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.