But always there's the current running to ground after the bulb or speaker or 8seg display?
I'm not sure what you mean by "after". Electricity (except for static discharge) flows through a circuit. Any output to a bulb, speaker or display will put a current through that circuit element (typically) to ground. It's not really "after" - it's an essential part of the single circuit function of making the speaker cone move or the bulb to light up.
But then i don't see how such a complicated scheme could work. You change one resistor value somewhere and won't that have a knock-on effect thoughout the rest of the circuit?? How can this be manageable in a complicated circuit? Or are there ways to sandbox parts of the circuit against this sort of chaos?
This is a good question - and leads us into the topic of impedance. Actually working through this in a "complicated circuit" can become a bit of a rabbit hole - so be careful about jumping in too deep! However, you don't need a bagful of maths to understand the principles.
Interaction between stages (where a "stage" is a group of components that do a specific function within a larger circuit) is, indeed, a consideration in circuit design and the primary factor involved is impedance.
Let's look at a circuit that processes a signal. It will have an input and it will have an output. That output will, in turn, be used as an input to a following circuit. Now the question is - how much will the
following circuit affect the circuit we are looking at when that input is connected to our output?
The answer is impedance.
If the output of our circuit has a *"high" impedance, then the signal it produces will look like it is coming through a *high value resistor and it will not be capable of providing much current, before the voltage drops down to an *"unusable" level. If it has a *"low" impedance, then the signal will look like it is coming through a *low value resistor and it will be able to provide a *fair bit of current with the voltage dropping slightly.
If the input of the following circuit has a *"low" impedance then it is going to look like a *low value resistor that is going to want to pull a *significant amount of current, but if it has a *high impedance, then it will not need much current for it to be able to process the signal.
The simplest way to look at this is with the following diagram. It is
very simplistic and will not reflect what you will actually see in schematics - but it demonstrates the principle.
Here, Z
1 will be the output impedance of the previous circuit and Z
2 the input impedance of the circuit that follows. How these impedances actually come about are the subject of some maths - maths that
can be bewildering - but, for the sake of simplicity, just think of it like this and you will get the "feel".
Plugging in High/Low Input/Output combinations, you should notice the preferred approach is for the output impedance of one circuit be low
compared to the input impedance of the following circuit as this will minimise the effect of connecting the following circuit to the one we are looking at. The greater this ratio, the less the interaction will be.
This principle applies not only to electronics, but to an extremely wide range of physics as well. However, sticking to electronics, this matter of impedance is pervasive. It is everywhere. If we take a simple example of a PA system, we have to consider the output impedance of the microphone and the input impedance of the pre-amp; the output impedance of the pre-amp and the input impedance of the power amp ... and then the output impedance of the power amp and the speaker impedance.
But this issue of impedances also exists
within any circuit. Each section of circuit that performs a particular job will have an input impedance and an output impedance. For example, inside the pre-amplifier, you might have a transistor or two to boost the signal from the microphone to a level that makes it easier to work with. This signal might then go through some tone control circuitry. Then it might need a circuit that looks like an amplifier, but doesn't increase the signal level. Such circuits are often used as a "buffer" where its input impedance is such that it does not affect the previous circuit (which may have a *high impedance) but provides a *low impedance output for whatever is subsequently connected. (It is not uncommon to see unity gain buffer amplifiers - and that is their specific purpose.)
As these chain along the circuit, these impedances must be taken into consideration to ensure the signal is passed along in a useful manner.
* These terms are relative. (This is important!) What's painfully high in one situation might be insanely low in another.This issue of impedances also extends into digital circuits and is the effective issue behind them as well. We see them expressed more as the current capability of an output or the specification for an input which makes life easier as it hides away the internals (unless you
really want to go deep) and just provides the numbers you need to know for a practical circuit.