My first project -- how to use a circuit board?

Discussion in 'General Electronics Chat' started by koggit, Aug 2, 2008.

  1. koggit

    Thread Starter New Member

    Aug 2, 2008
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    I'm extremely new to this circuit business. I'm about four weeks into an introductory physics course on E&M, and we're just getting into circuits (we start RC circuits next week). Since I'm considering majoring in electrical engineering, I decided to go a little deeper in circuits on my own, a little beyond the scope of the class. AAC has been a wonderful resource and I've been learning a lot. I've been going through the experiment guide, found here, and have been enjoying it (although my wallet has not -- $145 in three days!).

    I'm on the Make Your Own Multimeter experiment and made this schematic:

    [​IMG]

    I'm sure the symbols and stuff are wrong, but hopefully it'll give you an idea of what I'm trying to do if you're unfamiliar with the project. I'm pretty sure I'd know how to wire this just using wires, but my problem is transitioning it to the circuit board. I want to make this meter on a circuit board so that I can mount it in a project box, for a more permenant solution.

    My big problem is that I don't understand how a circuit board is wired. How do I know which holes are connected to one another? I just don't know how to position my components to achieve the desired circuit (series / parallel and combinations).

    I apologize if this info is available on AAC already -- if it is, please direct me to it. I looked, but couldn't find anything. The experiment guide available on AAC jumps straight into using a breadboard, then straight into using a circuit board. I tried Googling a lot of different terms but couldn't really find anything. Perhaps this is too basic a question to expect a guide on... but I'm too dumb to figure it out myself.

    Oh, and for clarity, I'm referring to circuit boards like this one: http://www.radioshack.com/product/index.jsp?productId=2102846
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Gee, it's too bad you didn't ask for suggestions first.

    You could've saved quite a few bucks.

    What you should've bought is the Electronics Learning Lab:
    http://www.radioshack.com/product/i...kw=electronics+learning+lab&parentPage=search
    It's worth it for the project board alone. It comes with a number of components, jumper wires and a couple of lab books that were written by Forrest M. Mims III (who also designed the project board.)

    Your symbology isn't bad at all - but you've mixed a European-stype potentiometer symbol in with US-style resistors, and a rotary switch is usually depicted somewhat differently than how you've shown it - but not bad at all for the first few tries ;)

    Here's a page that you'll find quite helpful for finding combinations of standard resistors in series or parallel to get close to the desired resistance:
    http://www.qsl.net/in3otd/parallr.html
    Plug in the value you're trying to achieve in Ohms, and then select E24.
    You'll get a variety of configurations that you can use.

    Radio Shack doesn't have much of a variety in trim pots, but they have this type:
    http://www.radioshack.com/product/i...&cp=2032058.2032230.2032275&parentPage=family
    10k Ohm, 15 turn. Try to come up with a combination of resistors that will give you about 5k Ohms less than is required, and use one of these pots to get "dialed in" to the exact resistance that is required.

    As far as layouts, that's through experimentation. Try to get your component count down to a minimum, and then see how you can make it use the least space on the board.
     
  3. koggit

    Thread Starter New Member

    Aug 2, 2008
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    Thanks, SgtWookie.

    I found the Electronics Learning Lab on eBay -- it has six days left, but I put it on watched items, hopefully it'll go for quite a bit less than the MSRP. I couldn't justify spending too much more than I already have, at this point.

    I think my current resistor combinations may be good enough, since I'm very limited in what resistors are available to me (I bought a 100 piece assorted resistor pack). The linked tool is handy, and I've bookmarked it for future projects, but I doubt I'd be able to make many of the combinations it gives. My project box is pretty large, so I don't think I'll have too many components.

    I actually have3 of the potentiometers you linked, I picked them up yesterday -- that's what I included on the bottom of the diagram. I guess my image wasn't so great: I already have the resistors/potentiometers set and ready to go, I just need to put them on a circuit board. It's not that I don't know how to maximize space, I just don't know of any layout that would work.

    Creating the circuit I want on a perfboard using wires to connect the components would be no problem for me. I understand how I want it connected, and space isn't an issue. My problem is simply that I don't understand how a circuit board connects components. You don't connect a wire from component to component, right? The circuit board connects your components? I don't understand how the components should be placed on a circuit board to make a complete circuit. I don't understand how a circuit board is wired, and I can't seem to find anything that explains it.

    [​IMG]
    (hopefully they don't block hotlinking)

    On something like this, even if I just had a power source and a resistor, I'd have no idea how where to put the +, - or R to make a complete circuit.
     
