I have an Adafruit Quad level shifter 74AHCT125 that I am trying to build into an Eagle Schematic and PCB for a project. I am very new to schematics as in...this is my first attempt. I piggybacked on a GitHub issue that defines out more of the issue with additional images and links.

Essentially when I add the 74*125 part to the schematic it just drops in a single transistor for each click that is performed. I believe these represent each of the quad level shifter as it labels A, B, C, D for each one.

I am looking for advice and guidance on how this should be wired in the schematic. I am also baffled how there are so many connections to the packages but I only use 1 level shifter and the transistor schematic only has 3 connection points.

Fritzing of the setup (blue is GPIO18): enter image description here

Current setup of schematic...pretty bare. enter image description here


Not sure what exactly you are asking about. Also you picture with breadboard schematics isn't complete. How do you think you are connecting power to the LEDs? It's somewhere on the right outside of the image.

The element, that you called "transistor" in reality is "buffer". There are 4 buffers in total inside 74AHCT125. If you need only one, you can use any of them. Read this datasheet first: 74AHCT125

So what you should do is rather straightforward, just connect pins with the same number as on your top picture. Something like on this picture: Connections

But remember that you should also provide power to 74AHCT125 and LEDs, you should add some power jumper or connector to your schematics and define ground and power line in your CAD program.

  • \$\begingroup\$ Just like that, except the other gates have floating inputs so these must be connected to ground or supply voltage. And power ports must be connected to supplies as well, this power instance needs to be invoked in Eagle somehow. \$\endgroup\$ – Justme Mar 1 at 7:19
  • \$\begingroup\$ @Justme You can leave them floating, it's really doesn't matter. It will work without any problems for such kind of application. About power instance - I wrote my concerns about this also, but this question goes to the author of the question. \$\endgroup\$ – cyclone125 Mar 1 at 7:46

The 74HCT125 is offically a quad (4 channel) bus buffer, not a "Quad Level Shifter", as Adafruit calls it.

Your "transistors" are the common schematic symbol for a tri-state buffer - a digital logic gate. for multi-channel parts such as this, it is common to show sections individually on a schematic, rather than one large block containing all the channels. That makes it much easier to understand the schematic without having to go find the datasheet to see what's inside the big box.

the 74HCT125 has four separate buffers, each with three terminals (in, out, and enable) as well as power and ground connections common to all buffers.

For IC1A, pin 2 is input, 3 is output, and 1 is enable. You should get the datasheet for the 74HCT125 to learn more about the chip. There is a link to the datasheet on the Adafruit page you linked to. (Don't just go by Adafruit's description)


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