Velleman's MK102 beginner-level flashing LED kit has adjustable flashing speed and two potentiometers.
Pros: Simple build
Cons: Must download schematic, Some experience needed
Best Uses: Robotics, Toy
Describe Yourself: Enthusiast
Primary use: Personal
Was this a gift?: No
Some solder experience required. Must download schematic from site. Must already know positive end of LED and electrolytic capacitor. Provides excellent adjustability for background noise level- if background too loud and LED stay on, turn potentiometer. Very small with battery attaching to back of circuit. I kept the leads on the LED long and will attach to a minisumobot as sound indicators.