Introduction: Ultra-Sensitive Spy Ear

Picture of Ultra-Sensitive Spy Ear

Have you ever tried to listen to a conversation on the other side of the wall? Or perhaps one across the street? Well, now you can!

**An interesting observation: The other day I was using this device to listen to mechanical movements inside a machine. While the machine wasn't moving, I began to hear some noises (the machine was not the source of the noises). The more I listened to the noises, the more I was able to distinguish. It turns out the device was picking up radio signals! I figured that it must have been because the long wires of the microphone were acting as antennae, so I connected a whole spool of wire to the block connector, where the microphone was connected to the device. Amazingly, I was able to hear people talking on the radio well (along with lots of static) !**

Step 1: Materials

Picture of Materials

Most of the parts used in this project can be found in your local Radio shack.

- small project box
- prototyping board 
- N-type battery holder
- A23 12v battery 

- LM386 op amp
- 100k Ω resistor
- 100k Ω variable resistor 
- 0.1 μF capacitor
- 0.22 μF capacitor
- 1 μF capacitor
- 100 μF capacitor
- electret microphone

- switch (optional)
- block connector (optional)
- 3.5mm(1/8") audio jack (optional)
- heat shrink tubes (optional)

Note: The values of the components listed above are not critical and may be replaced with other values, but testing them in the circuit on a breadboard before building would be a good idea.

Step 2: The Circuit

Picture of The Circuit

Here is the circuit that I used to build this device.

Again, I recommend you bread boarding this before building it. 

Step 3: Laying Out the Circuit

Picture of Laying Out the Circuit

Before constructing a circuit, it is always a good idea to lay out the components on a circuit board to see how well things fit together. The box I used is pretty small so everything had to be fitted tightly together. The blue lines marked on the board shows how big my circuit board can be in order to fit well in the box.

*The variable resistor and the microphone are not shown on the board because they will be connected off-board.

Step 4: Building the Circuit

Picture of Building the Circuit

Now, after laying out the circuit, it is time to solder the things together.

Like always, the first thing to solder onto the board is the IC. In this case, it would be the LM386 chip. 

Step 5: Building the Circuit

Picture of Building the Circuit

It is usually easier to build a circuit when the shorter components are soldered in first, and that is exactly what I did here. I soldered the 1μF capacitor in because it is shorter than the other components.

Step 6: Building the Circuit

Picture of Building the Circuit

The next shortest component is the 0.22μF capacitor. In the schematic, it is connected between Vcc & ground, so one lead has to be connected to ground and the other has to go to Vcc. Because the way I laid out the components only one lead can be connected to the IC directly. The other lead has to be connected to the IC via a insulated wire to prevent shorting out the circuit. This will be done later. 

There is one thing that I forgot to do in this step that I should have done to make things easier, that is connecting pin 2 to pin 4. Since I forgot to do this in this step, I will be doing that later

Step 7: Building the Circuit

Picture of Building the Circuit

The next thing to solder is 0.1μF capacitor. One of its lead has to go to pin 3 and the other has to go to the 100k Ω resistor and the microphone; this lead will be left alone for now.

Step 8: Building the Circuit

Picture of Building the Circuit

The next component is the 100μF capacitor. Please pay attention to the polarity. The positive side of this capacitor goes to pin 5 and the negative side will be "out". The red wire(positive) from the audio jack will later be connected to this, but for now, this will also be left disconnected.

Step 9: Building the Circuit

Picture of Building the Circuit

Remember the lead that was left alone on the 0.1μF capacitor? Well, now it's time to connect it to the 100k Ω resistor.

As I mentioned before, a connection to the microphone will be made between the 0.1μF capacitor and the 100k Ω resistor later, so the lead from the resistor that is connected to the 0.1μF capacitor has to be bend downward to an adjacent hole to be connected to the microphone later. The other lead of the resistor will also be bent downward to an adjacent hole for a connection that would be made later. This will be connected to the variable resistor.

Step 10: Building the Circuit

Picture of Building the Circuit

At this point, all the on-board components are in their designated spots, but like I mentioned before there are still some more connections to make. The first one is between the 0.22μF capacitor and pin 6. The second connection is between pin 2 and pin 4. This second connection could have been avoided if it had been done earlier by bending the 0.22μF capacitor's lead and connecting them between pin 2 and pin 4 instead of cutting it short like I did. 

