LED - 3. Let's make a large led display - Part 4 (connect multiple RGB LED Matrices)


Connect multiple RGB LED Matrix

Connecting the RGB LED Matrix is ​​really easy. When you connect the Hub 75 interfaces of the two modules, you are done. One thing to note is that you must connect Hub75 at the beginning of the next module to Hub75 at the end of the arrow. And be sure to connect 5V power cable to the second module.
If you want to connect three Matrix, connect Hub75 at the end of the arrow of the second module and Hub75 at the start of the arrow of the third module. This type of connection is called a daisy chain.




Daisy chain

From Wikipedia.

In electrical and electronic engineering a daisy chain is a wiring scheme in which multiple devices are wired together in sequence or in a ring,[1] similar to a garland of daisy flowers. Other than a full, single loop, systems which contain internal loops cannot be called daisy chains.
Daisy chains may be used for power, analog signals, digital data, or a combination thereof.
The term daisy chain may refer either to large scale devices connected in series, such as a series of power strips plugged into each other to form a single long line of strips, or to the wiring patterns embedded inside of devices. Other examples of devices which can be used to form daisy chains are those based on USB, FireWire, Thunderbolt and Ethernet cables.


<daisy flower and daisy connection>

Testing 2 RGB LED Matrix (32 X 128)

Test the same way as in the previous post. This time, however, there is one option that was not used in the previous test. Add the --led-chain = N option when daisy chaining multiple RGB LED Matrix.



pi@raspberrypi:~/rpi-rgb-led-matrix/bindings/python/samples $ sudo python image-scroller.py  --led-cols=64 --led-rows=32 --led-chain=2
Press CTRL-C to stop sample
^CExiting

If you connect it correctly, you will see two RGB LED Matrix work as one as shown in the following figure.



Trouble Shooting

Flickering

If you have flickering problems, use --led-slowdown-gpio=N options. Default N is 1, Increase this value to find the best result. The maximum value of N is 4. Generally the CPU speed is high, you need to increase the N value.(RPi 4 may need 4)

Tried all these, it still flickers.

Check the power supply. Connecting multiple RGB LED Matrix using daisy chains requires a lot of power. Unexplained errors are often due to poor power. I strongly recommand a good quality power supply and check the max power consumption of your chained RGB LED matrices.

Disable all the unnecessary services. For example



sudo apt-get remove bluez bluez-firmware pi-bluetooth triggerhappy pigpio


Switch off on-board sound (dtparam=audio=off in /boot/config.txt)

Adjusting Brightness

Use --led-brightness=<percent> options. Default value is 100, so to decrease the brightness, set this value to under 100.


Wrapping up

The original model of Adafruit's RGB LED Matrix Hat, Bonnet is Henner Zeller's adapter PCB.(See this page: https://github.com/hzeller/rpi-rgb-led-matrix/tree/master/adapter/active-3). In fact, the only thing the board does is convert the Pi's 3.3V signal voltage to 5V. For voltage level shifting, it uses four 74HCT245 chips.
It supports 3 parallel chains! So you can drive more RGB LED matrices.

Preview
<Henner Zeller's adapter PCB>

Real World
<Henner Zeller's adapter PCB on the Raspberry Pi>
He also shares the PCB Gerber file. So you can make your own. If you are interested in H/W, I hope you will challenge the PCB manufacturing.

The architecture of Adafruit's RGB LED Matrix Hat,  Bonnet has the same structure as his PCB, except that it supports only one chain. It has only two 74HCT245 chips.

Adafruit's RGB LED Matrix S/W uses Henner Zeller's library at https://github.com/hzeller/rpi-rgb-led-matrix. This github page have many useful tips that Adafruit homepage does not offer. So be sure to visit this page to read the readme.md file. There are many examples for Python developers in the rpi-rgb-led-matrix / bindings / python / samples directory. These examples use the Python rgb matrix module that is installed with the "rpi-rgb-led-matrix" software.



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