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
Check out my other Instructables:
MAKE A HIGH VOLTAGE SUPPLY IN 5 MINUTES
Super Easy E-mail Encryption Using Gmail, Firefox and Windows
Make a Voltage Controlled Resistor and Use It
Make a Ball Mill in 5 Minutes
Make a Rechargeable Dual Voltage Power Supply for Electronic Projects
SODA CAN HYDROGEN GENERATOR
Step 1: How to Reverse Engineer a Circuit
1. First take a picture of the front and back of the circuit.
2. Trace the pcb layout on the back using a graphic program like photoshop. Try using the "bucket fill" tool first. if that doesn't work color it by hand.
Don't color the whole pcb layout. Leave the areas where a solder is made clear, so that you can figure out which component's leg goes where.
3.Copy the pcb layout you made and paste it on top of the device's front picture. Flip it horizontally and adjust the scale and position so that the trace is super-imposed exactly on top of the components (see picture below).
4.Then comparing the different pictures and looking at the actual circuit, draw in the components' symbols from node to node. (see last picture).
5. Next, you'd need a circuit drawing program to rearrange the rough circuit that you drew by hand (see next step).
Count the components. Use the count as a checksum when you reconstruct the schematic. It is easy to forget something.