Hack the Spy Ear and Learn to Reverse Engineer a Circuit

Intro: Hack the Spy Ear and Learn to Reverse Engineer a Circuit

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:
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
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Make a Rechargeable Dual Voltage Power Supply for Electronic Projects
SODA CAN HYDROGEN GENERATOR

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.

LTspice
http://www.linear.com/designtools/software/switchercad.jsp

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
.backanno
.end

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.

Comments

[c4fmguru]

Nice try, but incorrect...

The reason the first schematic seems to amplify more than the second schematic doesn't have anything to do with the two missing capacitors from the second circuit. If you notice the resistor values used for negative feedback of the self-biasing configuration (connected between the base and collector of the first two pre-amp transistors), are greater in value than the second circuit. This means that the first circuit has less negative feedback than the second circuit, which means that the first circuit could be less stable and can begin to self-oscillate. This is why the first circuit needs the added capacitance. The capacitors keep the amplifier from oscillating (high pitched squeal). However, less negative feedback signal means more gain. The second circuit compromised a little gain using slightly smaller feedback resistor values, but the circuit is more stable. This means that the capacitors used to keep the amp from self-oscillating are no longer needed in the second circuit.

Also, the ceramic capacitors marked 100uF should be 100nF. Or, 0.1uF in other words.

[T0BY]

I am very impressed!

[anubhavtalukdar007]

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!!

[anubhavtalukdar007]

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

[achraf52]

Do I really need that transistor or use any type .

[Biotele]

yes any transistor will work.

[achraf52]

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 .

[static]

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.

[Biotele]

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

[Dave54321]

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.

[Biotele]

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

[Dave54321]

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 ?

[burdockwing]

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

[techh2]

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

[Vaxinius]

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?

[Biotele]

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

[Vaxinius]

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.

[Biotele]

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.

[Vaxinius]

Aha.

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!

[Vaxinius]

Aha.

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!