This is an easy to build 7x7 led matrix. You can use it as a casebadge, for example. The device comes to it rights the most when showing animations. See 7x7led.avi for a short movie showing a rotating torus. As you can see the schematic is quite simple. An AVR, a few transistors, com resistors and some logic to convert RS232 to TTL.

Features
7x7 led matrix
127 grades of brightness per led
Interface to PC: RS232
Microcontroller: ATTiny2313.
License: GPL

Construction
Let's start with the led-array. Best is to build it row by row, just keep in mind that you solder the led the right way around: anode to anode, kathode to kathode.
Then connect the transistors, mind the pinout! Emittors can all be connected together, they all go to ground. The bases of all transistors go to the uC, then also connect the columns.
Next, solder the crystal and it's condensators. Final thing to do is soldering the RS232 link. Alternatively you can use a MAX232 IC but the cost are higher and you only need one-way communication.
Some pictures of construction can be found inParts:
49 x led (preferably the all same colour)
8 x transistors (for example BC550 of BC547)
1 x IC-holder, 20DIP
1 x ATTiny2313
1 x kristal, 20MHz
2 x condensators, 15pF
8 x resistors, 1KOhm
1 x diode
1 x female DB9 connector
Misc: wire, connectors, mounting plate, etc.. Please keep in mind that the IC-holder in those pictures is soldered the wrong way arround (mirrored)!

Communication & software
Protocol is simple: RS232, 19200 baud, 8 bit, no parity, 1 stopbit.
First send a synchronisation byte (value: 129). Then send 49 bytes, values of these bytes represent the brightness of a pixel (led). That's it

The author has written 2 programs to control the ledmatrix. The Windows program can't do much, just something tickertape-like. The Linux progam can show a sequence of PNG-images. Both programs are intended to show how to interface to the 7x7 led display.

Downloads
- AVR firmware source and hex, Linux program source and Windows program source (VB6): software.zip
- Pictures of construction: pics.zip
- Short movie showing a rotating torus: 7x7led.avi

Copyright Sprite_tm, source: http://gathering.tweakers.net

49 x led (preferably the all same colour)
8 x transistors (for example BC550 of BC547)
1 x IC-holder, 20DIP
1 x ATTiny2313
1 x kristal, 20MHz
2 x condensators, 15pF
8 x resistors, 1KOhm
1 x diode
1 x female DB9 connector
Misc: wire, connectors, mounting plate, etc.