There are still some more connections make across the board, but we'll do that later.

Step 11: Wires for the Variable Resistor

Picture of Wires for the Variable Resistor

The variable resistor is the volume control for the output("out"). It is important to connect things correctly to avoid blowing out our eardrums. To make sure the right connections are made, I numbered the pins on the variable resistor to make things easier to understand.

Pin 2 will be the input, and depending on how you want to control the volume, either pin 1 is connected to Vcc and pin 3 to ground OR pin 1 connected to ground and pin 3 to Vcc. If you want to increase the volume by turning the screw counter-clockwise, do it the first way. If you want to increase the volume by turning the screw clockwise, then choose the second choice. I chose the second one and soldered a black wire to pin 1, a yellow wire to pin 2, and a red wire to pin 3. Heat shrink tubes were added after the wires were soldered to their pins.

*For those who are still wondering, Vcc is positive(+) power, and ground is negative(-)

Note: in the first picture the variable resistor says "103" but I actually used "104"

Step 12: Variable Resistor to Circuit

Picture of Variable Resistor to Circuit

If you followed the previous step, you should have 3 wires coming from the variable resistor: 1 black, 1 yellow, 1 red. In this step, we will be soldering them to the circuit.

1. solder the red wire to Vcc (pin 6 of IC)

2. solder the yellow wire to the lead of the 100k Ω resistor that is not connected to the 0.1μF capacitor

3. solder the black wire to ground (pin 2 or pin 4)

Step 13: Power for the Circuit

Picture of Power for the Circuit

In this step, the wires from the battery holder is connected to the circuit. The positive wire, or the red wire, is connected to pin 6 and the black wire, or ground is connected to pin 2 & 4 via another wire. The black wire was soldered to a random hole because I was running out of holes around pin 2 and pin 4.

Step 14: Wires for the Audio Jack

Picture of Wires for the Audio Jack

This step is optional if you're using an audio jack to plug in your earphones or speakers. I only connected one wire to one of the signal tabs. This means there will only be audio signal to one ear bud. 

The black wire is connected to the "ground" tab, and the red wire is connected to one of the signal tabs. Heat shrink tubes were used after the connection were made to prevent any unwanted connections with other components.

Step 15: Audio Jack to Circuit

Picture of Audio Jack to Circuit

The audio jack will now be connected to the circuit. Remember the pin on the 100μF capacitor that was not connected to anything? Now it is time to connect it to the audio jack. Thread the red wire through a hole next to the 100μF capacitor and bend it towards the negative(-) pin of the capacitor. Solder them together. After doing so, solder the black wire from the audio jack to ground. 

Step 16: Wires for the Microphone

Picture of Wires for the Microphone

This step is optional. The microphone can be soldered to the board directly. If you do so, you can skip this step; if not, read on.

Now that you are done soldering the audio jack to the circuit, set that aside and get out your microphone. Solder two wires to the pins on the microphone. Once the connections are made, use some heat shrink tubes to protect them. I also added some extra layers to take some tension off the soldered joints.

*For the microphone, it is best if you have a cable with 2 separately insulated wires wrapped inside it. Unfortunately I did not have that, so I took two wires and twisted them together. You can learn how to do that here. I used two black wires for this job and if you do so too, please remember to pay attention to the polarity of the microphone. I marked the polarity on one of the wires with a small section of a red heat shrink tube.

Step 17: Wires for the Block Connector

Picture of Wires for the Block Connector

This step is also optional.
I wanted to be able to disconnect the microphone from the circuit quickly so I added a block connector. Take two wires and solder them to the pins on the block connector. I did not add heat shrink tubes to the soldered joints, because they will later be covered in hot glue anyways. 

Step 18: Drill Holes

Picture of Drill Holes

There are some holes that are necessary to drill before the block connector can be connected to the circuit. But before drilling the holes, I fitted everything into the box to find the proper places for the holes. I found the best way to drill holes into plastic is to make a pilot hole first by melting it with a hot nail and then drill the hole to the desired size.  

Step 19: Block Connector to Circuit

Picture of Block Connector to Circuit

Now that the holes are drilled, the block connector is ready to be connected to the circuit. Thread the two wires from the block connector through the two small holes that were made on top of the box. Then, solder one of its wires to ground and one to the joint between the 0.1μF capacitor and the 100k Ω resistor. 

