Hack the Spy Ear and Learn to Reverse Engineer a Circuit




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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:
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

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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
R§VR1 N002 N003 5k
R§VR2 N003 N006 5k
R2 N001 N002 220
C§BigC N002 0 10µ
R8 N006 N009 200k
C1 N007 N009 .1µ
R1 N002 N007 3.3k
C2 N003 N008 .1µ
V§Microphone N007 0 SFFM(0 1u 2000 100 100) AC .1u
R§Earphone N001 N005 75
C3 N001 N005 .1µ
.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.

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    125 Discussions


    2 years ago

    I am very impressed!

    Can you give me a schematic of the spy ear with 6v as the source? i m dying to get one!! plz giv me! thnx in advance!!

    in step 4 , the capacitors that you have marked 2x100 uf did you use tantalum capacitors which have the number 107 written on them???


    Reply 7 years ago on Step 2

    Great you helped me from having to buy those type as I have so much ones but no one is this type, now I only have to grab capacitors and resistors and put them together .


    7 years ago on Introduction

    Back in the day, and even in the modern era I simply shine a pen light from the solder side through to the component side, to aid in tracing out the circuit. There are time when you will need a stronger light though, and that fatigues the eyes quickly, even young eyes. Nice idea here, but bucket fill generally gives me fits. I have learned to use [control]+S often.

    1 reply

    Reply 7 years ago on Introduction

    yes bucket fill is a pain. I am investigation using raster to vector programs as way to automatically trace the lines.


    8 years ago on Step 2

    Wow that's great, here is another diagram of the Spy Ear from area 50
    it allows you modify the Spy ear by removing a transistor and powering
    it by the computer to make a super duper sensitive Condenser Mic
    which worked well for me as I wanted a better mic than I could find at
    radio shack.

    here is the link to several directional Mic's the last one is the Spy Ear
    http://www.techlib.com/area_50/PTM/audio.htm#super and also includes
    the schematics.

    also the .1 uf caps don't appear to be electrolitics so observe the DC potential
    in this circuit when using electolitic caps by putting the + to the higher potential
    so they don't blow up, which I doubt at that low voltage.

    2 replies

    Reply 8 years ago on Introduction

    this is awesome. they also use the photoresistor/led pair from my other instructable as ACG.


    Reply 8 years ago on Introduction

    I think you mean AGC automatic gain control. I don't know how the
    photo resistorwould work in that circuit.

    What lead me to the spy ear was I wanted to make my computer
    mic actually put out some audio to make a series of affermation
    statements to play while I sleep under my pillow. The little condenser
    mic's that are powered by the computer audio circuit board lack
    sensativity so you must speak closely to the mic and you pick
    up a lot of unwanted noises made by the mouth that are not normally
    heard in conversation.

    What is the title of you other Instructable ?


    8 years ago on Introduction

    i have noooo idea what you are talking about its literally like you're talking gibberish


    8 years ago on Step 4

    why this circuit dosent work please send me explication for this circuit plz plz i ned this circuit


    8 years ago on Step 2

    Im trying to open your schematic in LTSpice and all I get is a bunch of text grid locations in a list. Need a bit of help with this. Bit new to this program but im familiar with programs like it. When i download the file it saves as a .tmp? Should that happen?

    5 replies

    Reply 8 years ago on Step 2

    just rename them with their proper names "spyear.asc" and "spyear.net", then open them with ltspice.


    Reply 8 years ago on Step 2

    nope even when i rename the file i still get a spice error message that says theres "multiple instances of [symattr]"

    then it lists a whole bunch of text of wire locations etc. same thing as before.

    your help is appreciated as im a training electronics technician in the canadian forces and getting the edge on software like this can only do me good.


    Reply 8 years ago on Step 2

    rename the extensions of both the files as ".asc" and try to opening them. I think the files are mixed up.

    If that does not work Google "multiple instances of [symattr]". You might find the problem. keep me posted, thanks.


    Reply 8 years ago on Step 2


    Turns out my problem was i didn't know how to properly rename an extension. Kept renaming the file the whole time. ...to the sandbox!


    Reply 8 years ago on Step 2


    Turns out my problem was i didn't know how to properly rename an extension. Kept renaming the file the whole time. ...to the sandbox!


    8 years ago on Introduction

    I'm trying to pair spy ears circuits with appropriate speakers. Do we know what a well-matched speaker would be? I know that under-powering speakers are a good way to damage them. I'm seeing .2 watt, 2 watt, 5 watt speakers out there.