Electronics > Beginners

Talk me through this simple circuit?

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LateLesley:
As others have said, Voltage is just potential difference between two points. The concept of "ground" comes literally from the earth - IE one connection of the power supply is connected to a metal rod in the ground, and that becomes the reference point for voltages - so the ground you stand on is literally the reference point. Now, in small circuits that beginners mess about with, many are battery driven, and there is no connection or reference to ground. But We usually, if it's a single voltage supply take the negative pole as being ground. In multiple rail supplies (like your circuit which has +4.5V and -4.5V) it gets more complicated. You could look at your circuit as being +4.5  0  -4.5, or as 0  4.5  9 , depending on what we decide is "ground". The reason they choose the midpoint in this case, in an oscillator, is to get the idea of positive and negative voltages, and the main reference point of the circuit has the most connections, and Op Amps are conventionally run from a positive and negative supply. (That's not always the case, but common in older circuits and audio circuits). Also, the equipment you connect to, may be grounded (amplifier for example), and it may provide a path to ground via it's PSU and circuit reference point. I'm probably over complicating it now of a beginner, but in general, the negative in single rail supplies, or the midpoint in dual rail supplies are mostly (but not always) considered ground.

The first pic of your circuit, is a wee trick to generate a dual rail supply (+-4.5V) from a single +9V supply, plus an LED with current limiting resistor for power indication. The other two resistors are creating the "Ground" of the dual rail supply for the oscillator circuit in the next pic. We would call that a "Virtual Ground" in this case. Compared to the negative of the 9V battery, it sits at 4.5V, but the rest of the circuit references off that line, and if properly grounded via a wire to actual ground or through other equipment connected to the output, its suddenly seen as 0V, and the battery negative is -.4.5V in respect to ground, and the positive is +4.5V.

Mr D:
Hi folks,

I believe i understand ground and virtual ground etc.

The piece of the puzzle i'm still missing is this:

I keep reading about the ground being needed to "reference" to.

Could someone please give me a ultra simple example of why this is needed in a circuit and how (and what) components or parts of circuits do this referring?
Like i said before, the word "reference" suggests to me a thinking agent that will "refer" to something, like we might refer to a user manual or a wikipedia article.
But an electronic circuit doesn't contain thinking agents, so how can anything in the circuit "refer" to anything else?

This is (i believe) the only part of this ground story that i'm still having trouble with.

timelessbeing:
A bit of English misunderstanding there I think.

Here we use the noun form of the word "reference", not verb.

"The use of a source of information in order to ascertain something."
For example a dictionary.

When you ask the question "what is the voltage?", the correct answer is "compare to what?" because it's always measured in reference to another voltage. The correct question should be "what is the potential difference between these two points?". That's why multimeters have two probes.

You can think of voltage like your weight. When you tell someone you weigh 200lbs, they are able to get an idea of your mass. But how? Your weight is different at home from what it is in an airplane, or in orbit, on the moon etc. We can imagine 200lbs because we all live at the surface of the Earth where gravity is the same. The surface has become the "reference" point.

In the same way, we can tie a segment of a circuit to Earth's electrical potential as a reference point, to which all other potentials can be compared to and measured.

Mr D:
You're not understanding where my remaining difficulties lie, please re-read my previous post carefully.

timelessbeing:
I read your post.

Can you please rephrase the question?

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