Here's a fun little project I did while bored. Then I got even more bored and made up this instructable. I describe the parts needed, the schematic, and the breadboarding process. The rest is up to you.
Step 1: What You'll Need.
1. an LM339, or LM393 comparator IC
2. two different color LEDs
3. two LED resistors ~300ohm to 1k in value
3. piezo buzzer (not transducer/speaker)
4. 4 identical "high value" resistors, about 5-10k or so
5. 1 "low value" resistor, somewhere's between 20 and 600 ohms
6. A breadboard and some leads.
7. A power supply. AFAIK the LM339 can handle anything up to about 14V. But you will need a minimum of about 3V to drive the LEDs and the piezo good 'nuff. Too much voltage and you risk damaging the circuit you are working on. Batteries will do fine. A couple of alkaline button cells or a single lithium coin cell would do nicely.
8. About ten minutes of free time.
Step 2: Schematic
Don't forget to clik on pic 2.
Step 3: Build
Check out the pics for step by step instructions.
If your circuit has "issues" try increasing the value of the sensitivity resistor. If it's too low, it might be swamped by variations in the 10k resistors. Alternatively, swap around the 10k resistors until it works, properly.