This is an easy to build 7×7 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.
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
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