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
- 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
Again, I recommend you bread boarding this before building it.
Step 3: Laying Out the Circuit
*The variable resistor and the microphone are not shown on the board because they will be connected off-board.
Step 4: Building the Circuit
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
Step 6: Building the Circuit
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
Step 8: Building the Circuit
Step 9: Building the Circuit
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
There are still some more connections make across the board, but we'll do that later.
Step 11: Wires for the Variable Resistor
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
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
Step 14: Wires for the Audio Jack
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
Step 16: Wires for the Microphone
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
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
Step 19: Block Connector to Circuit
*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
Step 21: Use It
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!
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.