Hi again, thank you masked!

Here's a new diagram that I've labelled with arrows representing the flow of electrons through the circuit:

I understand that at any given time only two of these bolded arrows will be in action, when A is low and B is high or vice versa, and that the current will go from X -> Q4 -> M -> Q2 -> +.
So, for instance, when the base of Q4 is high, a small current will flow from Q4's emitter to it's base allowing a larger current to flow from emitter to collector, through the motor and from collector to emitter in Q2 (when Q2's base is low) and back to the positive side of the voltage source.

Is that the general idea?

1. Im a bit confused as to the current coming from X up through Q4's emitter and to its base. Where does it go? A and B will be each be connected to a pin of the parallel port (after some protective intermediary which I have yet to investigate). As a result the flow of electrons from paralell port pin A will be heading south to X, which is connected to the parallel port ground pin and the negative terminal of the main voltage source. But this cant be right, as the parallel port ground pin should be positive (defecit electrons) and the negative terminal of the main voltage should be negative (surfeit of electrons). So I think something is quite incorrect in my interpretation. Have I at least gotten the flow of electrons through the main voltage correct?

2. Ive also labelled a branch with 5mA, because my parallel port provides a little less than 5V high and with a thousand Ohms of resistance thats 5mA. Im looking at this now though and because of the above flow direction issues reconsidering the validity of that notion.

3. Im also still a bit confused as to the inverters. If they are to prevent the main circuit from effecting A and B, why could'nt there simply be diodes on each input?

4. Here is a diagram with new diode locations added in:

Im pretty sure thats correct now. And I've added an arrow designating the backwords flow of electrons. How about that?

5. One quick less relevant question: will I be able to switch out different motors in this same configuration, controlling the main Voltage accordingly but leaving everything else the same?

 

I'm just guessing, but:
I think questions 1,2, and 3 are all related. the type of "protective intermediary" you use as a buffer will affect the answers.

I'm not really sure why the parallel port ground would be positive or be connected to the negative of the main voltage source in question 1, so I don't think I'm understanding the concern.
In general, I take it that the purpose of the inverter is to isolate your port from the H-bridge circuit. In the case of an opto-coupler, the signal gets turned into light, and a phototransistor closes/opens a circuit between your main +V and ground, but the circuit is separate from the signal operating the LED and grounding elsewhere. For an inverter chip, I don't know the mechanics.

Yes, the diodes look correct to me.

In general, yes I think you can swap similarly small motors. It depends a bit on your input signal and transistor ratings though. From what I understand, you'll be sending a 5v signal which should be more than enough for any load handled by small transistors. The general purpose ones I've seen max out at about 100mA across ce, and have a gain of around 100. I think that means that if you draw the maximum current of 100mA, you would need a base current of 10uA to drive it.

If you start trying to drive a higher voltage motor than your inputs, you run into different concerns though. Normally, when your PNP transistors have +5v at the base, they are turned off, but if your mains are +12V, then the 12v emitter will be higher than the 5v base, and the PNP will turn "on" even in the high-signal state, and allow the 12V from the emitter to flow back into the base and into your inverter.
It also means that Q1 and Q2 would turn on at the same timne as Q3 and Q4. ...which creates a short circuit.

Hope that helps. Wish I knew more about interfacing parallel ports, so I could have been more useful.

-masked

 

I still don't understand entirely, but I think I have only a few more questions to ask and then everything will become clear.

I know electrons have negative charge, so in real life they move from the negative pole of a battery or voltage source to the positive pole. The negative pole having a surfeit of electrons and the positive having a defecit.

BUT, do we then by convention simply label the negative pole as positive and in all our symbology act as though the electrons are positively charged?

So in the circuit here:
http://www.solarbotics.net/library/circuits/driver_4varHbridge.html
The electrons will actually be flowing from +2-5V to the ground. So they will be following the arrows of the bipolar junction transistors instead of going against them?

But then why do my diodes pictured here:
http://forum.allaboutcircuits.com/attachment.php?attachmentid=21643&d=1281209829
make sense in this context?

I hope someone can clear it all up for me, this I think is the main confusion ive had from the very beginning, I need to finally understand exactly in which direction the electrons are heading, and what we've labelled them as.

Thank you!