Ultra-Sensitive Spy Ear

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

Materials:
- 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.

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

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

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.

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. 

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.

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

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.

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.

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.

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.

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"

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)

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.

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.

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. 

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.

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. 

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.  

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.

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.

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.

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! 
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