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

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.
<p>I am very impressed!</p>
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??? <br>
Do I really need that transistor or use any type .
yes any transistor will work.
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 .
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 <em>often.</em>
yes bucket fill is a pain. I am investigation using raster to vector programs as way to automatically trace the lines.
Wow that's great, here is another diagram of the Spy Ear from area 50<br>it allows you modify the Spy ear by removing a transistor and powering<br>it by the computer to make a super duper sensitive Condenser Mic<br>which worked well for me as I wanted a better mic than I could find at<br>radio shack.<br><br>here is the link to several directional Mic's the last one is the Spy Ear<br>http://www.techlib.com/area_50/PTM/audio.htm#super and also includes<br>the schematics.<br><br>also the .1 uf caps don't appear to be electrolitics so observe the DC potential<br>in this circuit when using electolitic caps by putting the + to the higher potential<br>so they don't blow up, which I doubt at that low voltage.<br>
this is awesome. they also use the photoresistor/led pair from my other instructable as ACG.
I think you mean AGC automatic gain control. I don't know how the <br>photo resistorwould work in that circuit. <br><br>What lead me to the spy ear was I wanted to make my computer<br>mic actually put out some audio to make a series of affermation<br>statements to play while I sleep under my pillow. The little condenser <br>mic's that are powered by the computer audio circuit board lack<br>sensativity so you must speak closely to the mic and you pick<br>up a lot of unwanted noises made by the mouth that are not normally<br>heard in conversation. <br><br>What is the title of you other Instructable ?
i have noooo idea what you are talking about its literally like you're talking gibberish
why this circuit dosent work please send me explication for this circuit plz plz i ned this circuit
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?
just rename them with their proper names &quot;spyear.asc&quot; and &quot;spyear.net&quot;, then open them with ltspice.
nope even when i rename the file i still get a spice error message that says theres &quot;multiple instances of [symattr]&quot;<br><br>then it lists a whole bunch of text of wire locations etc. same thing as before.<br><br>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.
rename the extensions of both the files as &quot;.asc&quot; and try to opening them. I think the files are mixed up.<br><br>If that does not work Google &quot;multiple instances of [symattr]&quot;. You might find the problem. keep me posted, thanks.
Aha. <br><br>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!
Aha. <br><br>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!
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.
the output impedance of the spy ear is ~70 ohms. Use an audio transformer. Beware, you will just have a lot of feedback.
Just bumping this to let UK readers know &quot;Poundland&quot; has been selling these units for the last couple of weeks - I'm sure there must be a lot of ways to re-purpose these...
Please visit here: <a rel="nofollow" href="http://www.deltasoft.com.cn/english">http://www.deltasoft.com.cn/english</a><br/><br/>That is a good software of pcb reverse engineering<br/>That is too easy for pcb reverse engineering.<br/>
It says &quot;buy now&quot;. Buying already made stuff is antithesis of DYI enthusiast. Is it only in chinese? Make an instructable showing how to use your software. <br />
can you wire this up to a lm3916 vu meter?<br />
so, which one the true..? i dont understand about these.<br />
the first model. This is to show that there are variation.<br />
merci +ok
do anyone know where i can get a spy ear in the uk (shops/stores) coz i really want to give this a go but cnt get a spy ear and i not old enough to buy online so if anyone could help that would be fab lewis94 :D
eBay UK has it.<br/>You do not need a credit card to buy online.<br/>You can pay by Postal Order from your local Royal Mail Post Office.<br/>Here is a link:<br/><a rel="nofollow" href="http://preview.tinyurl.com/lfl4o4">http://preview.tinyurl.com/lfl4o4</a><br/>
there's something fishy about the schematics and the PCB Picture. They Don't Match!!! Please fix. I can't find where the 47uF capacitor is. in the schematic
Where do I get it? (i live in usa)
i believe that toys r us sells spy toys including this one, check online i also think u can just build the circuit yourself according to the schematic in this instructable and an online order from an electronics shop (digikey, allelectronics, the goldmine are some of my faves)...
I just want to make a pocket headphone amp for cheap, so could i just solder wires from a headphone jack to where the mic used to be? thanks
yes that will work. Except it will not be HiFi. You can change the capacitors like I did in one of the steps and increase the fidelity.
Try to use layers in Photoshop. What I am doing is taking the pictures of bottom and top then flip the bottom one and cover the top PCB picture with flipped bottom as different layer. Then you can set brightness and transparencies of layers (each one separately) for the best result. Even if they aren't exact same size you can stretch pictures to match the layers.
that is the best way to do it.
I want to make this wireless, any ideas?
sure. if you are lazy hook it up to an ipod car radio transmitter or an fmrs walkie talkie. if you want to build the circuit google "fm transmitter".
hi, do you think you could send me a picture of where the mic is linked to? cos mine came off and i dont know where to solder them. thanx
The microphone goes where the green dots are on the reverse side picture in step 4.
I just got home from Walmart and could not find these. I checked electronics, toys and the gadget area by the registers, None of the workers in those areas had ever heard of them. They had a more expensive one in the toy area, but I didn't want to pay $10 Where do you find them? I bought one years ago at a truck stop, but can't find it now.,
I always find them at super sketchy 99 cent stores that sell Chinese bootleg transformers toys
i saw some in hungates, a craft store.
Hello biotele i was very impressed by you and your instructable and tried to construct the circuit myself. i used BC547 instead of 2n3904(that's what i have but the specs are the same) and used 220K instead of the 200K resistors(didn't think it would matter) but it just didn't work for me ,as hard as i tried i made noise and talk to the mic and there is no sound through the speaker. :-O i checked the mic,speaker and the VResistor and they are fine. also i checked to see if i didn't do a mistake of connection the wires and component. but still no sound :( was it OK to use 2 AA instead of the 2 LR44?
You need to trouble shoot it stage by stage. First, hook the oscilloscope to the microphone output. Is there a waveform? Then test the output of the first transistor, the second transistor and finally the third. Make sure all your transistor Have the proper DC bias, use a voltmeter to test these. it is ok to use 2AA.
by the way the only test measurement i have i DMM.
Ok then, measure the voltages on around the transistors. Try injecting a tone from a mp3 player (don't use an Ipod, get a cheap one, just in case you ruin it), before each transistor stage.
results:<br/>(I had tried 5 different mics i had lying around, which one of the was new,and non worked.)<br/><br/>i put(instead of the mic) a tone out of my radio through a headphone (one of the channels) and <strong>i could hear real good the sound</strong> coming out of the circuit speaker,amplified.<br/><br/>the volt around each transistor was:<br/>q1 Vce=0.52v<br/>q2 Vce=0.7v<br/>q3 vce=2.6v<br/>(those values where also given when there was nothing attached where the mic is)<br/><br/>the volt when the mic was there<br/>q1 Vce=0.06v<br/>q2 Vce=0.7v<br/>q3 vce=2.6v<br/>
this also resemble to the pc sound card inputs<br/>the <strong>line in</strong> &amp; the<strong> mic in</strong><br/><br/>my circuit works as a line in. i don't understand much of it ,but this analogy seems the same here.<br/>

About This Instructable




Bio: Make a better Instructable, and the world will beat a path to your website.
More by Biotele:Easy to Build WIFI 2.4GHz Yagi Antenna Hand Cranked Flashlight Mod plus Joule Thief Ganzfeld: Hack Your Brain the Legal Way 
Add instructable to: