Make your own automatic shower timer. via instructables

We’ve all been told to take 5 minute showers. However, do we really know how long our showers actually are?
Here’s a simple weekend project that is both fun and practical. This circuit uses an ATtiny85 chip that automatically starts a timer as soon as you close your shower door. When 5 minutes are up, a piezo speaker starts beeping to let you know that time is up. If you finish your shower early, the timer resets.

Project Materials

• ATtiny85
• 8-Pin IC Socket
• SPST or SPDT Limit Switch (also called Microswitch)
• 8 ohm Piezo Speaker
• Green LED
• Red LED
• LED Holder – 5mm (2x)
• 220 ohm Resistors (2x)
• Male and Female Header Pins
• Mini Solder-able Breadboard
• Stranded Hook-up Wire 22 AWG
• 2x AAA Battery Holder
• AAA Batteries (2x)
• Adhesive Velcro Strips
• Small Project Box or Enclosure
( Most of these materials can be purchased from Sparkfun Electronics )

Tools

• Soldering Iron
• Diagonal Cutters
• Rotary Tool (with drill bits)
• Screwdriver
• Multimeter

To program the Attiny, you will also need an Arduino and a few jumper wires, as well as the latest version of the Arduino IDE.

Full tutorial