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Thread: How do a share a single PCB ground across multiple digital switches in parallel?

  1. #1

    Default How do a share a single PCB ground across multiple digital switches in parallel?

    I'm new to using perf boards and am trying to figure out how to ground multiple digital inputs using a single ground wire. I have an 20 pin bus cable going from a FI GSA-010 main board to an extender board that splits the bus into the PCB/perfboard; 16 inputs, 4 grounds. How do I share a single ground with the 16 digital inputs so they run in parallel vs running in a series?

    I've tried creating a single ground strip and attaching each digital switch ground to the ground strip I've created. Unfortunately, that creates a circuit that shares the switch inputs, and doesn't isolated each switch. This image has the extender board using one ground (which connects to me perf board ground strip on the left hand side), and the red wire connecting to a switch input on the perf board. https://ibb.co/vdG0Wm2

    Example; using two switches, switch one or switch two will only turn on if both switches are turned on. Do I need to create a separate ground for each digital switch/input? Here is the backside of the switch panel that I'm working on: https://ibb.co/dchZ6w1

    I've taken a couple photos. My soldering skills aren't very great, so bear with me. I'm also new at this, so any help is appreciated. I'm sure this is a simple question to most. Top side: holding the PCB ground that will feed into ground strip: https://ibb.co/12rtKQK. The underside showing how my solder connects the grounds to the ground input on the left hand side: https://ibb.co/xCFN6Fr

    This is the solder and tools I have available right now: https://ibb.co/QX02yxF . I suspected the solder gauge might be too heavy, but it does seem to work when I connect a single switch. It's when I add a second switch to the shared ground that the circuit then requires both switches on, which then activates both switches. If one switch is on, and the other is off, both switches are then off.
    Last edited by gnarly; 01-25-2020 at 03:58 PM.

  2. #2
    Join Date
    Feb 2014
    Location
    Klammath Falls, OR
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    4,088

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    Well, first off, get rid of the soldering gun, and the solder. Silver solder has it's uses, but these aint it.
    A 30W iron is all you need. Otherwise, you stand a good chance of lifting a run when soldering to it.
    You want the lead/tin solder, resin core is fine.

    Another good tool to have is solder wick to remove a solder join you don't want, and a sponge. Yes, a small square of regular, kitchen use sponge. Get it moist, not wet but moist before you start. Every single time you pick up the iron for any use, wipe the tip on the sponge to get it clean, bright silver clean. Then touch the iron's tip to the solder, to "tin" it.
    Personally, and this is from my mini/micro comp soldering days back in the Corps, I use flux. A bit on the wire, a bit on the item the wire's going to. A decent, liquid flux. Just a little drop is all it takes.
    Grab the iron, wipe it on the sponge, re-tin it, touch it to the fluxed wire to tin the wire, just enough that the portion at the end is silver. Maybe 1/4" of wire is all you need to tin. The solder on the tip of the iron should just flow easily onto the wire, leaving almost none on the iron, and a short piece of the wire a nice bright silver. Wipe the iron again, tin it, set it down until you need it again. You'll use 3 times as much solder tinning the iron as anything else.
    The part that will be used to attach the wire to whatever you're attaching to as well. Same for the target. Just a drop of flux, a touch to tin it, leaving it a nice, bright silver is all it takes.
    Use a small, stiff brush and some alcohol to clean any flux residue off.
    Make a good physical connection of the wire to whatever you're soldering it to, then a drop of flux, clean and retin the iron, and just a quick touch onto the connection. The solder should flow easily onto the connection, leaving it a nice, bright silver, with just a small amount off solder on it. It should be a strong enough connection, once done, to stand up some pulling. If it pulls off, it's no good, and you start over, cutting the wire back to a clean area, retinning it, etc.
    If soldering onto a pad on a PC board, you obviously can't make a good physical connection, just push enough tinned wire through that about 1/8'' is sticking out, flux and solder it, leaving a nice, bright silver cone. Cut away any excess wire still sticking out.
    A connection that is a dull silver when cleaned off with alcohol is a "cold" or high-resistive solder join. This is no good and must be redone. Here's where the solder-wick comes in. Flux about 1/2" to 3/4" of the wick, put it against the bad solder, and hit it with the iron. The solder on the iron's tip will flow onto the wick, but so will the bad solder on the connection, leaving the wire free to pull off. Don't take the iron away until all the bad solder is drawn into it, using a clean, freshly fluxed section of wick until the wire comes free. Don't leave the iron on so long it lifts a pad, though. Just a quick touch should be all it takes. Don't solder the wick onto the connection
    Cut off the soldered portion of wick, clean and retin the iron, etc.