*The wire connected to ground will be connected to the negative side of the microphone, and the wire connected to the resistor and the capacitor will be connected to the positive side of the microphone. If you wish, make a note to yourself on which side of the connector is positive and which side is negative.

Step 20: Glue Everything In

Picture of Glue Everything In

Once everything is in their designated spot, we may glue them in. The variable resistor will fit into the bigger hole on the top, the leads from the block connector will be pushed through the two small holes, and the audio jack will fit into the big hole on the side. Once they are there, secure them with some hot glue.

Step 21: Use It

Picture of Use It

We are now finished with the construction and the device should be ready to go. 

How to use it:
If you haven't done so already, please screw the wires from the microphone into the block connector (make sure the polarity is correct), pop in a 12v battery, and plug your earphones in. Don't put on your earphones just yet! Make sure the volume is on the lowest level by turning the screw on the variable resistor. Also, if you are using a switch, turn that on now. You can now put your earphones on and slowly increase the volume to a desired loudness. This is a procedure that should be done before each use to prevent blowing out your eardrums.

Step 22: Have Fun!

Picture of Have Fun!

This device is truly amazing. I could hear a conversation in the other room, one across the street, and even hear a pin drop on the other side of the wall!  

If you have built one already I hope you have fun with it. If you haven't built one yet, I hope you have fun building one (if you are planning on doing so). 

Thanks for reading! 
Please rate and comment.


NigelJ8 (author)2017-08-21

How sensitive is this circuit? I once had an FM transmitter which could listen to people talking in the next house. I have failed many times trying to make an amplifier that is equally good. Can anyone point me in the right direction?

ruecianfate (author)2013-12-21

can i use other IC if i cant get LM386?
cause i cant find that part here, yet i found LM386n and LM386 9pins?

or what else can i use, please help.

ernie666 (author)ruecianfate2013-12-30

I'm pretty sure LM386n is the same as regular LM386.

akshaya ladwa (author)ernie6662017-03-19

I tried this circuit 2 times but all I heard was simply noise ??

T0BY (author)2016-12-08


mrwillcreates (author)2016-09-15

How would one reduce the volume? mine is WAY too loud

jbrittain2 (author)2016-08-04

also the mic cable is twisted pair, makinf it a "balanced cable". The circuit is unbalanced, so there is a mismatch. Use a shielded RCA cable instead and that will help alot. (Im experienced in audio equipment)

jbrittain2 (author)2016-08-04

The noise is not a bonus, if you're trying to hear acoustic sounds... It is interference made by close by AM radio transmitters, sometimes FM but mostly AM is easiest to pick up in microphone cables etc. To resolve this, it needs a sturdy ground plane around the circuit, metal plate underneath, or a Faraday cage (metal box) Altoids can?? instead of the plastic enclosure shown. The circuit should be grounded via a jumper and insulated from the box.

AbishekE (author)2016-06-14

can i use lm358 instead of lm386?

shanren (author)2014-12-18

hi ernie666, I made one, not very fine for the testing. I can only have big noise, what's wrong?


gussmith001 (author)shanren2015-04-22

Did you perhaps put this on a breadboard? Maybe try soldering it on a perfboard.

gussmith001 (author)2015-04-22

I just tried this circuit with the same components except used 10uf between pin 1 and 8. It works really great using earphones and have little noise.

gussmith001 (author)gussmith0012015-04-22

I also hooked the output to another LM386 and I can hear my own breathing.

BeenShockedOnceOrTwice (author)2015-03-13

Try this out, and see if it works for ya. I got a little higher gain with less noise.

Oh, and I saw that post earlier about using 18v. Only the LM386N-4 can take 18v, and even then you could cause damage to the chip. Be careful. You may not even want to take it to 12v. At least 9v, but closer to 10v would probably be good.

mr.ante.sepulveda made it! (author)2015-02-24

Did not work great with a 9V battery. Changed to a 14,4v one and almost blew my eardrums. :D But getting a 12V battery tomorrow and already built a case with an on-off button

BelchingDragon (author)2015-02-05

Very well explained.

ruecianfate (author)2013-12-23

hey dude :D
can i use other IC's if there is no lm386 available in my country??

will it be same diagram?

anubhavtalukdar007 (author)2013-03-05

Can anyone give me a schematic of a spy ear circuit with 6 volts as source???

hrhtml99 (author)2013-03-05

Can someone please help me? I have constructed this and double checked everything, but it does not work. All i hear is a clicking buzzing sound! Please help!

hrhtml99 (author)2013-03-04

Please help! It can only pick up audio if i am talking like 1 inch away from the mic, i have checked my circuitry, and i have fiddled with the volume! Please help!

anubhavtalukdar007 (author)2012-08-09

Where will i get a LM386 chip?

