If you're looking to reduce the amount of time you spend in the shower, this DIY setup starts a timer automatically as soon as you close the door.
If you catch yourself just standing under the hot water a little too often, a timer can tell you how long you're really taking in there. This build from Instructables user GeckoDiode has a set five minute timer that starts beeping when the time is up. It uses an ATtiny85 microcontroller, speakers, LEDs, and a small limit switch that attaches to the backside of your shower door. When you get in and close the shower door, the five-minute timer starts, but if you finish up early, the timer will automatically reset itself.
It's a nice little setup if you're trying to conserve water, or if you waste unnecessary time in the shower. Check out a complete list of parts and instructions at the link below
DIY Automatic Shower Timer [Instructables]