Over on another forum (EEVBLOG), there's a REALLY good thread going on "how transistors work" that is more detailed than "it's an amplifier" and less detailed than "Starting with the state equation of an electron in a quantum well..." In particular, read the posts by "free_electron": How transistors work - Page 1
Another good resource for how to use a transistor as a switch:
It includes things like how to determine if a particular transistor will meet your requirements.
Thanks guys.
I keep reading various things, but I don't really grok the transistor yet. Mostly, I think, because while I typically get the gist (functionally -- amplifier or switch), most articles either get into math I haven't studied, or present things in too much of a physics explanation, rather than practical. Also, I just don't spend time working with them, so I don't get the reinforcement of repeated lessons. That, and it seems like my brain isn't as agile or adept as it was back in the 80's, when first studied electronics. One thing I was wondering, just the other day, was if there's a wall chart sort of like these Ohm's law charts, but for transistors. I'm not sure exactly sure what I'd expect to find, but I figure there are some basic rules and characteristics which could be expressed.
As an example, if I'm reading a thread, and someone says something about using an emitter-follower, I can go read about that (just a random example) but it doesn't seem to mesh well with the context. Well, anytime I encounter greek letters, I wish I'd studied calculus, and figure I mostly won't be able to follow the discussion, because I can't read the equations properly.
Anyhoo, I'll check those links out. At some point, I'll have enough various bits tucked into my head that it'll start to mesh.
(But I'm really visually oriented, so a poster would be really cool.)
I think part of the problem with transistors is that there are so damn many of them that have been created since the early 1960's. This leads a novice to conclude that each one must have had a purpose and ideal application... so how on earth are they suppose to be able to pick the right one for their project. Picking the right one is about 1) knowing what your circuit is doing. 2) having a clear idea what you need and creating set of search parameters from those items.
In my mind there are only a few kinds of electronics personalities. (yeah, Ok, there are others, but lets stick with 3... and yes, this somewhat tongue in cheek)
- The hobbyist
They know enough to apply some basic concepts and turn them into reality with maybe some trial and error involved. Basic Skills, can sort of read a datasheet, understands most circuits and most components and can whip up a simple circuit without too much trouble or maybe using a reference like a book, magazine or datasheet. Probably has a soldering iron, some basic tools and a multi-meter. Most probably only follows design work other people have documented. How a transistor or FET works is eluding them but they will gladly take a transistor circuit design from someone and build it.
- The technician
Won't start a project without all the data sheets. Can create a circuit that is a variant of what is in the data sheet. Can repair a lot of electronic things without too much trouble or be able to know when to toss an item in the trash. Knows what transistor "beta" is and understands the concept that "MAX" data sheet values are not where you push your design towards. Understands basic math related to circuit design. Probably has a good soldering iron, tons of tools, multiple meters, home made test gear, a scope and also wants things like a spectrum analyzer.
- The engineer
Knows enough calculus to make the technicians head spin. Will gladly spend 4 hours figuring out how a 2 transistor oscillator works and can tell you, given random values, what frequency is that it operates at. Believes that spectrum analyzers are for sissies since you should be able to whip up a FFT computation and be done with it. Believes that if you don't do the math, you don't know Jack. Overkill.
So really, not everyone is going to be a #3 and a lot of people are already #1's. I think the goal here should be to shoot for the middle where you have the basics down and understood without the need to talk a whole lot "atomic structure" beyond some of the very basics.
Knowing WHEN to use a PNP or NPN or Darlington or FET and how to read data sheets are key goals to getting beyond step #1. In my opinion, most people messing around with Arduino should aiming for Step 2. At least that's where I think the answers should be aimed at.
justjed:
...but I don't really grok the transistor yet.... it seems like my brain isn't as agile or adept as it was back in the 80's, when first studied electronics. One thing I was wondering, just the other day, was if there's a wall chart... I'm not sure exactly sure what I'd expect to find, but I figure there are some basic rules and characteristics which could be expressed.
(But I'm really visually oriented, so a poster would be really cool.)
After 30 years, man, it's like... well, hope springs eternal.
So, here's a "poster" (Take this, may it serve you well) ?
One of the important things about USING transistors is that most of the actual circuits are designed to minimize the effects of variations between transistors. In some sense, the standard topologies like "common emitter" are designed around "ideal" transistors, and/or behavior that is common to ALL transistors. Then you put a 'real' transistor in the circuit while being careful not to exceed its REAL limits, and everything should be basically OK. So you design an amplifier with a gain of 20, and pretty much any transistor with a "beta" (raw gain) of 40+ should work. Assuming max voltages, currents, frequencies, and so on are also within the specs of the transistor.
Lets be clear here - we are only talking about bipolar junction transistors in this thread - there are other kinds too(!).
I think the best intuition about bipolar transistors is that the collector current is a large multiple of base current (unless saturated) and the Vbe is about 0.5V. That's all you need to design most circuits really.
If you want something a bit deeper then think of the base as a tax-collector on the collector current - it takes its percentage or nothing is allowed to happen.
Of course the better explanation is about minority charge carriers flowing across the base, but that explanation only helps you design a better transistor, not use one.
(although the posting I originally referred to is about how/why transistors WORK, internally as a semiconductor, not about how they BEHAVE in circuits.)
Bah...
They work on MAGIC! Le the MAGIC SMOKE out and they quit.
So There!
Hey, I'm only 51! 
Anyways, part of why I'm messing with this stuff is to engage (or re-engage) my gray matter. I could point to lots of people who maintained mental acuity into advanced age. I do have hope.
Thanks. I will endeavor to understand those. Suppose, despite:
westfw:
(although the posting I originally referred to is about how/why transistors WORK, internally as a semiconductor, not about how they BEHAVE in circuits.)
(What, us, stay on topic? XD )
that we attempt to examine that common collector voltage regulator, in terms what free_electron wrote over at EEVblog:
If i am not pulling anything out of the base i have those two darned depletion layers in the way that prevent current from flowing. if i start pulling some electrons out of the base the depletion area is being broken down
So, once the capacitor is fully charged, there will be no base current to "pull electrons out". I have to assume that there's a return through the load which keeps some base current flowing there, so I guess the cap is just for smooting out ripple.
Now he also writes:
If you read mine attentively you will see that i never use the word 'base current'. It is the voltage difference ( potential) that causes the electrons to flow , and the electrons are charge carriers. it i indeed the pressure ( the 'voltage' , technical definition 'potential' that cases charge to move ) i control the direction it is allowed to flow by applying pressure ( or void thereof ) to the base
even though he did talk about base current. Well, I think electrons flowing is current. So anyway, I see this as the load being parallel to Vbe and Vz, with Vbe (i.e. the junction voltage) in this context being -0.8V. As the resistance, or impedance, of the load decreases, then the base current would have to increase, in order to maintain 6V by allowing more Ice to flow. I initially thought that R1 was supplying to current to D1, in order to develop the 6.8V there, but now I have no idea. ETA: I did some more reading. R1 is there for bias?
There. How's that? 
Did some more reading. Still don't fully grok it, but I've figured out that my first impression of R1 was, in fact correct. Actually, I think I'm pretty close; it's just that my mental grasp of it hasn't fully congealed.
Its good to wrap your mind around all this technical stuff so that you get a better grasp of it.