Microphone, Spy Microphone

When I was a young boy, often intrigued and I always found it amusing to catch glimpses of some girls looking my way, whispering and giggling. Also, when I knew that my parents discussed matters concerning me, I couldn't help but wonder if it was about my Christmas present or perhaps my mischievous behavior. Then one day l made promise to myself – that once I acquired enough knowledge and skills, I would create a device allowing me to eavesdrop on those delightful conversations of those cute girls, what is so funny about me.

• PAM8403 Amplifier module (See circuit or order online)
• 100uF Capacitor
• 10kOhm Resistor
• Electret Condeser Microphone
• On/Off Switch
• Power Supply (Battery pack Up to 5V max.)
• 3.5mm Audio Jack (Female)
• Headphones (3.5mm)
• Spherical shape Plastic Bowl (For reflector)
• Screw, Bolt, Washer, Magnets, Box.. (Optional for casing)

The working principle of this project is based on the natural phenomenon of wave reflection. Sound waves behave similarly to light waves. Perhaps you remember from school learning about the reflection of sound and the rules associated with common shapes. In this case, we will use a spherical surface because it is best suited for this project. We consider sound waves traveling from a distance as parallel lines. Just like light waves, when sound waves hit a reflective surface, they reflect at a 90-degree angle.

A spherical surface, from the inside, possesses the property that every straight line makes a 90-degree angle with the line that intersects the hitting point, forming what is commonly known as the 'focus' or 'focal point.' For us, this means that every audio wave traveling from a distance and hitting our reflector will bounce toward our focal point. We will place a microphone at this focal point so that the device can capture more waves due to the wider surface area. The waves will bounce off the reflector, concentrating them into the microphone.

An important component of this project is the electronic circuit, which is simple. The electronics in this project serve the purpose of amplifying the audio signal from the microphone. I opted for a straightforward design that you can either construct using the electronic circuit provided above or easily order online (as it is more cost-effective to order). While I experimented with various designs and preamplifier circuits, I decided not to complicate the electronics because the sole purpose of this project is to harness wave behavior. Additionally, nowadays, there are inexpensive and tested modules available, and I don't see a challenge in building a simple amplifier.

Whether you decide to build the amplifier module following the schematic or purchase it online, you'll need to make some modifications to adapt it for an electret microphone input.

• For this purpose, connect an electrolytic capacitor of 100 micro farads and a resistor 10 kilo ohms as indicated in the schematic diagram above. Pay careful attention to the capacitor's polarity, where the negative pin is typically marked with a bold strip. Solder the negative pin to one of the inputs of the amplifier (in my case, the left -L). The positive pin should be connected in series with the resistor, which is then soldered to the positive power supply input on the module.
• Regarding the resistor, note that the order of the color rings should be as follows: brown, black, orange, gold.

• Next, solder the microphone with a 20cm-30cm cable (I highly recommend slim shielded cable), paying attention to the polarity of the electret microphone. The negative output of the microphone should be soldered to the ground input on the amplifier module, while the positive is connected to the junction point between the capacitor and resistor (see the diagram above).
• After that, solder a 3.5mm audio input jack (female) to the chosen input on the amplifier module (in my case, the left -L).
• Then, connect two batteries (3.7V) in parallel (keeping the voltage the same but extending battery life). Connect the positive wire to the positive power input of the amplifier, while the negative is soldered in series through the On/Off switch to the negative power input of the amplifier module. Remember, the voltage cannot exceed 5 volts.

In this step, I challenge you to unleash your creativity. There are only a few rules to consider; everything else is left to your imagination. The key rule for this step is the positioning of the microphone, which should be at the focal point of the reflector, in this case, a plastic bowl. As seen in the pictures above, the focal point of spherical surfaces is always halfway along the radius defined by the reflective surface.

• In my case, I selected a plastic bowl, ensuring its spherical shape. After measuring its diameter (16cm), I determined the radius to be 8cm (half of the diameter), placing the focal point at 4cm from the bottom (half of the radius). I affixed a screw with a bolt longer than 4cm, providing the option for fine-tuning the distance if necessary. In this instructable i also included printable 3D design files for tinkerers, similar than bowl i used.
• For a protective casing to shield the fragile electronics, I repurposed a small broken plastic box. I cut it into the desired shape, bent it to fit my needs using the heat from a soldering iron, and crafted openings for the main switch and headphone jack.
• Additionally, I repurposed a flexible plastic mount (holder) from a broken device, attached a magnet to one end, and glued it on top of the casing.
• On the outside of the reflector (plastic bowl), I affixed a washer.

• Next, I carefully place the electronics inside the plastic box, securing them in place with hot glue. Afterward, I fasten the box together with the battery pack, forming a convenient handle. Additionally, I attach a small magnet to the electret microphone, making the entire project modular and easy to calibrate.
• The final steps involve attaching the reflector by securing the washer onto the bowl and the magnet on the holder. I fine-tune the distance using the bolt on the screw, attach the microphone with a small magnet to the bolt, connect the headphones, and proceed to test the device.

This modular design not only facilitates assembly and calibration but also enhances the overall versatility of the project. Choosing flexible joint holder give this project dual working mode, as a hand holding device and as a stand for listening on fixed direction. Feel free to make any adjustments based on your specific implementation and preferences.

Now, you can plug in your headphones, tune into focus, attach the microphone, and direct it toward something in the distance. I enjoyed listening to the birds in the nearby trees.

Disclaimer:

The information provided in this publication is intended for educational and entertainment purposes only. The author does not endorse or encourage any illegal or unethical activities, including the use of the described techniques or devices for unauthorized surveillance or any other unlawful purposes. Readers are solely responsible for their actions and should ensure compliance with all applicable laws and regulations in their jurisdiction. The author and publisher disclaim any liability for the misuse of the information presented herein.

In addition I also added 3D files ready to print in *.STL format which you can download and tinker further more in Tinkercad.com or Fusion360.