This instructable introduces the venerable Spy Ear in details and my way to reverse engineer a circuit. Why does this device deserves its own instructable?: -You can buy a Spy Ear for a dollar! -It can amplify sounds up to 60 dB or a factor of a 1000. -It has a self limiting property and adjusts the gain so that the amplified signal volume is always just right. -It runs of two LR44 1.5 volt button cell alkaline battery, so it's perfect for portable projects. -Many of today's projects, such as in robotics, require analogue front end for sensing the environment and the Spy Ear circuit is just right to fill in as a multi-purpose front end amplifier. -It is simple enough to reverse engineer. -I am making another instructable using this device. So the Spy Ear is a fantastic cheap,small and rugged circuit for modding and hacking
http://www.instructables.com/id/Hack-The-Spy-Ear-and-Learn-to-Reverse-Engineer-a-C/