you probably need to add more spacers or increase the size of each spacer.
My mat is is concealed beneath a litter-trapping mat, so I needed to test this exact scenario.
If your mat is indicating an electrical connection under the weight of its concealing mat, you probably need to add more spacers or increase the size of each spacer.

The Press Test*
Press on the matt with your palm. The multimeter should indicate an electrical connection while you're applying pressure, and it should stop when you stop.
If the mat doesn't activate, you may need to remove spacers and/or check your wires.
If that mat activates but doesn't stop after you're removed your hand, you may need to add more spacers, use larger spacers, or use thicker spacers.
Pressing directly on a spacer will probably not activate the mat. If your cat is Indiana Jones or a super-ninja, this may not work out for you, but congratulations on having an awesome cat!

The Cat Test
Once your mat passes the hand test, it's time to test it with your cat.
Ideally, you should command your entice your cat to walk over the mat (good luck!).
Given that your cat is a cat, it will probably do the exact opposite of what you want, so you may need to pick up your cat and place it on the mat.
Again, the multimeter should show an electric connection while the cat is on the mat, and that connection should disappear as soon as the cat steps off.

Test every inch of the mat! One dead, stuck, or overly-sensitive spot can ruin the whole project. You may need to experiement with different patterns, shapes, sizes, and thicknesses of spacers to get the perfect effect.

*It was during the Press Test that I discovered how inconsistent posterboard is. In the end, I glued the top posterboard layer to foam board. This increased the thickness of my mat and the minimum activation weight, but provided much more consistent results.

Step 6: Cat Mat: Finishing the Mat

With every inch of the mat well-tested, we're ready to make it permanent.

Place the top layer of the mat (the one without the foam) onto your bottom layer, making sure they are well-aligned.
Add hotglue between the craft foam border of the bottom layer and the top layer.
I did this with several glue spots all the way around the mat, you may choose to do a full line of hotglue.

If you didn't added the wire plug connector during the testing step, now is the time to do it! This isn't necessary, but having it will make it much easier to connect and disconnect the mat in the future.

Congratulations, you're done with the cat mat!

Step 7: Control Circuit

Our circuit needs to provide power to the lights and fans, but it needs to power itself off to save energy. It will work like this:

Cat steps on the mat which closes a circuit.
The closed circuit provides 12v to the coil of a relay, closing its circuit.
The closed relay circuit provides power to three things:
    The lights
    The fans
    The 555 Timer chip
The output of the 555 chip is also connected to the coil of the relay.
When the 555 timer runs out of time, it cuts power to the relay coil.
When the relay coil is no longer powered by the 555, it opens its circuit, which cuts power to the lights, fans, and the 555 itself.

The 555 chip is configured to run in monostable mode. This means that, once triggered, its output pin will remain high (providing voltage to the relay) until its timer expires.

The 555 timer starts its countdown when it encounters a high-to-low voltage transition on its trigger pin. To keep the circuit simple, we'll activate this high-to-low transition almost immediately after it is receives power.
To pull the trigger pin high, we'll use a resistor tied to the trigger pin and the 555's input voltage.
To pull the trigger pin low (and officially start the timer), we'll use a capacitor between the trigger pin and ground.
In layman terms, the capacitor will take a moment to "fill up", and once it does it will bring the trigger pin low, activating the 555 timer.

The duration of the timer is dependent on a separate resistor/capacitor pair. I'm aiming for 3 minutes, and using a calculator I found online, I determined that I could achieve this using a 350kΩ resistor and a 470μF capacitor.
In the diagram, these components are labled as R1 and C1. You'll want to use values which provide your desired timing.

Step 8: Putting It Together

With your mat assembled and the circuit completed, you can complete your project! Attaching the fans and lights will vary based on your enclosure. The graphic on this page demonstrates my configuration.

Here are a few more notes to keep in mind:

* If you use two fans like I did, make sure the fans blow air in the same direction (one should blow in while the other blows out).

* Cats love strings and wires, and most will play with anything dangly. Make sure your wires are tidy and/or tucked away.

* Consider finding a simple enclosure for your circuit. This will help prevent feline-sabotage, especially if you're circuit lives on a breadboard.

* Cats aren't usually fond of changes to their homes, and it may take them a little while to get used to your upgrades.