I am close to completing a device which will have a 16x32 led matrix as well as quite a few lights/buttons/sensors. All these lights and sensors go through an MCP23017.

I am using the Adafruit led matrix from here: https://learn.adafruit.com/connecting-a-16x32-rgb-led-matrix-panel-to-a-raspberry-pi/wiring-the-display

As well as a MCP23017: http://microelk.azurewebsites.net/Images/MCP23017/MCP23017_PINOUT.jpg

Right now I get both devices (the LED matrix and everything that goes through the MCP23017) to work perfectly when hooked up directly to my raspberry pi. However, I cannot figure out how to use the SDA/SCL for both. I am a novice when it comes to circuitry so I am worried I may be doing something wrong. Because it is a lot going on I have attached some pictures.

There are a lot of wires but just including to show the SDA and SCL coming out of the pi. Other than the ground and power which go to the breadboard with the MCP, the rest of the cables go directly to the MCP. enter image description here

This is the most important picture. When I have it set up like this the 16x32 LED matrix works perfectly. If I remove the two wires that say SDA/SCL to screen/matrix then the MCP works perfectly. However, when those wires are in the MCP does not work and is not detected when I run i2c detect in the terminal. enter image description here

Just included this if needed. The back of the screen. The wires from the board actually go to a separate breadboard where I use a resistor to help get a clear signal. How do I wire up the screen and the MCP, which both need SDA/SCL, with a single SDA/SCL output from the raspberry pi?

I have seen the guides that say things like "split the signal" or something but I have not been able to use it for both at once with success. Any clear advice would be helpful! I worry that the solution is simple but the jargon I have seen used I do not understand and have not been able to determine the source of my error.

  • No one can tell from that rats nest what you have done. To make matters worse it is unclear what you are trying to do.
    – Milliways
    Commented Apr 2, 2019 at 5:27
  • Hey there is there anything you have to add or a suggestion of what to include? I took the time to put pictures with labels and arrows pointing out the connections that are of issue so that those willing to help would know what I am talking about. I'm not sure your snarky comment belongs on here otherwise.
    – PinMan
    Commented Apr 2, 2019 at 5:31
  • You have posted rather unclear photos most of which show one end of a wire - there are multiple wires of the same colour. I would defy anyone to work out what you have connected from the pictures. Start by making a list of what is connected to what. How many is "quite a few"?
    – Milliways
    Commented Apr 2, 2019 at 5:44
  • All lights and sensors that go through the MCP work correctly (before the screen is attached). The screen works correctly. The problem is when I take the SDA and SCL wires (from the screen) and plug them into the breadboard to connect to the SDA and SCL (going from the pi to the MCP) the MCP stops working.
    – PinMan
    Commented Apr 3, 2019 at 0:35
  • 1. Create a proper schematic and post that. Don't post (badly focused, badly cropped) pictures.
    – JayEye
    Commented Apr 3, 2019 at 2:59


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