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
Check out my other Instructables:
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Step 1: How to Reverse Engineer a Circuit
This step shows you how to reverse engineer the 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.
Step 2: Draw the Schematic
To draw the circuit and simulate it I used Linear Technology's LTspice. It's free and it is great.
I make the Spy Ear schematic available for the first time on the web in this instructable.
V1 N001 0 1.5
Q1 N006 N009 0 0 2N3904
Q2 N004 N008 0 0 2N3904
R6 N001 N004 4.7k
Q3 N005 N004 0 0 2N3904
R7 N004 N008 200k
C4 N008 0 5n
RÃÂÃÂ§VR1 N002 N003 5k
RÃÂÃÂ§VR2 N003 N006 5k
R2 N001 N002 220
CÃÂÃÂ§BigC N002 0 10ÃÂÃÂµ
R8 N006 N009 200k
C1 N007 N009 .1ÃÂÃÂµ
R1 N002 N007 3.3k
C2 N003 N008 .1ÃÂÃÂµ
VÃÂÃÂ§Microphone N007 0 SFFM(0 1u 2000 100 100) AC .1u
RÃÂÃÂ§Earphone N001 N005 75
C3 N001 N005 .1ÃÂÃÂµ
.model NPN NPN
.model PNP PNP
.lib C:\Program Files\LTC\SwCADIII\lib\cmp\standard.bjt
.tran 0 100ms 0 1ms
Step 3: Simulations
Here are the simulations that I ran from the previous netlist and they show the characteristics of the Spy Ear.
You'd notice the frequency response is not even which produces distortions in the output (see next pic).
But this is ok, because Spy Ear is designed to focus on speech. The main spectrum of speech is between 300 and 3000Hz and if you are trying to spy on someone's conversation as the package claims, the goal is to amplify speech frequencies while cutting out ambient noise.
There is an advantage of having a schematic for simulation because with a few clicks you can investigate the effect of modding the components without actually doing it physically. For example, if C1 and C2 are replaced with larger capacitors, like on the order of 100u, the response approaches HiFi (see last picture). HiFi requires that the frequency response be flat and wide.
Step 4: Picachu's Spy Ear
Picachu bought spy ears that are different than the one I used. It turn out they are missing two capacitors that makes it amplify less.