How to represent free hanging components in schematic? - Page 1
Forum section: electronics / general

author: Tinynja
author_rank: Newbie
timestamp: February 07, 2023, 04:57:46 am
content: Hello,I'm learning to draw schematics for my hobby projects, and I have a situation where I have a PCB that will be connected to some LEDs through a connector, but the thing is I want the current-limiting resistor of the LED to be free-hanging, so it won't be part of the PCB (refer to picture below).My question is, what am I supposed to do in this situation?Not include the resistors/LEDs in the schematic? (to me this seems bad since they won't be in the BOM either)Include them in a separate "block" of the schematic, but delete them from the PCB?Any other solution?Am I doing something wrong and you're not supposed to have "free hanging" components?Or in a more general term, how do we deal with situations where we have components that are part of the system but not part of the PCB itself?Thanks for your help!

author: Kim Christensen
author_rank: Super Contributor
timestamp: February 07, 2023, 05:40:34 am
content: Well, you could draw the "chassis" as it's own schematic complete with your "free hanging components", and all the PCB interconnections.

author: redkitedesign
author_rank: Regular Contributor
timestamp: February 07, 2023, 06:09:20 am
content: I would not include the resistor and LED on the schematic, since they won't be on the board.I would make a separate mechanical drawing (think AutoCAD, or any cheaper alternative) with the (populated) board, the resistors and LEDS and how to connect them (and how to bend the wires, where to cut them, where to solder etc.That drawing would also have a BOM associated with them, containing the board, the resistors, the leds and solder.Such a mechanical drawing is quite common to specify all the non-board things that go into a project. E.g. the casing, mounting material, external components (knobs, connectors, switches) that are not on the board but connected through wires/connectors.Optionally, I would draw the resistor and led as drawing/illustration on the schematic. That would make it better documented, provided you keep it up to date (i.e. change the resistor details if the resistor changes on the mechanical drawing/BOM.Theoretically, you could put the resistor and LED on the schematic as unconnected components. That would bring them into the BOM, but would give you PCB design errors, and won't let you document how to connect them.For a one-off private project, that can be acceptable.

author: Tinynja
author_rank: Newbie
timestamp: February 07, 2023, 06:26:51 pm
content: Thanks a lot for both your responses, I did not realize it's quite common to have components off of the board as you mentionned. Now I have better terminology that yields better google results.And thank you@redkitedesignfor this very complete answer, this was very enlightening! I didn't know about "mechanical drawings", and since I want to do this the "right" way, I will definitely look into those to learn.Have a good day!

author: Feynman
author_rank: Regular Contributor
timestamp: February 07, 2023, 08:13:03 pm
content: For a one-off hobby project it doesn't really matter too much, I guess.From i professional point of view (or if someone else is buying your parts and assembling your board) such components should never be part of the schematic hence the BOM. It just creates confusion.What you can do is create a sketch of those components on a documentation layer so they aren't actual parts that appear on the BOM.It's actually very common to draw such sketches next to connectors for example to provide some hints on what is going on off the board.

author: Mattylad
author_rank: Regular Contributor
timestamp: February 10, 2023, 08:21:19 pm
content: I used to not include them as part of the schematic as symbols/components but instead drew them so they were just lines/circles etc.That way they were clearly on the schematic for information but not included in the PCB BOM etc.