Daisy Chaining MAX7219: How to Build a 3-Unit 'I Heart U' LED Display

I recently came across MAX7219 IC, which is a fascinating component with immense potential for driving LED displays. I think this IC can be useful in my future project so I was eager to understand its capabilities and practical applications, I wanted to try it out and so built a multi-segment LED matrix display to show "I heart U" (with a heart symbol). This not only deepened my understanding of the MAX7219, but I also learned its power in creating scalable, dynamic visual outputs. The resulting circuit, which uses three MAX7219 ICs to drive three 8x8 LED matrices, forms a 24x8 display, perfect for personalized messages or real-time data visualization.

MAX7219: A Powerful LED Driver

The MAX7219 is a compact, serial input/output common-cathode display driver that interfaces microcontrollers to 7-segment numeric LED displays, bar graph displays, or 64 individual LEDs. For our project, its ability to drive an 8x8 LED matrix with minimal pins from the microcontroller is invaluable. Instead of requiring 16 pins for each 8x8 matrix (8 rows + 8 columns), the MAX7219 uses just three serial communication pins, making it incredibly efficient for complex displays.

Key Components of Our 24x8 Display

To construct our "I heart U" display, we utilize a few core components:

• Arduino Nano: This serves as the brain of our operation. The Arduino Nano, a compact and robust microcontroller board, sends the serial data, clock, and load signals necessary to control the MAX7219 ICs. Its small footprint is ideal for projects where space is a constraint. For more details on this versatile board, check out our article on the Arduino Nano.
• MAX7219 LED Display Driver (x3): These integrated circuits are the workhorses, translating the serial commands from the Arduino into the parallel signals needed to illuminate the LEDs in our matrices.
• 8x8 LED Matrix (x3): These are the visual output units. Each matrix contains 64 individual LEDs arranged in an 8x8 grid. By controlling these LEDs, we can form characters, symbols, and simple graphics.
• Connecting Wires: Standard jumper wires are used for all signal and power connections between the components.

The Circuit Explained: Daisy-Chaining for a Bigger Picture

Our circuit's brilliance lies in the MAX7219's daisy-chaining capability. By connecting the output of one MAX7219 to the input of the next, we can control multiple 8x8 matrices using the same three Arduino pins. This creates a larger, continuous display—in our case, a 24x8 grid capable of showing more elaborate messages like "I heart U."

Step-by-Step Wiring and Operation

Let's break down the connections shown in the schematic:

1. Powering the System: The Arduino Nano provides the necessary 5V power and ground to all three MAX7219 ICs. The +5V pin from the Arduino connects to pin 19 (VCC) of each MAX7219, and the GND pin connects to pin 4 (GND) of each MAX7219.
2. Serial Communication: The Arduino Nano communicates with the MAX7219s using a 3-wire serial interface:
   • Data In (DIN): Arduino Nano's Digital Pin 11 connects to pin 1 (DIN) of the first MAX7219.
   • Clock (CLK): Arduino Nano's Digital Pin 13 connects to pin 13 (CLK) of all three MAX7219s in parallel.
   • Load (LOAD): Arduino Nano's Digital Pin 10 connects to pin 12 (LOAD) of all three MAX7219s in parallel.
3. Daisy-Chaining: This is where the expansion happens. The Data Out (DOUT) pin of one MAX7219 connects to the Data In (DIN) pin of the next. Specifically, pin 24 (DOUT) of the first MAX7219 connects to pin 1 (DIN) of the second MAX7219. Similarly, pin 24 (DOUT) of the second MAX7219 connects to pin 1 (DIN) of the third MAX7219.
4. Connecting the LED Matrices: Each MAX7219 directly drives one 8x8 LED matrix. The MAX7219's segment pins (A to G and DP pins 14, 16, 20, 21, 22, 15, 17, 9 respectively) connect to pins 8 through 1 of the LED matrix. The digit pins (DIG0-DIG7, pins 2, 11, 6, 7, 3, 10, 5, 8 respectively) connect to pins 16 through 9 of the LED matrix. This direct connection simplifies wiring, as the MAX7219 handles all the complex multiplexing.
5. Brightness Control: The ISET pin (pin 18) on each MAX7219 is shown connected directly to GND. Typically, an external resistor is connected here to set the segment current, thereby controlling the LED brightness. As no resistor value is visible in the schematic, it appears to be directly grounded, possibly using a default internal setting or for maximum brightness.

