Build a DIY Spy Microphone: The Acoustic Telescope
By Robin Mitchell
Like spies in the movies and real life hearing a conversation from a distance can be the difference between war and peace. Of course, your intentions may be more honest such as wanting to hear an animal from a faraway distance. In this DIY Hacking project, I will show you how to build an Acoustic Telescope!
- 5.6K Resistor (R4)
- 10K Resistor (R1, R2, R3)
- 47K Resistor (R5)
- 100K Potentiometer
- 1nF Capacitor (C8)
- 10nF Capacitor (C9)
- 100nF Capacitor (C1, C2, C3, C5)
- 100uF Capacitor (C4, C6, C7)
- 1N5817 Diode (D1, D2)
- LM358 Op Amp (U1)
- NE555N Bipolar Timer (IC2)
- Electret Microphone (MK1)
You can see the full-sized schematic here.
How Does it Work?
To detect sound from a large distance a two prong attack is used. The circuit starts with an electret microphone (MK1) which uses a 10K pull-up resistor to produce the DC + AC signal. However, in the audio world, DC signals are not good at all, so this must be removed! This is the job of C3 which couples the microphone with the amp stage (removes the DC offset). The op-amp U1A is configured as a non-inverting amplifier with a gain dependent on the value of RV1.
With the signal amplified, this is then fed into a buffer (U1B) which is then coupled by a 100uF capacitor to a speaker or headphone set. One problem exists with the sound signal, it’s AC by nature which means that there are negative portions of the input waveform. Therefore, our op-amp circuit needs a negative voltage and thus a negative voltage generator is used. This negative voltage is generated by the 555 IC (IC1), which is configured as an astable multi-vibrator. As the 555 output swings from power to ground (and vice versa), the diode and capacitor arrangement (C6, D2, D1, and C7) creates a negative voltage equal to the supply (for example, a 5V supply would produce -5V).
The sound amplification cannot just come from a gain circuit because increasing the microphone signal also amplifies noise. Therefore, to make the microphone more sensitive to distant sounds, the microphone is built into a parabolic reflector. The parabolic reflector focuses sounds waves (in a similar fashion to a radio dish) onto the electret microphone which helps the microphone to pick up weaker signals. Therefore, distant signals are more easily received with the help of the dish. To improve the sensitivity, a larger dish can be used but the microphone must be at the focus point.
Constructing Your Acoustic Telescope
The circuit itself can be made using just about any construction technique. The circuit demonstrated here is made on a custom PCB which was fabricated on a CNC machine (you can find the CNC files in the provided project’s zip file). The project is put into an open case mount to demonstrate the wiring and setup of the circuit and the potentiometer is put on the front panel for ease of use.
The completed circuit and dish.
Setup from the side.
Close-up of the main circuit.
The main circuit.
The parabolic reflector itself is made using a paper cut out which is then glued into a specific angle. One piece of plywood is taken from the perimeter of the parabolic dish to the focal point where the microphone is mounted. The parabolic dish itself is mounted onto a simple stand that allows it to be placed down. However, the dish could be controlled by a pair of stepper motors to give an electronically controlled dish which would allow for precise movement.
You can download the project files for the acoustic telescope here: DIY Spy Mic Project Files.