Are you still worried that your keyboard is "fear of water"? The piezoelectric disc keyboard is here to solve all your concerns. This innovative design uses a piezoelectric disc as both a sensor and a buzzer, capable of detecting even the slightest pressure on a thin stainless steel plate—just 0.4mm thick. Not only is it waterproof, but it’s also highly resistant to vandalism. In this article, we’ll walk you through the design and working principle of this unique piezoelectric wafer keyboard. At the heart of the system is a piezoelectric disc, commonly used as a buzzer. For this project, we’ve selected Murata’s 7BB-35-3 model, which has an outer diameter of 35mm and a sensing area of about 20mm. The PCB includes electronic components and has round holes to allow the ceramic material to move freely. A 3mm thick self-adhesive foam rubber is used to secure the disc to the PCB, and the entire assembly is clamped to the back of the front panel with appropriate pressure. Below is the PCB layout and opening for reference:
Design of piezoelectric disc type waterproof and tamper-proof keyboard
When you press the outer panel, the steel (or other material) will flex slightly, transferring the pressure through the rubber to the piezoelectric disc. This small amount of pressure is enough to generate a detectable voltage, which the microprocessor can then recognize. At that point, the processor activates the piezoelectric disc as a buzzer, producing a beep sound. This example uses four buttons, and the microcontroller employed is the uPD78F0513 from Renesas Technology (formerly NEC). However, any compatible microcontroller can be used instead. The smaller electrode of the piezoelectric wafer is connected to the ADC input of the microcontroller, while also being pulled up via a large resistor. It is also connected to other port inputs (P7), which are initially in a high-impedance state. The larger electrodes of the piezoelectric disc are connected together to several parallel port bits (P3), ensuring a low impedance value. If the ports don’t have sufficient output current, this step is necessary. These ports are initially set to low. When you press the wafer slightly, the voltage supplied to the ADC input decreases. Below is the schematic of the piezoelectric keyboard/buzzer:
Design of piezoelectric disc type waterproof and tamper-proof keyboard
At startup, P3 is set to low, and P7 is high. The piezoelectric disc is quickly charged, with a capacitance of approximately 30nF. After a few microseconds, P7 is switched to a high-impedance input. The program continuously scans the ADC input. Because the piezoelectric disc has a large capacitance, the voltage changes slowly, so fast scanning isn't required. In this application, a measurement is taken every 1ms, meaning each disc is checked every 4ms. When the input voltage drops below a preset threshold due to button depression, the controller processes the signal and activates all the piezoelectric discs as buzzers. With a 0.4mm thick steel plate, a threshold of 1.5V below the 5V supply is sufficient for good sensitivity. If a thicker panel is used, there's still room to increase sensitivity further. To produce a beep, P7 is set to a low-impedance output, and P3 is set to the opposite polarity, creating a square wave at the piezoelectric disc’s resonant frequency (2800 Hz in this case). The buzzer lasts for 250ms, with the drive current limited by R5. After the beep, P3 returns to 0, and P7 is briefly set to 5V again to recharge the disc.

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September 28, 2025