Dual Colour LED Matrix Shield (Educational)

Introduction 
This blog shows the hardware used for an Arduino-compatible shield featuring a dual-colour matrix LED.

LED Matrix Shield Mounted on Arduino Uno

Purpose of the LED Matrix
The design was created for educational purposes and to train school-aged programmers. For hobbyists looking for an off-the-shelf solution using LEDs, some related products featuring RGB LEDs are the NeoPixel or for an LED matrix, the D1 Mini from Wemos.

Technology Used
Three shift registers, part 74HC595, were used to drive the 8 rows and 10 columns (matrix 5 red and 5 green LEDs) of the LED matrix from the Arduino Uno. To maintain the design simplicity, no LED buffers were included which would increase the drive current. The 74HC595 from Nexperia fitted to the prototype could provide 25 mA per pin although the maximum current of the shift register is limited to approximately 70 mA.

A buck converter, TI part LMR51606YDBVR was chosen to provide 5V power to the shift register and LED matrix. A higher current DC-DC converter and LED drivers could be utilised for a brighter LED matrix.

Kingbright Dual Colour LED Matrix

For the LED matrix, Kingbright part TBC24-11EGWA was used. At the time of writing the LED Matrix from Kingbright part had become End of Life.

Connections to Arduino
Details are provided on the Arduino website for connecting shift registers such as the 74HC595 to the Uno board. One such example with connections and code is shown in the tutorials.

The shift register connections to the Arduino are the same as the tutorial with the addition of the output enable signal.

To adjust the LED matrix intensity and depending on the characteristics of LEDs, the value of the resistor packs could be changed. During testing the values 68 R and 56 R for red and green LEDs respectively achieved a good balance between current consumption and light output.

LED Matrix Shift Register Connections

Shown in the image below are the pins used on the Arduino Uno for the shift registers and power.

LED Matrix Shift Register to Arduino Connections

 

Lastly, a DC-DC converter was used instead of the linear regulator on the Arduino Uno. This meant less heat dissipation on the Uno board.

LED Matrix Power Supply

Printed Circuit Board (PCB)
The PCB for the LED matrix shield was designed with four layers however the board layer count could be reduced. Shown below are the external power and internal signal layers. The top layer was a solid copper pour for the 0 V layer and therefore not shown.

LED Matrix Bottom Layer PCB

LED Matrix Mid Layer PCB

LED Matrix Mid Layer PCB

Populated LED Matrix
The completed prototype board used pin headers for the LED matrix although the PCB was designed to allow flush mounting of the LED matrix.

Populated LED Matrix Shift Register Side

Populated LED Matrix (Display) Side

When connecting the LED matrix shield to an Uno or similar board, a space between the two boards of at least 13 mm was required. This spacing ensured the LED matrix pins did not cause a short against the USB B connector housing.

Arduino Uno and LED Matrix Fitted Together

LED Matrix Operation & Code
For the example below the Arduino pin mapping shown below was used.

void setup() 
{
  pinMode(latchPin, OUTPUT);
  pinMode(clockPin, OUTPUT);
  pinMode(dataPin, OUTPUT);
  pinMode(oePin, OUTPUT);
  digitalWrite(oePin, LOW);
}

Some videos of the operation with example code are shown below.

Counting with Rows

  for (int numberToRows = 0; numberToRows < 255; numberToRows++) 
  {
    digitalWrite(latchPin, LOW);
    shiftOut(dataPin, clockPin, MSBFIRST, 0);           // red = 0, all off 31 show green
    shiftOut(dataPin, clockPin, MSBFIRST, 0);           // green = 0, all off 31 show red
    shiftOut(dataPin, clockPin, MSBFIRST, numberToRows);   // turn on all columns
    digitalWrite(latchPin, HIGH);
    delay(100);
  }

Counting with Columns

for (int numberToColumns = 0; numberToColumns < 32; numberToColumns++) 
  {
    digitalWrite(latchPin, LOW);                                 
    shiftOut(dataPin, clockPin, MSBFIRST, numberToColumns);   // red = 0, all off 31 show green
    shiftOut(dataPin, clockPin, MSBFIRST, numberToColumns);   // green = 0, all off 31 show red
    shiftOut(dataPin, clockPin, MSBFIRST, 255);                        // turn on all rows               
    digitalWrite(latchPin, HIGH);    
    delay(100);
  }

Dual Colour Change

 

  digitalWrite(latchPin, LOW);
  shiftOut(dataPin, clockPin, MSBFIRST, 0);   // red = 0, all off 31 show green
  shiftOut(dataPin, clockPin, MSBFIRST, 31); // green = 0, all off 31 show red
  shiftOut(dataPin, clockPin, MSBFIRST, 255); // turn on all rows
  digitalWrite(latchPin, HIGH);
  delay(200);
  digitalWrite(latchPin, LOW);
  shiftOut(dataPin, clockPin, MSBFIRST, 31);    // red = 0, all off 31 show green
  shiftOut(dataPin, clockPin, MSBFIRST, 0);     // green = 0, all off 31 show red
  shiftOut(dataPin, clockPin, MSBFIRST, 255); // turn on all rows
  digitalWrite(latchPin, HIGH);
  delay(200); 

Downloads

LED Matrix Gerber

 

LED Matrix Schematic

LED Matrix Top Overlay

LED Matrix Bottom Overlay