In India we don't have RADIOSHACK-like stores, so I want your help.
Thanx in advance for answering!


I hv a question- I got the lm386 but wat can i use instead of the 1uf tantalum and 0.22 uf capacitors? i dnt hv dem!!

ok thnx dude

HavocRC (author)2012-12-18

One question. Which side is the front of the IC? I'm assuming its on the right of the schematic. What is the pin load out? Where is 1 and where is 8? Thanks (I'm new to this :D)
Would a 9v battery work fine?

ahebbar (author)2012-12-10

Can spy ear be done with wireless earphones???

ernie666 (author)ahebbar2012-12-15

I don't see why it can't be done. You should try it

KhajagB. (author)2012-09-03

thanks :)

pavithraa (author)2012-07-17

we didn't get any output for this circuit with 9v. whether we need to change any resistor or capacitor value? advise us with value.

ernie666 (author)pavithraa2012-07-20

You might want to check your circuit. LM386 chip has a wide supply voltage range. Maybe you connected the capacitors or microphone the wrong way, or maybe a component is broken.

pavithraa (author)2012-07-17

reply us as fast as u can

Alatec (author)2012-06-16

On the schematic, what are the pin numbers?

ernie666 (author)Alatec2012-06-23

oops. I forgot about that. The bottom left pin is pin 1 and the rest of the pins are numbered like it would be on a regular 8 pin DIP chip if you were to look straight down on it from the top

DoctorDv (author)2012-05-11

You should enter this in the kit challenge. You would have a chance!

ekay3 (author)2012-05-11

hey! I just wanted to ask that how much was the distance from which you can catch the sound and don't the wall's muffle the sound? I'm making it as my final semester project so want a pronto reply. :) waiting...

nodoubtman (author)2012-05-10

I'm sorry, i tried that with hooking up a speaker, i heard nothing sorry.. :(

KhajagB. (author)2012-05-06

can i use instead of 12V...9v ! if i replace the resistors with another numbers

ernie666 (author)KhajagB.2012-05-06

yes with 9v and different value resistors, but you should probably test them first because too much input power could cause distorted output

KhajagB. (author)2012-05-06

or the capacitors!? plz reply ASAP

ernie666 (author)KhajagB.2012-05-06


Arjun123 (author)2012-04-22

guys try shielded cable for noise interference definitely u will get results...

Ram g (author)2012-04-17

its good

slamdaddy (author)2012-03-18

The problem I've always had with anything that allows you to pick up sounds at a distance is that they amplify the background noise as well.
Would it be possible to add something similar to the technology used in noise-cancelling headphones to cancel out at least the sounds generated closer to the microphone?

ernie666 (author)slamdaddy2012-03-18

Noise cancelling headphones simply block sounds around you, so no this could not be used to solve this problem. If the noise is at a certain frequency, you could add a high/low pass filter, otherwise I don't think this problem could be solved without software. But you could focus on what you want to hear by adding a parabolic reflector like blinkyblinky suggested.

slamdaddy (author)ernie6662012-03-29

I used a parabolic dish but still there was background noise, most of which came from much closer to me than the sounds I was trying to pick up. I didn't mean use noise cancelling headphones themselves to counteract this, I meant employ the technology that they use and integrate it into the circuit.
As I understand it noise cancelling headphones use a small microphone to pick up sounds just outside the headphones then play the inverse to cancel out the external noise with destructive interference. Would it not be possible to integrate this somehow in order to filter out the noise coming from closer to you so you're just hearing the sounds from further away?
Other than that, I think you're right and having various filters to tune out the frequencies you don't want would be the way to go.

ernie666 (author)slamdaddy2012-04-01

That sounds like a brilliant idea but I can't think of how to do that just yet. I'll try to figure out how to do this, and if I ever do, I'll write about it.

slamdaddy (author)ernie6662012-04-08

Thanks, that'd be really awesome. I wish I could help but electronics is still on my to-do list.

Pranjal Joshi (author)2012-03-29

can i connect signal output to 2 pins of earphone to turn on speakers in both ears?? reply fast

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