MAX 7219 Boards
- Max7219 & 7 Segment Displays
If you're struggling to wire 7-segment displays to the MAX7219 boards, don't worry; this guide is here to help. These displays are essential to bringing various panels to life in the simulation.
When I first started building simulations, I was fascinated by these simple boards. It took me some time to understand how they could light up so many LED segments and boards using only five pins.
​
7 Segment Displays
The first thing you need to know about 7 Segment displays fitted to the MAX7219 Boards from eBay or other providers are:
-
The operating voltage is 5VDC.
-
The connection type is Common Cathode!
​
This is important if you want to connect the different sizes of the 7 segment displays or adjust the number of digits driven by the board. Another favourite is to change the colour of segments attached to the MAX7219 boards.
​
Different Types of MAX7219 Boards
Predominately, there are three different types available on eBay. Their PCBs are colour coded to help identify the type:
​
A GREEN PCB typically has detachable displays and comes with attaching hardware. This is great for easy removal, so you can change the size or colour as required. This also allows you to place the two halves (4-digit displays) anywhere you like. These are my preferred type.
​
​
The BLUE PCBs are great for a narrow space on top and bottom. But the displays will need to be removed with a heat gun.
​
​
​
The RED PCBs are excellent for applications where space at the ends is a priority. They have the shortest width. Again, you will require a heat gun if you want to separate the displays.
​
​
From my personal experience and much testing, I would only try to run one or two MAX7219 boards from a single Arduino. I have found that connecting any more boards than this is too much for the Arduino mega on the current draw, and strange things started to happen to the displays. The whole display would switch off or start to indicate gibberish. This was all fixed by wiring the MAZ7219 boards to an external 5v power source.
Remember! When using an external 5VDC power supplyYou always need to connect the ground from the external power supply to the Arduino to complete the circuit!
​
7 Segment colour
When I was working on creating version 1 SIM, I didn't pay much attention to the color of the LED segments. My focus was on building a prototype that would work effectively and be cost-efficient. Initially, I used red LED segments as they are the most common and easily available. They also tend to be the cheapest and come with Max7219 Boards. However, as time went on, I discovered other colors such as blue, yellow/amber, and white. I recently bought multiple colors, and now I have a blue MCP and the option of yellow, depending on my preference.
7 Segment Wiring - Using Ardunio Power
Daisy chaining MAX7219 boards using Arduino-supplied 5v power is possible. But I would really limit yourself to two boards maximum. People will tell you that they have 4 or 5 boards connected to an Arduino, and this is possible. The problem comes when you start to power on all the LEDs simultaneously when large numbers are required. This will draw more current than the Arduino can provide.
​
There are generally five input pins and five output pins on each board. The 5v VCC must be connected in parallel to each board and not daisy-chained through the board itself. This is because the VCC line has a diode fitted and will reduce the voltage through each board until it stops working.
​
The ground can be daisy-chained, but I prefer to wire in parallel like the VCC.
​
The following 3 input pins (DIN, CS, CLK) can be daisy-chained through the board by using the same output pins, as shown below.
7 Segment Displays - Daisy Chaining
After several weeks of testing every combination possible, I concluded that the more modules you daisy chain together, the less reliable they become. This is regardless if you are powering from the Ardunio direct or external power source.
Here are my current recommendations for Daisy Chaining 7 Segment displays:
7 Segment Wiring - Using External Power
Wiring the MAX7219, 7 segments displays boards to an external power supply helps reduce the current draw on the Arduino.
As you can see from the diagrams, the 5v VCC line is connected in parallel to all 3, 7219 PCB's.
The GND line in also connected in parallel and also connected to the Arduino.
The data pins are then daisy chained as normal.
COMMON GROUND
​
Please Note: You must connect the ground cable from the 7 segment boards, the external power supply and the Ardunio board for everything to work.
Test, Test & Test Again!
It is important to always test your seven segments in the configuration you plan to use before installing them. This includes checking the length of the cables and the number you want to daisy chain. If you are still experiencing issues with unusual digits, there is one last thing to check. Make sure the power at the last 7-segment unit is at 5V.
If not, adjust your power supply carefully until it reaches 5V.