[MUSIC]

So we're gonna talk about a few more components that you commonly see in

circuits that we have in our kits actually, switches and potentiometers.

Right here we've switches.

Push buttons and switches.

They're basically serving the same purpose.

Now, on the left side of the screen, you see a switch.

There are many different types of switches, okay?

Many different looks, but that's one right there.

That's called a rocker switch, but there it is and you can see below it,

you can see what its symbol looks like.

It's basically just a little, I don't know, a door on the circuit, right?

And then, next what you see is a push button,

which, you actually should have a push button like that in your kit.

Push button serves a similar purpose and its symbol is below.

Now the point of a switch and

a push button, both of them, is really to complete a circuit, to close a circuit.

So understand that if you've got a battery, let's say,

and you got wires coming out of the positive and negative side.

If those two wires are not connected, you don't have a circuit.

No flow, no current flow.

You gotta have a full path from positive to negative.

So if you put a switch in between that path, if that switch is open,

meaning it is off, right?

Then the switch is open and the circuit is not connected.

But if you close that switch or if you push that button,

which is doing a similar thing, you push that button.

Then that connects the two ends of the circuit, and

it makes a complete path from positive to negative and you can get flow.

So this is how pushbutton and switches are used, right?

They're used to close circuits.

So basically if the circuit is open, no current will flow, nothing's gonna happen.

You close the circuit by flicking a switch or pushing a button, and

then current flows and something happens, whatever you want to have happen.

So those are switches and push buttons, they close the circuit.

Note that these switches that we're showing.

Okay, so let's look at this switch on the top left, right?

That switch has got two terminals, right?

One left, one right.

And when you flick that switch, those two terminals are connected.

So when the switch is off, the two terminals are not connected.

When it's on, then the terminals are connected.

When those two terminals are connected, their voltage is exactly the same, right?

It's a consequence of Ohm's Law, right?

If there's no resistance between two points.

Or actually, not Ohm's Law.

It's Kirchhoff's voltage law.

[LAUGH] Different law.

But if you've got two points and they're connected with no resistance in between

through a switch like this, then they will have the same voltage.

Where if they are not connected, then they can have different voltages.

Oh and one note I should just bring up right now, the push button that we have

there, that actually is very similar to the one you have in your kit.

That thing has got four legs on it.

It's hard to see.

You can only see maybe three of them here, but there are four legs.

So it doesn't really just have two terminals.

This one has four.

But two of the terminals are connected to each other.

So the two on the left are connected to each other, and

the two on the right are connected to each other.

So when you press the button,

the two on the left become connected to the two on the right.

But there are many different varieties of switches and push buttons, but now,

potentiometer.

Now this is an interesting component.

We've got one, there are many different looks for potentiometers.

I believe yours looks like this one.

Now, this potentiometer, hard to see, but it has three leads in it.

Now, the blue thing, that's the potentiometer.

And notice it has a knob, hard to see, but it has a little knob that you rotate.

All these potentiometers, they have something that you move,

either like a rotary potentiometer, like this one, has a knob that you rotate.

You can have sliders that go back and forth that do the same thing, right?

Now, it has a schematic symbol and you see that in the middle there.

So it's like basically a resistor, with a little arrow in the middle, right?

That's a potentiometer, that's its schematic symbol.

And notice that schematic symbol has three leads also.

The resistor has a top lead, a bottom lead, and then there's that arrow,

which is a third lead, right?

So the three leads in that potentiometer correspond to those three leads in

that schematic symbol.

Now then, over on the right, that circuit that we have there with the two resistors

and with the voltage in and the voltage out and all that.

That is the circuit that I'm going to use to describe the behavior of

a potentiometer.

Cuz a potentiometer acts like that circuit.

That's what I'm telling you.

That circuit is called a voltage divider.

So, remember the potentiometer has three terminals.

And that circuit's called a voltage divider.

Now if you look at that circuit, at the top, there's this V in, okay?

And at the bottom there's ground.

So those two, a V in and ground, those are two of the leads on a potentiometer.

The top lead and the bottom lead in the potentiometer are the V in and the ground.

And then the middle,

that little arrow in the schematic, that is that V out right there.

That's what you're actually measuring typically, okay?

So the resistance from the top to the bottom, from one side of the potentiometer

to the other, so from V in to ground, that resistance is constant no matter what.

You've turned the knob.

The total resistance from the left side of the potentiometer to the right side of

the potentiometer, or top to bottom is gonna be constant.

Now the total resistance from V in to the ground is gonna be R1 plus R2,

those two resistors add together.

That sum is constant.

So for instance, if you've got a potentiometer, it is a 10k potentiometer,

10 kiloohm potentiometer.

Then the sum of those two resistances from one lead to another is always 10 kilohms,

but what changes is that V out right there.

Cuz what changes is as you turn that knob, the ratio between those two resistors,

R1 and R2, the ratio of their resistance changes.

So if you turn the knob one way, R1 gets smaller and

R2 gets bigger, even though the sum stays the same.

So and you could turn the knob other way and the reverse happens,

R2 can get smaller, R1 can get bigger.

But the sum stays the same.

So as you turn this knob,

the voltage that's observed at V out changes as you turn the knob.

Because what happens is, that voltage, okay,

let's go back to our Ohm's law, right?

V equals IR.

So we wanna measure V at V out, that point V out in between those two resistors.

That V goes I times R, now I is the same for both resistors,

cuz there is only one wire that everything has gone through but the R changes, right?

If you could make R get bigger and smaller, R2 specifically.

The R2 is the resistor between V out and the ground, okay?

That R is changing as you turn the knob.

And since V equals IR, if R is changing and

current is constant, then V is changing.

So as you turn this knob, you're changing resistance,

which changes the voltage that's perceived at V out.

So that's how we're gonna end up using potentiometers in general, and

I'll show you a circuit like that.

Thank you.

[MUSIC]

Lecture 2.3 - Switches, Potentiometers

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