    The biggest mistake people make is not keeping the iron's tip clean, with a fresh drop of solder on it. Whenever you touch the iron, get into the habit of wiping it on the sponge, and retinning it right away. Whatever you're going to do with it. Even just before you unplug it for the day, clean it and retin it. When you plug it in, let it hot up, wipe it and retin. Always, always clean and retin the iron's tip. It'll pay off in good, strong, clean solder joins.

    Sorry, I taught this for a while...
    I do ramble, though.
    Without some schematics, it's difficult to tell just what you're trouble is. One thing, though: Make sure the switches are wired up in parallel, not series. It sounds like you wire two switches in series, that's why they are affecting one another the way they are.

    Good luck to you!
    Pat☺

    Had a thought...then there was the smell of something burning, and sparks, and then a big fire, and then the lights went out! I guess I better not do that again!
    Sgt, USMC, 10 years proud service, Inactive reserve now

  3. #3

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    Wow, what a fantastic reply, thanks PhantomTweak.

    So I did realize my problem was that I was trying to wire in a series of switches that were already part of a pre-built circuit board switch panel, thus, only allowing me to wire in series vs parallel. The good news is, that I did finally figure this out after a late Saturday night and managed to work out all my switch issues and have been able to wire the toggle switches that weren't part of the switch panel circuit board in parallel.

    However, as you can tell from my soldering examples, I have very little experience when it comes to soldering. Your tips are definitely appreciated and I will make sure to put them to good use. Thank you!

  4. #4
    Join Date
    Feb 2014
    Location
    Klammath Falls, OR
    Posts
    4,088

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    I'm glad you got it set up right!

    I am also glad I was a small help to your soldering skillz
    I hope you don't need them TOO much

    Just a heads-up: You can get everything you need for a decent soldering kit from McMaster-Carr. Irons, solder, flux...everything.
    For scrubbing off flux residue, I use a medical alcohol dispenser, one of those small, white, square-ish bottles with the top that pushes up a shot of alcohol when you press down on it. Doctors used to use them to put alcohol on a cotton ball for scrubbing off the place to give a shot. HERE.
    For a brush, I use what we called "acid brushes". Silver handle about 5-6" long, bristles about 3/4"-1" long. Cut them off fairly short, like 1/4" and they are the perfect stiffness to scrub flux residue. A quick tap on the dispenser bottle, the brush is soaked, and will take off flux quickly and easily. Just throw them away when you get done. Simple! FLUX HERE. Acid brushes HERE.
    Many soldering iron holders, mainly a plastic base with a spiral of heavy wire to hold the iron, you've seen them, come with a small square of sponge, but like I said, simple to cut a small square to use, and quick and easy to replace. Find it HERE.

    I keep all my soldering gear in a small tool bag. Works great
    Thus: TOOL BAG

    BTW: if you're going to be soldering either wire to wire or wire to connector, like a spade terminal, always ALWAYS use what we call FIT-300 heat shrink. It has a meltable inner lining that will keep harsh, corrosive atmosphere out of you solder join, so you don't wind up with darn hard to find, intermittant problems.
    There are also pieces of heat shrink with a loop of solder built in, so all you need to is put in your two wires, overlapped, and heat it up with a decent heat gun. The solder melts, joining the two wires, and then the inner wall melts, protecting the join. Kind handy for doing in cars, especially under the hood, where all kinds of nasty can "get" your solder joins.

    OK, I shut up now...
    Good luck, and have fun!
    Pat☺

    Had a thought...then there was the smell of something burning, and sparks, and then a big fire, and then the lights went out! I guess I better not do that again!
    Sgt, USMC, 10 years proud service, Inactive reserve now

  5. #5

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    Once again, great advice that I'll happily take on board Phantom! The world of soldering and electronics is still fairly new to me, so I appreciate all the feedback. It is definitely being used during my build.

    I will make some updates to my soldering/wiring toolkit based on your response. I do have a box of heat shrinks I used for both wrapping a series of wires (to help organize the wiring), and the odd wire-to-wire connection when not using spade connections.

    The Cessna 172 project is slowly wrapping up. Only thing left for me to do is wire up the slide and rotary pots.

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