Monday, December 31, 2012

Buying Parts and Using Digikey

Before I dive into my circuit modifications, I'd like to talk about how to buy parts.  As a non-professional electronics person, it can be hard to know what specific items to buy.  I mean, if someone says that you need a 10K resistor, where do you get such a thing?

Well, it's great if you can find what you need at hobbyist places like Sparkfun or Adafruit.  They really limit the choices to just the core essentials.  Having limited choices sounds like a bad thing, but it really is a blessing.  They have done all the work of sifting through the thousands of choices available and have narrowed it down to just the few choices that will likely fit what the hobbyist needs.  If you can find the part that you need at one of these places, it'll probably be the right one for you and you should buy it.

If they don't have what you need, you can go to a place like Jameco.  They've got more choices, which can be daunting, but they show a lot of pictures, so you can often shop by the pictures.

If you still can't find what you want, it's time to put on your big-boy pants and step up to "real" stores like Mouser and Digikey.  They're the places that professionals go to buy their electrical parts.  Their stores are ridiculously deep.  They're scary places to go at first, if you don't know what you want with an engineer's precision.  But, with a little practice, they get more comfortable, and then the whole world of electronics is open to you.

Let's go through an example of shopping at Digikey.  Nearly any time that you work with integrated circuits (aka "ICs" or "chips"), you'll have to use 0.1 uF capacitors to provide filtering ("decoupling") of high frequency transients on the power input line.  On most designs (even hobbyiest designs) 0.1 uF caps are everywhere.  Let's say you need to buy some.  In this case, they're so common that Sparkfun does carry them, so you should just buy them there.

Buying a 0.1 uF Cap at Sparkfun

But shopping at Sparkfun is not the point of this exercise.  The point is to try Digikey.  So go to Digikey and search for "capacitor".  You get 275,000 options.  Umm.  OK.  Now what?

Shopping for Capacitors at Digikey.  Lots of choices!

Well, for caps, you need to know what type (composition) of capacitor you want.  How do you know that?  Well, sometimes the schematic tells you ("electrolytic" or "polypropylene").  But, usually, it won't say.  What you need to know is that, for a given cap size, or for a given application, everyone seems to use the same type of cap.  It's tradition.  If you search around the web enough for people using a cap in a similar way as you, you'll be able to find out what everyone uses.

For synth hacking, most caps will be ceramic caps because they're small and cheap.  The biggest exception is for high capacitance caps (1 uF and bigger).  These caps are almost always electrolytic caps.

Returning to our example case of finding a 0.1uF cap for use around ICs, everyone seems to use "cheap" caps, which definitely means "ceramic" caps.  So, on the Digikey page, click on "Ceramic Capacitors".  This gets you down to 126,000 choices.  Note that the webpage has changed...

Digikey's page for filtering through all the choices for "Ceramic Capacitor"

This new page is giving you all sorts of filtering options.  Now's when we really start cooking:

  1. First, click on the checkbox for "In Stock".  Now we've got only 41,000 choices.
  2. Under "Capacitance", scroll down and select "0.1uF".  2,000 choices.
  3. Under "Voltage - Rated", use Ctrl-click to select "25V", "35V", and "50V".  944 choices.
  4. Under "Mounting Type", choose "Through Hole".  159 choices.

At this point, you've narrowed it down to a few pages worth of choices.  If the part you needed was a little less generic than simply a "0.1 uF capacitor", you'd probably only have a handful of choices now, instead of 159 choices.  So, you'd look at the pictures, maybe look at a datasheet or two, and then just pick one and go.

In the case of these caps, you probably want to down-select a little more.  Look under "Tolerance".  We probably don't want to pay for the best caps (smallest tolerance) and we never want the worst (biggest tolerance), so select "10%" and "20%".  That got us down to 128 choices.

Looking down through the first page of choices, the pictures all look like parts that I could work with.  Any of these would probably be fine.  Because I'm likely to be hand-soldering on a crappy proto-board, I think that I prefer the look of the caps with the long leads.  So, I'd probably buy some of Digikey Part BC2665CT-ND.  They're 37 cents individually.

Hopefully, a decent choice for a 0.1 uF capacitor from Digikey.

For cheap general-purpose components like caps, you should always buy more than you need right now.  I always buy extras.  That way, I might not need to order more when I go to my next project.  Having the parts on hand means that you can dive right in when the inspiration strikes and not have to wait for the shipping.  It's so much more fun when you can dive right in.

For something super general-purpose like 0.1 uF caps (or 10K resistors), I might buy 10 or more.  For these caps, you'll see that the price drops to 25 cents each when you buy 10.  Or, if you're really bold, you could buy 100 for only 10 cents each!  I'm not that bold....I'd probably just buy 10.

So, that's how one buys parts.  It's a very important skill to have when you're going to be hacking synths.  Anyone have any sites that they really like to use for parts?

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