Hardware
Features (firmware v.1.0):- 50 x 7 LED:s
- Computer link (serial port)
- Scrolls either static text (without computer) or dynamic text (from a computer)
- Automatic text source selection
- Maximum static text length 768 characters
- Programmable font, 256 different characters
- Character width 0-5 pixels (+optional blank column between the characters)
Note: Once you have constructed the device, you must write a font in its memory for the display to show anything meaningful. Both static and dynamic text modes use the internal font to transform the character codes to visible characters.
Files:
- Schematics, PCB and component layout:
- v.1.0.1
Note: The control electronics is compatible with the display of an older similar project by Mikko Kursula.
Thanks to him for the display PCB (for 3mm leds).
- v.1.0.2 (source code
included)
Note: The firmware code is built with WinAVR.
Images:
- Display
- Electronics
Note: The electronics in the image has been built with an earlier version of the PCB, but the changes to the current version are only slight. - Display wiring
Software
Features (v.1.0.1): (Here are listed the changes from the previous version)- Integrated text editor
- Integrated font editor
- Dynamic text (shown in the display without altering the programmed text) can be played
- from the text editor
- directly from a file
- from the Internet (HTTP)
- Hides to the Windows system tray when minimized
Files:
- LedDisp for Windows binary:
- LedDisp v.1.0.1
Note: The program is built and tested in Windows XP but should work in earlier 32-bit Windows versions, too.
- LedDisp v.1.0.1 project for Borland C++ Builder 6
Note: The source code is still partially uncommented. Some of the code was written in a hurry and needs rewriting anyway.
Images:
Known bugs:
- The program may prevent Windows from shutting down. I have no
idea why this happens, I'm not really a Windows coder. Any suggestions are
welcome.
Hardware operation
ElectronicsThe display has seven rows containing 50 LEDs each. The display controller consists of seven transistors and seven 8-bit shift registers which all are controlled by the MCU (AT90S2313). Each transistor drives one row selecting whether it’s enabled or not. The shift registers are chained together, so their serial input is converted to parallel output of 56 bits, 50 of which is used to drive the columns. The last six bits remain unused.
The MCU updates the screen a single row at a time. While displaying one row, it feeds the contents of the next row to the shift registers. After that it disables the current row, enables the next one and commands the shift registers to show their data on the outputs. This cycle is performed hundreds of times in a second forming the whole image on the display.
The device has an EEPROM memory of 2 kilobytes (24c16) that is used to store the static text and the font. The contents of the memory is read and written by the MCU that either follows the commands received via the computer interface or works independently reading the static text and showing it on the display over and over again.
Computer interface
The computer serial interface uses the normal UART (with 9600bps, 8 bit, 1 stop bit, no parity) to communicate with the computer software. The device keeps sending the state of its receive buffer to the computer: each byte sent tells how many bytes of free space there is in the buffer. If the device receives a byte from the computer, it checks whether it is a command byte. If not, the byte is considered to be a text character and the corresponding character image in the EEPROM will be shown on the display. If a command is recognized, the MCU will act as asked. In a case of serial buffer overflow, the extra bytes are ignored.
Updated: [15.11.2005]
Tuomas Tuononen
t u o m a s . t u o n o n e n @ g m a i l . c o m