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.