How It Displays "I Heart U"

The Arduino sends the pixel data for each character ("I", "heart", "U") serially down the chain. The data for the last matrix ("U") is sent first, then "heart", then "I". This data shifts through the MAX7219s until each IC holds the correct bitmap for its respective 8x8 matrix. When the LOAD pin is pulsed, all three MAX7219s simultaneously latch their data and update their displays, rendering the full "I heart U" message across the 24x8 grid.

#include <SPI.h>

const int CS_PIN = 10;
const int NUM_DEVICES = 3;

// Patterns
byte char_I[8]     = {0x3C, 0x18, 0x18, 0x18, 0x18, 0x18, 0x18, 0x3C};
byte char_Heart[8] = {0x00, 0x66, 0xFF, 0xFF, 0x7E, 0x3C, 0x18, 0x00};
byte char_U[8]     = {0x66, 0x66, 0x66, 0x66, 0x66, 0x66, 0x3C, 0x18};

// Function to send command to ALL chips in the chain
void sendGlobalCommand(byte reg, byte data) {
  digitalWrite(CS_PIN, LOW);
  for (int i = 0; i < NUM_DEVICES; i++) {
    SPI.transfer(reg);
    SPI.transfer(data);
  }
  digitalWrite(CS_PIN, HIGH);
}

// Function to send different row data to each specific matrix
void updateDisplay(int row) {
  digitalWrite(CS_PIN, LOW);
  // Matrix 3 (The 'U') - Send first
  SPI.transfer(row + 1);
  SPI.transfer(char_U[row]);
  
  // Matrix 2 (The Heart) - Send second
  SPI.transfer(row + 1);
  SPI.transfer(char_Heart[row]);
  
  // Matrix 1 (The 'I') - Send last
  SPI.transfer(row + 1);
  SPI.transfer(char_I[row]);
  digitalWrite(CS_PIN, HIGH);
}

void setup() {
  pinMode(CS_PIN, OUTPUT);
  SPI.begin();
  
  sendGlobalCommand(0x0F, 0x00); // Test Mode Off
  sendGlobalCommand(0x0B, 0x07); // Scan Limit (All rows)
  sendGlobalCommand(0x09, 0x00); // Decode Mode (Matrix)
  sendGlobalCommand(0x0A, 0x0F); // Medium Brightness
  sendGlobalCommand(0x0C, 0x01); // Shutdown Off (Wake up)
  
  // Clear all
  for (int i = 0; i < 8; i++) {
    digitalWrite(CS_PIN, LOW);
    for (int j = 0; j < NUM_DEVICES; j++) {
      SPI.transfer(i + 1);
      SPI.transfer(0x00);
    }
    digitalWrite(CS_PIN, HIGH);
  }
}

void loop() {
  // Update all 8 rows for all 3 matrices
  for (int row = 0; row < 8; row++) {
    updateDisplay(row);
  }
  delay(1000); 
}

Practical Applications Beyond "I Heart U"

This daisy-chained MAX7219 setup is incredibly versatile. I envision its use in various projects:

• Sensor Data Display: Imagine displaying real-time temperature, humidity, or pressure readings from sensors. This provides a clear, bright visual output, much like an LCD, but with potentially better visibility and impact.
• Scrolling Text Displays: The 24x8 grid is perfect for scrolling messages, news tickers, or custom alerts in a DIY smart home system.
• Simple Animations and Graphics: With careful programming, you can create engaging animations or display simple icons.
• DIY Scoreboards: For homemade games or small events, this display can serve as an effective digital scoreboard.

Compared to traditional character LCDs, these LED matrices, driven by the MAX7219, offer higher brightness and a distinct aesthetic, making them a compelling alternative for many display needs.

Conclusion