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Are you still worried that your keyboard is "afraid 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, detecting even the slightest pressure on a 0.4mm-thick stainless steel plate. It’s not only waterproof but also highly resistant to vandalism. In this article, we’ll walk you through the design and functionality of this unique keyboard.
At the heart of the system is a piezoelectric disc, commonly used as a buzzer. In this example, we’re using Murata’s 7BB-35-3 model, which has an outer diameter of 35mm and a sensing area of about 20mm. The PCB features holes that allow the ceramic disc to move freely. To secure the disc in place, a 3mm thick self-adhesive foam rubber is used, and the assembly is clamped firmly against the front panel with just the right amount of pressure.
The PCB layout and openings are carefully designed to ensure optimal performance. When you press the outer panel, the steel (or other material) slightly deforms, transferring the pressure through the rubber to the piezoelectric disc. This small pressure generates a detectable voltage, which the microcontroller can sense. At that point, the microcontroller activates the piezoelectric discs as buzzers, producing a beep as a response.
This design includes four buttons, and the microcontroller used is the uPD78F0513 from Renesas Technology. Of course, other microcontrollers can also be used depending on the application.
The smaller electrode of the piezoelectric disc is connected to the ADC input of the microcontroller, while the larger electrodes are tied together and connected to several parallel port bits. These ports are initially set to a low state. When the disc is pressed, the voltage at the ADC input drops, triggering the microcontroller to respond.
The schematic of the piezoelectric keyboard and buzzer is shown below. At startup, P3 is set to low, and P7 is high. The piezoelectric disc is quickly charged, and after a few microseconds, P7 is switched to a high-impedance input. The program then continuously scans the ADC input. Because the piezoelectric disc has a large capacitance, the voltage changes slowly, so a scan every 1ms is sufficient. Each disc is checked every 4ms.
When the voltage drops below a set threshold due to button activation, the controller processes the signal and triggers the buzzer. With a 0.4mm steel plate, a threshold of 1.5V below the 5V supply is enough for good sensitivity. If a thicker plate is used, there's still room to adjust the sensitivity.
To produce a beep, P7 is set to a low-impedance output, and P3 is set to the opposite polarity, generating a square wave at the piezoelectric disc’s resonant frequency (in this case, 2800 Hz). The beep lasts for 250ms, and the current is limited by R5. After the beep, P3 returns to 0, and P7 briefly goes high again to recharge the disc.
September 28, 2025