This is my first instructable - I didn't plan on writing one, but about 1/2 way through my project I realized this might be helpful for other people so I'll try to catch you up on my pictures as best as possible. I use a lot of parenthesis to explain the concepts (such as what I'm doing right now) so hopefully you'll be able to get the idea to do with your own scrap parts instead of just using what I used.

Some kids struggle to have dry nights longer than their friends do, and it can be very embarrassing for them with sleepovers, siblings, or just a bad feeling. Although it can be frustrating for the parent who has to keep doing the sheets and cleaning up, this is never the kid's fault. The major cause of bed-wetting is a very sound sleeper. This can be caused by many issues - in our house it's because of a health issue that causes the "out like the dead" slumber.

One thing to point out - the device doesn't work on its own. It's a partnership between you, the kid, and the device (we all ours R2). The idea is the device goes off, the kid wakes up, associates the urination feeling with waking up. With that, you (the parent) gets up, and instead of making it a disappointing thing (as in "too bad - another accident") you make it a positive thing ("Great! You got up!"). Additionally, show the kid the wet spot - it'll get smaller after a couple of nights as the kid wakes up sooner in the cycle, and seeing the wet spot shrink (AFTER giving said child time to go to the bathroom and change of course) will be a form of progressive encouragement. I can't stress this enough - ALWAYS STAY POSITIVE WITH YOUR CHILD ABOUT THIS! One negative comment or feeling is going to give the child an embarrassing association with the whole thing, halting the progress.

Name-brand companies have been making these and selling them for a TON! This show you how to make it for ~$10
Special thanks to Jack Brockman who gave me this idea originally, and provided me with a couple of parts.

Onward ho!!!

Step 1: Gather Materials and Tools

Sorry again for the lack of pictures at this point - pictures didn't start until later :(
A part of this project was wanting to do it all myself with things lying around the house, but I'll throw some suggestions out there for the harder to find stuff. This seems like a long list, but don't be daunted - it's all pretty easy to come by!

Materials:

Two ~4'x4' aluminum or copper screen (taken off of old windows or you can buy a roll very cheap @ Lowe's)
One and a half 1" PVC pipes (taken from my old sprinklers)
~9' wire (I used old speaker wire pulled apart to make my pennies stretch!)
Some sort of electric bell or alarm (this instructable is with an analog bell, but you can make a digital one work). Umm...this was from a junk pile...I don't know where to buy one
9v battery (or whatever is appropriate for your bell) and battery clip/holder
A switch (can be simple 2 way switch - I used a 3 way pull switch that was sitting around, but any type of switch that a kid can easily operate will work)
Some screws
[optional] carpet padding
Last item - a box/container. Only one rule here - can't be made out of metal. Can be any size you want, but if you decorate it it will be a nice touch for your son/daughter

Tools:

A drill or screwdriver
A Dremmel (or other rotary tool)
Wire stripper or a knife (be careful - they are sharp!)

Step 2: Put Your Switch in Place

Yay! First usable picture!
Take your container and drill a hole the right size for your switch, and screw your switch into place (or hot glue, or super glue, or steel band, etc...) The important thing here is that the switch is easily accessible, and sturdily mounted to your box so when a groggy child is fumbling for it, it can be easily switched off without knocking it out of place.

Step 3: Put Your Bell Where You Want It

Find a place to put your bell and screw/glue/chewing gum it into place. I used a box with plenty of room, so it was easy. One thing I learned with this old bell is that if it was screwed down too tight then the ringer would get stuck after hitting once, so attach a wire to each lead and test it out with your battery after screwing it into place to make sure it's going to work NOW instead of AFTER putting it all together. If you're using a digital bell, that second half wasn't for you :)

Step 4: Wire in Your Switch to Test

Before you move on, you'll want to test out that your system is good to go. All you're doing here is completing a circuit.

*The drawing shows the negative (-) as the constant with the positive (+) as interrupted, but it doesn't really matter.

Step 5: Cut Your PVC

Time to cut the PVC - YAY!!
What we're doing now is making a protective edge for the screen as cut aluminum tends to be sharp (if I showed my cut finger, I would have to give this a PG rating, and I don't want to be all edgy-like). I'm sure there's a better way to do this, but I used a Dremmel on 1" PVC pipe. Here's your goal: To use as small of a diameter PVC as possible (small size makes it easier to cover comfortably with pillows) and to cut a THIN line down the length of the PVC for the screen to slide into.

*TIP: If using a Dremmel, take it SLOW (the movement, not the speed). I went through quite a few cutoff discs before I realized a slow hand at a medium high speed was the best, otherwise the disc broke FREQUENTLY
*TIP 2: make sure the child this is for is not around for this part, because 12 broken discs on 2 pipes will teach new words to said child

Step 6: Put the Protection Pipes on the Screen

Cut your screen if it's all on one roll to ~ 4'x4' (width of bed x length of child)
Bend 1/4" of the 2 cut edges of the screen back on itself (not shown). BE CAREFUL - IT'S SHARP! What you're doing here is creating a small catch for the PVC to keep it from pulling out easily *EDIT - use a board or some hard object to bend this so you don't hurt yourself
Slide the screen into the PVC (I shouldn't have to say this, but....to this from the side) The non-cut edges (the ones that aren't cut and bent) can slight straight in, but do it slowly to avoid bending the aluminum because it's a ROYAL PAIN to feed it in there once it's bent (child is learning more new words at this point...)

