Created: Sep 17, 2014
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The numeric side keypad is usually used to input a series of numbers, or working with applications like calculator, spreadsheet and accounting programs. Most laptop manufacturers do not provide a numeric keypad area on their keyboard due to space optimization. The USB keypad project is an easy plug-and-play USB keypad that can be plugged into a computer or a USB host to make the numeric keypad available. The USB keypad is designed using the PIC16F1459 device, which is part of Microchip's 8-bit USB-capable family, PIC16F14XX.
Initially, when the USB keypad is plugged into a USB host, the LED backlight is turned ON and the USB keypad is ready for operation. Touch any button on the keypad to send the corresponding character to the host. The buzzer beeps for every recognized key touch. The beep frequency is set to 1kHz when Numlock is ON and 2.5kHz when Numlock is OFF. Numlock ON and OFF operations produce a slightly longer beep. To disable the buzzer beep, press and hold the clear button for two seconds or until a longer beep is heard from the buzzer. Touch and hold the clear button again to turn on the buzzer feedback.
The Capacitive Voltage Divider (CVD) technique is used for implementing 18 touch buttons and the proximity sensor. Nine CVD sensors are arranged in a 5 x 4 matrix that allows a maximum of 20 touch buttons. However, the USB keypad implements 18 touch buttons. Each touch button includes a pair of row and column. A distance of 1.5 mm separates the rows and columns in a touch button. A guard ring to shield the buttons from parasitic capacitance, thus increasing the sensitivity of the button, surrounds each touch button.
Backlighting of the USB keypad is done using 12 side-firing LEDs soldered on the top layer of the PCB. Six LEDs each are soldered on both the left and right sides of the PCB. The LEDs are arranged on the PCB in away to ensure proper lighting of the touch buttons. An external drive buzzer is provided to generate an audio tone whenever a key is pressed. The audio feedback helps the user to realize that a button is pressed. The buzzer is connected to the PIC16F1459 through the PWM2 peripheral. The PWM2 peripheral generates rectangular waves to drive the buzzer. Varying the PWM period changes the tone frequency. The buzzer volume can be adjusted by varying the PWM duty cycle.