Tobi’s Corner

What can you do with 100 common anode rgb leds? Build a 8×8 matrix and interface it!

The circuit uses 74HC595 shift registers with output latch. Each shiftregister interfaces one color + one shiftreg for the matrix rows. The interfaces uses 6 I/O lines for faster speed and easier interfacing. An AVR running at 8MHz can update the matrix with 8 brightness levels displaying nice fading colors patterns or colored text (or both). The sample program creates fading color patterns using a sine table. For smoother display a double buffer was implemented.

Download: rgbmatrix.c

For my 7-segment display AVR clock i was searching for a way to keep the clock accurate. One option would be using a DCF77 standard time receiver. The module I had seemed to be broken so I decided to use an ethernet interface and NTP. For this I purchased a small ENC28J60 + AVR board from Simon Küppers. This small board is only about 38×30mm in size and includes an ATMega168, ENC28J60, Ethernet-Jack + Magnetics and a voltage regulator. The 14 available I/O Pins are connected to a standard 0.1″ header, so there are possibilities to extend the board.

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I’ve bought some 14cm high red 7-segment LED Displays on EBay and started to build a clock based on AVR ATTiny2313.

The circuit is build with an AVR ATTiny2313 a UDN2982 high side and a ULN2803 low side driver/switch

The LED Module is powered with 18-20V to give a brighter display. The forward volatge of a single segment is about 13V. 13V + voltage loss at the switches (worst case 2V + 1.6V) is about 16.6V. I used 39Ohm resistors for the segments and so 16.6V + 39Ohm x 0.08A = 19.72V. Because of multiplexing 4 modules to get an avarage current of 20mA per segment the current has to be four times higher ( 20mA x 4 = 80mA ). The dots on the modules have a lower forward voltage resulting in a higher limiting resistor: (16.6V - 3.6V - 3.6V) / 80mA = 120Ohms.

Here you can see my IKEA Storm Lamp Mod using the powerRGB circuit and the tinyRGB firmware.

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powerRGB is the extension of tinyRGB to drive high power LED’s. For this project I used a 3 x 1W common Anode RGB LED.

Q1 to Q3 are N-Channel HEXFet Mosfet’s with logic level drive and a RDSon at about 50mOhms. R1 to R3 are at about 2k2, R4 to R6 at about 15k and R7 to R9 depend on the LED used and VCC. If you use FET’s with higher RDSon you have to consider RDS in your calculation!

Rx = (Vcc-Vf)/Im - RDSon

[Vcc: Volatge of power supply, Vf: Vorward Voltage of the Diode, Im: Max current of LED]

The software for the AVR is the same as for tinyRGB.