    Last edited: Aug 2, 2008
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Well, general purpose pre-drilled perfboards sometimes have traces that can be used for power/ground busses, possibly pads for connecting a few components to the leads of a DIP IC, etc - or may be just lots of holes with copper pads around the holes, or may not even have copper pads. Of the type with copper busses for use with IC's, take a look at Radio Shack's cat no 276-150.
    http://www.radioshack.com/product/index.jsp?productId=2102845&cp=
    [​IMG]

    When you're connecting things up, you use buss wire or hookup wire as appropriate. Having a good soldering iron, 63/37 solder, rosin flux, and a decent set of tools is a big help.

    But before you even get that far, it helps a lot to use a software tool to capture the schematic, and test it - then convert the tested schematic to a board layout using a PWB layout software tool.

    Check out our Resources section for a variety of schematic capture and PWB layout tools.

    I notice that you have a current setting on your range switch - and your meter seems to have a 0.2mA next to it. Is that the meter's actual range? If so, you really should consider using shunt resistance across the meter, or the first time you try to use it as a fractional milliameter you are likely to fry the movement. I strongly recommend that you include a fuse in your circuit to protect the meter movement, or your meter movement will act as a fuse! :eek:
     
  5. koggit

    Thread Starter New Member

    Aug 2, 2008
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    Oh, thanks, that clarifies a lot. My confusion before was because I thought circuit boards were wired sort of like a breadboard, where certain rows/columns are already connected to one another in ways that aren't immediately visible (I thought there were conductive pathways in the board).

    So, the perfboard I have doesn't have any metal on it at all, and it's the same on both sides. Is it okay to solder on this board for what I want to do, and simply connect each component with wire, or should I invest in a board with metal or a pad to aid soldering?

    This is the soldering iron I went with: http://www.radioshack.com/product/index.jsp?productId=2062758

    I'm not sure what quality the solder is, they don't have any type of number. I read that it includes a small amount of flux in the middle of the solder. This will be my first soldering project as well, although yesterday I soldered together some resistors/wires to get a feel for it.

    Thanks for the tip regarding the shunt. I will include a shunt to raise the ammeter range.


    Edit: My calculations show that for a 25 mA scale I'd need a shunt with resistance 5.424 milliohms... I don't think there's any way I could accurately get a resistance that low with the resistors I have, which don't go any lower than 10 ohms. The big problem seems to be that my meter itself has such low internal resistance -- only 0.678 ohms... to get a shunt that takes any significant portion of the current, it would have to have significantly less resistance than that.
     
    Last edited: Aug 2, 2008
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Nope, when you buy pre-drilled perfboard, it may be partially clad on both sides, partially clad on one side, or have no copper on it at all. Basically, what you see is what you get.

    Well, your board will basically just serve as a platform, or a "spacer" if you will, to keep your components more or less held in position. You can't solder to the board, as it's fiberglass. You can solder the leads to each other, and additionally use buss wire to connect other portions of the circuit. Precisely how you connect it up is up to you. As you experiment with connecting things up, you'll find neater ways to do things.

    That'll do for a while. Try to not leave it plugged in for longer than you need it. A copper or stainless steel scrubbie pad (available in the cleaning supplies section at the grocery store) is great for cleaning the tip of the iron.

    It's most likely their 60/40 rosin core solder. 63/37 is the easiest solder to use, as it has no "plastic" state; it cools directly from a liquid to a solid. It's jiggling the parts during the "plastic" state that's the cause of "cold" solder joints. The solder will look dull instead of bright and shiny.

    It's a good idea to get practice before you try actually putting together a real project.

    You can use solid copper wire of a known gauge.
    AWG 20 wire has a resistance of 10.3632 Ohms per 1,000'
    So, 1.911 feet of AWG 20 would serve.
    AWG 30 has roughly 10 times the resistance.
    I typically see brass shunts being used in meters. Brass has a much higher resistance than copper - except brass wire is somewhat hard to find nowadays.
    [eta] Brass has 4.822 times the resistance of copper.
     
    Last edited: Aug 3, 2008
  7. koggit

    Thread Starter New Member

    Aug 2, 2008
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    Thanks for all the help -- I'm just about done.

    [​IMG]


    The main thing that sort of confuses me is the circuit board doesn't fit the project box at all. Oddly enough, none of the circuit boards at radio shack fit any of their project boxes -- the perfboards didn't fit either. I asked the store manager and he didn't really have an answer. Is it okay to just cut the circuit board to size, then mount it using hot glue or something?