Step 7: Wire Your Box, Connect Some Wire to Your Screens

This part seems like an extra step, but I think it's a good one for two reasons:

Allows for easy transport of the system
Makes it easy to remove the screens for washing (muy importante!)

So everything on the inside works now, right? You now have a bell that turns on and off with a switch! Sadly, this isn't enough to begin rejoicing...it's quite boring, in fact. We need to now interrupt one of those wires to allow for the urine to be the second switch. I used the same line (positive +) that I wired to the switch to make it easier to troubleshoot in case something went wrong.

Using your wire strippers or a knife, strip ~ 2" of wire from the wire going out of one side of your switch, then wrap it around a ready-made or newly created (you can use your drill for 2 holes right next to each other here) hole in your box. When wrapping it through, make sure it is tightly wrapped around itself (there can be slack, by you don't want it to come off if it's tripped over). You can crimp it if you have the stuff for that - I didn't, I just wrapped it tightly.
Do the same for another wire connecting back to the bell this time
Take 2 pieces of ~4' of wire (longer if you like) that is going to attach from the screens to the box, and strip ~2" off each end. Because I wanted to be able to remove them easily, I put alligator clips on them (which was great in theory, but they ended up coming off. I think the dog is starting to learn bad words by now)

Step 8: Test Your COMPLETED Connections!

If this works, you're almost done!

Using the 4' wires, connect one end to one screen, the other to one of the 2 exposed wires on the outside of the box. Repeat for the other screen and other exposed wire.
Turn the switch to the ON position
Touch the screens together - if the bell works, you're golden! If not, go back and check your connections

Step 9: Attach Your Battery to the Box

Batteries are heavy. A battery flopping around inside your box is both loud and runs the risk of yanking out the wires inside. I used 3m hook-and-loop (it's Velcro - I don't care what the unlicensed name is supposed to be!). It made it easily removable, and is re-sticky for the next battery!

*Since we're on the subject, let's talk about the battery for a minute...
It was originally suggested to me to use a transformer and just plug this in. My big qualm with that is sometimes transformers malfunction sending a spike, and my kid is going to be laying on this! Really, the only way this will work well is through a positive experience, and an occasional jolt just isn't an option so I opted for a battery (or battery pack if you're using AA's). If you use too weak of a battery, the bell won't work, and too strong of a battery and the circuit might jump giving false readings. I found a 9v works best for my bell, and has a decent battery life.

After attaching your battery, close up your box and move on to the final testing stage!

Step 10: Lay It Out, Test It Out

So now we want a real world test.
You're going to need all the normal bedding, plus 1 extra sheet. Here's how your'e going to do it from the bottom up:

Mattress pad
Screen 1
Fitted (or bottom) sheet
Screen 2 (the order doesn't matter on the screens)
A 2nd fitted (or bottom) sheet
Child
Top sheet

For our testing, we'll skip the child and top sheet. Now, you can either pee on this yourself (that's pretty gross - why would you even think that?!?) or mix some salt into some water, and pour a small bit onto the sheet and lightly press on the wet spot.
The water should soak through, adding the weight of the child, causing the bell to ring waking up the child (and hopefully you) to get up to turn the switch off and go to the bathroom.

*I want to point out again that this is a joint effort between you and your child. For the first couple of nights, plan on hanging out on the couch next to your child's room or having him/her sleep with his/her door open so you can hear the bell, come help take the kid to the bathroom, and encouraging at the same time (i.e.: Great job! You got up! Now let's go to the bathroom, change the sheets together, and you can go back to sleep). The association is going to quickly come that having that "I've gotta pee" feeling goes hand-in-hand with getting up to go to the bathroom.

Step 11: Modifications, Changes, Etc...

The PVC on the uncut edges made the screen really hard to maneuver. If you can make yourself a custom sized window screen, using 2 of those may be a good option. What I ended up doing was using a couple of old towels and folding them in half to make them thick, then lining the edges of the screen (the uncut edges - you need to keep the PVC on those sharp cut edges!!). Even though they're uncut, they still need SOME protection as they're sharp when they bend, and can poke through the sheets giving false connections (and unpleasant pokiness)
The bell ended up being far too loud for a quiet room, and was scaring him awake which does NOT make for a positive experience. I went back to the box and added carpet padding to muffle it a bit which worked perfectly!
Mine is pretty dull and humdrum, but decorate the box however you like to make this as positive as possible!
I haven't learned Arduino yet, but a great add-on to this would be to have it send a feed to a private Twitter address, sending a notification to your phone. This would alleviate the need to have to be there waiting, and could also help you not miss an accident you might sleep through

I hope you enjoyed my first instructable and more importantly, I hope it helps some people out there!