Are you still worried that your keyboard is "fear of water"? The piezoelectric disc keyboard is designed to solve this problem once and for all. This innovative keyboard uses a piezoelectric disc as both a sensor and a buzzer, capable of detecting even the slightest pressure on a 0.4mm thick stainless steel plate. It's not only waterproof but also highly resistant to vandalism, making it ideal for harsh environments. In this article, we’ll walk you through the design and functionality of this unique piezoelectric wafer keyboard.
At the heart of the system is the 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 approximately 20mm. The PCB includes electronic components and carefully placed holes to allow the ceramic disc to move freely. To secure the disc in place, a 3mm thick self-adhesive foam rubber is used, ensuring the wafer remains fixed without restricting its movement. The entire assembly is then clamped onto the back of the front panel with just the right amount of pressure.
The PCB layout and opening are crucial for the performance of the keyboard. When you press the outer panel, the thin steel (or alternative material) bends slightly, transmitting a small amount of pressure through the rubber to the piezoelectric disc. This pressure generates a detectable voltage, which the microcontroller can interpret. Once triggered, the microcontroller activates the piezoelectric disc as a buzzer, producing a beep to confirm the input.
In this example, four buttons are implemented using the uPD78F0513 microcontroller from Renesas Technology. While this specific chip is used here, other microcontrollers can also be adapted depending on the application. The smaller electrode of the piezoelectric wafer is connected to the ADC input of the microcontroller and biased with a large resistor toward the positive supply. The larger electrodes are tied together and connected to multiple parallel port pins to create a low-impedance path. These ports start in a low state, and when the disc is pressed, the voltage at the ADC input drops, signaling a key press.
The schematic of the piezoelectric keyboard and buzzer is shown below. At startup, P3 is set to low, and P7 is high, allowing the piezoelectric disc to charge quickly. With a capacitance of around 30nF, the charging occurs within microseconds. Then, P7 is switched to a high-impedance input to begin the detection process.
The program continuously monitors the ADC input. Because the piezoelectric disc has a large capacitance, the voltage changes slowly, so there's no need for fast sampling. In this specific application, the microcontroller checks the voltage every 1ms, scanning each disc every 4ms. When the voltage drops below a preset threshold due to a button press, the controller processes the input and triggers the buzzer.
To produce the sound, P7 is set to a low-impedance output while P3 is set to the opposite polarity, generating a square wave at the piezoelectric disc’s resonant frequency—2800Hz in this case. The buzzer lasts for 250ms, and the current is limited by R5. After the beep, P3 returns to a low state, and P7 briefly goes high again to recharge the disc, readying it for the next press.
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Shenzhen Sichuangge Magneto-electric Co. , Ltd , https://www.scginductor.com