    I didn't go with the shunt (yet) -- I figured I can use it exclusively as a voltmeter for now, and next weekend add a second terminal with a shunt to use an ammeter.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    It's absolutely OK to cut the perfboard to size. :)

    I'm not surprised that the store manager didn't know about standoffs. I'll bet he knows all about cell phone plans though. :rolleyes:
    Basically, Radio Shack carries two kinds of standoffs; metal and insulated.
    Metal: http://www.radioshack.com/product/i...origkw=standoff&kw=standoff&parentPage=search
    Insulated: http://www.radioshack.com/product/i...origkw=standoff&kw=standoff&parentPage=search

    Since your switch is rather deep, you'll probably need to use the metal standoffs which are somewhat longer.

    Basically, you drill holes in the lid to match the holes in your perfboard, and the standoffs keep the PCB separated from the lid. In your case, you'll also need to drill a hole for your rotary switch and the jacks for the meter leads.

    It will be much easier for you if you mount everything to the aluminum lid. Otherwise, you'll have to cut wires and re-solder them if you mount it to the box.


    That's fine. You have enough of a challenge right now just figuring out how to mount what you have ;)
     
  9. AllVol

    AAC Fanatic!

    Nov 22, 2005
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    Actually, the board shown in this photo fits the project box exactly. See the channels in each end of the box? The board slides into those, just like the shelf in a refrigerator or oven.

    For future reference, smaller boards, if desired, can go into the channels located on each side of the box.
     
  10. koggit

    Thread Starter New Member

    Aug 2, 2008
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    Actually what I was referring to is the board is just slightly longer than the box -- all of RadioShack's circuit boards were slightly different sizes than their project board's slots.

    I cut a little from the length of the circuit board (about 1 cm) and although it now fits in the slot, the width of the circuit board is greater than the depth of the box. It won't be hard to do, just time consuming -- cutting the short side took about 5 min.

    [​IMG]

    My obstacle right now is cutting a meter-shaped (and sized) hole in the lid. I bought a hot knife -- that's useless on this plastic, a waste of money (financially I'm definitely regretting this project). I started to use the drill to create holes, thinking I could score it and push it through, but I don't think that's going to work -- too imprecise. I see a "nibbler" on RadioShack.com that might do the job, but it's web-only... and an extra $20 is really hard to justify at this point, after over $200.

    *sigh*

    I think I'm done. I don't need an enclosure. I have a more accurate digital multimeter anyway -- building this was just to learn, not to gain a useful tool, and I've learned, so mission accomplished.

    Thanks again for all the help.
     
  11. Voltboy

    Active Member

    Jan 10, 2007
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  12. AllVol

    AAC Fanatic!

    Nov 22, 2005
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    All is not lost.;)

    At least you did learn... the initial goal itself... about electronics with a side lesson in economics.

    You still have some of your purchased items that can be used in future projects, which will amoratize the cost. Keep plugging.:D
     
  13. p.marrero

    New Member

    Aug 5, 2008
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    Is the circuit Board single sided or double sided?
     
  14. Wendy

    Moderator

    Mar 24, 2008
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    Dremels are your friend when it comes to sculpting holes. You have a little problem with melted plasic around the cutting tools, but that can be removed easily.
     
  15. koggit

    Thread Starter New Member

    Aug 2, 2008
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    I went ahead and finished it last night, including a shunted line on the side for <250 mA current.

    [​IMG]
     
  16. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Hi Koggit

    There are a gagillion different formats for pre-made circuit boards, or you can roll your own from scratch using copper clad blanks (my favorite method).

    PCB layout isn't too critical at lower frequencies, or low parts-count situations, but can become very crucial at radio frequencies, or high speed digital (which really become quite related!)

    Since there is a bit of an art form to this, it's really helpful if you can work with an experienced hand. You certainly can learn this on your own, but it's a lot faster and less frustrating if you have a mentor.

    eric
     
  17. Wendy

    Moderator

    Mar 24, 2008
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  18. Mike Mandaville

    Active Member

    May 27, 2009
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    LMA*








    *Lost Me Again
     
  19. ELECTRONERD

    Senior Member

    May 26, 2009
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    Hey Koggit,

    Although you may not like it...failure can be your friend too!;) You'll learn from your mistakes and gain more knowledge. I've attemped numerous projects and they didn't work, but I troubleshooted them and found out what was wrong. Now that I experienced it, whenever I come across similar projects that aren't working, I might know the problem! You'll be a lot more satisfied with your project if you were able to fix what was wrong with it.

    As for enclosures, we're in the same boat! :p I've wasted a lot of enclosures because I didn't get them exactly like I wanted them. I definitely would suggest a dremmel tool or you could even use an electric drill and drill holes through the plastic.

    You're doing great! Keep up the good work!
     
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