In contrast to conventional microphones, a contact microphone offers a unique perspective on the world around you. At the core of the contact microphone featured here lies a ceramic piezo-disc. This disc can be affixed to a surface using double-sided adhesive tape, allowing it to capture structure-borne sounds effectively.
The Gold Coin!
Ceramic piezo discs are cost-effective, serving as sonic magnifying glasses that enable us to create straightforward contact microphones for uncovering hidden sounds. In fact, piezo elements typically come in two material varieties: ceramic and polymer. Ceramic piezos are characterized by greater self-resonance, efficiency, and affordability.
In scenarios where a flat frequency response isn’t a crucial requirement, opting for a ceramic piezo disc is a wise decision. This choice is advantageous as it can generate a more substantial signal and offers improved shielding thanks to its larger disc on one side. Moreover, a ceramic piezo disc has the capability to adapt to irregular surfaces with ease.
Processing Circuitry?
In fact, the piezo disc can produce a substantial signal output and usually requires minimal amplification. However, it’s crucial to provide buffering, which can be achieved through various methods. Due to the piezo disc’s exceptionally high impedance (typically around 1 MΩ), buffering is necessary to prevent potential impedance mismatch with an existing audio system. Failure to match impedance can result in various nuisances like humming, buzzing, and unwanted noise, especially with low-level signals.
Creating the necessary electronics for this task is relatively straightforward using a field-effect transistor (FET). Nevertheless, I’ve chosen a more efficient approach. Let me provide a brief overview of the schematic:
The incoming signals via the “PIEZO_IN” connector are managed by the first op-amp, IC1A, which is part of the IC-TL072P chip. Selecting the appropriate resistor value for R1 largely depends on your specific application. For instance, if you intend to capture sub-audio signals, a value of 10M is suitable. However, if your interest lies in other domains such as audio or voice, you can start experimenting with resistor values ranging from 10K to 100K. The configuration of resistor R1 and potentiometer VR1 is entirely at your discretion, as long as R1 is a metal-film resistor (MFR) and VR1 is a logarithmic (LOG) type. The second op-amp, IC1B, is equipped with a minor filter and gain adjustment, which can be quite handy for minor signal processing by the end user. Lastly, the AC coupling capacitor C5 and the audio output “AMP_OUT” can be directly extended to external devices.
To power your contact microphone, you can utilize a single 9-V alkaline battery or an external DC input of up to 15 V. Connect the battery’s [+] terminal to DC-1 and the [–] terminal to DC-3 connection points. Please note that V+ and V– denote the positive and negative power rails of the circuit, while GND/0V represents the virtual ground employed to simulate a dual-rail supply. It is important to emphasize that GND/0V does not connect to the negative terminal of the battery but rather connects to the midpoint of the voltage divider formed by R5 and R6.
Construction Hints
Consider employing a short length of high-quality two-core shielded cable when connecting the piezo disc to its amplifier. It’s essential to treat the outer metal rim, often resembling a gold coin, of the piezo disc as the ground (-ve) connection. This practice minimizes the impact of undesirable hum and other electrical noises, resulting in a cleaner output signal. Furthermore, enclosing the electronics within a grounded metal casing significantly reduces the presence of the 50-Hz hum.
For securing the piezo disc to a surface, you have the option of using double-sided adhesive tape or certain adhesives. It’s worth noting that I’ve observed a drop of “superglue” provides a sturdy connection and yields better results compared to double-sided tape.
Overall, this project is not overly complex. Enjoy your construction and experimentation!
Tailpiece: Piezo Notes
The term “Piezo” is derived from the Greek word meaning “to squeeze,” and this principle is at the core of how piezos operate. Piezo elements exhibit remarkable sensitivity, generating substantial voltages with even minor movements. The schematic symbol and equivalent circuit for a piezo are depicted below. The capacitance formed by the parallel plates is primarily what can make piezos a bit challenging to employ effectively.
This source capacitance can exhibit a range spanning from 500 pF to 20 nF, forming a high-pass filter in conjunction with the input impedance of the connected amplifier. To achieve a response down to 20 Hz, it becomes necessary to maintain an input impedance within the range of 15M to 400K ohms. However, the greater challenge arises from the susceptibility of the circuit to pick up the 50-Hz hum and other disturbances due to this high impedance. Therefore, effective shielding becomes crucial in mitigating such issues.
If you are interested in learning more about piezos, look into this Piezo Film Guide: http://www.openmusiclabs.com/wp/wp-content/uploads/2011/11/piezo.pdf
