• USB Display with different Brightness per Digit

    USB Display with different Brightness per Digit

    Last month I finally came around to order a lot of circuit boards that piled up over the last years. One of those boards was made to use some parts I just bought because they were on sale (the 16-segment LED digits) and a sample part I ordered just because I though it would be neat to have (the MAX6955).

    The MAX6955 from Maxim Integrated is an LED display driver for a combination of 7, 14 or 16-segment digits with up to 128 LEDs. The chip provides a font,  global or per-digit brightness control and two-speed blinking between two text buffers. Additionally, 32 switches in a matrix configuration can be scanned and debounced. To talk to the chip, the I²C-protocol is used.

    USB Display Software

    USB Display Software

    To control the display driver, an ATmega168 with the V-USB software USB stack is used. It is supplied with 3.3V and clocked at 12MHz (which is not possible, according to the datasheet, but works fine). Only four buttons were attached to the MAX6955 and placed on the backside of the PCB.

    At the moment, the PC-side software (which uses libUSB-win32) can send text and commands to the AVR which just passes them on to the MAX6955. In the future, I want to add a text memory on the AVR  so it can display and scroll messages independently from the PC. Maybe I will add some functions to dynamically vary the brightness of the digits to produce some cool effects.

    USB Display Backside

    USB Display PCB Backside

    I wanted the circuit board to be as small as possible, meaning it has the size of the eight 16-segment digits. All components were fitted between the pins of the display digits. To program the AVR, a card-edge ISP connector was used. To protect the circuit and to improve the contrast of the red LEDs a case was lasercut from red plexiglas.

    USB Display

    USB Display with Text

    You can download the circuit board layout (Eagle) here, the AVR firmware (C) here, and the software (Delphi) here.

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