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So this little guy I have here is ah, Transistor and ah, this schematic symbol for it over here is is this diagram here?
Ah, and you can see just kind of orient.
There's the three leads on the transistor.
And those correspondent that three things here.
So we've got an emitter, a bass and a collector.
And just to kind of orient ourselves the this would be the emitter.
The middle lead is the base, and this lead is the collector s.
Oh, that's just gonna have this corresponds to the schematic symbol for it.
And what a transistor is is.
It's basically a current switch.
What that means is that if we establish a current from the base to the emitter in this direction, that'll switch the transistor on, and the transistor will allow a much larger current toe Flo from the collector to the emitter.
So what we could do with that is something like this, which is the circuit that we've got set up over here.
And basically what this circuit does is we've got a switch that allows us to complete a circuit that allows current to flow like said from the base to the emitter from the base to the emitter.
So if we close this this little switch, push this button.
Um, that establishes a circuit like this and causes current to flow from the base of Demeter that turns the transistor on once the transistors turned on, then current is then allowed to flow from the collector to the emitter in this direction.
Ah, and that allows this sort of second part of the circuit thio work where current will then flow from the battery here, through this resistor, through this light emitting diode or led which will turn that on will see this light up on, then through the transistor.
And then if we aren't putting current from the base to the better than transmitter shuts off.
And so this this other part of the circle shut off in the lady was off s so we can see in the circuit the first part of the circuit we've got from the positive of altered rail.
We've got this resistor that goes through the button.
Oh, and then into the base of the transistor on then that will allow us to get that first current flowing to turn the transistor on and that'll go back out through the negative terminal here.
And once the transistors on that will allow current flow from the positive rail here through the resistor through the led into the transistor through the transistor.
Now that it's on on then out through this this negative rail.
So if we push the button transistor turns on and consequently the led turns on Okay, so far, so good.
Let's take a look at another circuit, which is on the other half of this bread board here and on this circuit.
Um, I've got something a little bit different going on.
We've got the, uh the transistor is is off.
But with the transistor being off, we have this part of the circuit here which actually allows current to flow through this resistor and then through the l E D S O.
The led is actually already on which you can see right here to the led is on right now.
But if we turn the transistor on there, Well, actually, um, sort of allow current to flow through the transistor here, and that'll lower this voltage potential at this note on.
So instead of having ah, positive voltage here that allows current to flow through the led we We won't have a voltage differential between the two parts of the transistor when the transistor is on on so that I actually have the effect of turning the led off.
So if we do that, we see the lady turns off.
So in this case we're using this transistor is a switch to kind of invert the, um the voltage here.
So if we look at the voltage here right now, it's zero volts because this switches is open and the led is seeing, you know, a positive voltage.
And then if we turn the switch on, we have a positive voltage here.
But then we get zero volts differential here across the led and led turns off.
And so this is it.
This little circuit is called an inverter, and there's Ah, there's actually a symbol for and that's this little triangle with a circle.
And there's a sort of a truth table for the logic.
It's pretty simple.
If the input is off, the output is on or if the input zero, we kind of use that as a represent off.
The output is on.
If the input is on and the output is off Pretty straightforward.
So let's look at some other logic circuits that we can make with transistors.
Okay, this next circuit that will look at we've got two transistors, just kind of this left half of the board here.
We've got two transistors, and the way it works is we've got these inputs which, which again or kind of haven't drawn the whole the whole circuit out boat.
It's We still have this resistor coming from the positive rail here through the push button switch and then into the base of the transistor, and so that allows us to put a current through that transistor.
And we've got two of those.
And so what happens is that if both transistors are turned on, then that will complete the circuit through this resistor here on then through this led and then through the first transistor and then over here and through the second transistor on.
So if both trends of areas both transistors turned on, then the led will turn on.
And this is, ah, little piece of logic that is called an and gate and ah, this is the truth table for it.
This is the symbol that we would use if we were drawing up more complex logic circuit.
We would we would actually, instead of drawing all the transistors, we would just kind of draw this this whole circuit as just an and gate using this symbols kind of rounded on this side and squared off on this side and the truth table, for it says that if a and beer off, then the output is off, and that's kind of what we have now.
So both of these are often not pushing either one, and the output is off.
Um, if B is on an A is off, then the output is also often we contest that just pushing the button and the lady doesn't come on.
Same thing if a is off, receives me today is on, and B is often The output is also off.
We contest that by pushing the button here, but if both of them are on, then the output is on.
That makes sense both of these air on then that completes the circuit all the way through.
And so if I push both the buttons, then you can see the lady comes on because we're completing the circuit all the way through.
So how about this other circuit over here?
It's a little bit different.
Show you the circuit diagram.
First on the circuit diagram for this is a little bit different so that we still have two transistors.
We still have our led, Um, but they're wired up just a little bit different.
So in this case, uh, the transistors are kind of in parallel on.
So if either of the transistors is on, then then the lady would come on because we can complete the circuit through this transistor or through this transistor.
So we look at the circuit kind of closely here.
We can see that from the positive rail we come through this resistor and then through the led, and then we can go through this first transistor.
So from the ER, you know, the collector here to the emitter, Or we can actually kind of come over to this transistor in through the collector and out through the emitter on.
And then so if either of these transistors is on than the lady will be on it so we can test that out.
So if we turn this one on comes on.
If we test turn this one on it comes on as well on.
So this is something called an or gate.
Um, which makes sense.
If this or this is on, then then the output is on on.
So this is the truth table for it.
So if both are off, the output is off, which is what we see here.
But if one of them is on, then the outputs on if the other one is on the output also on and also have both of them are on because then that will just complete the circuit.
Either way and course, Either way, current will flow.
And so it will be on.
So we push both buttons, you can see it's on.
So either way for both doesn't matter.
It'll it'll turn on so you can see here.
We've got two different circuits.
They're very similar.
Very similar, but slightly different.
Different Operation one isn't and operation the others and or operation.
So let's try.
Let's take a look at something a little bit Maur a little bit more complicated.
So this is a different board here, different circuit, and let me just power this up So this we've got actually five transistors in this circuit and ah, it is basically the same kind of idea here.
We've got two inputs that weaken.
You know, in this case, turn on and off with these little push button switches and we have got an output over here, which is this led and what we've what we've built is is what's called an exclusive or gate or an X or a gate.
And it's similar to the or gate, which is, you know, it's off if if both A and B are off and it's on if a or B is on.
But what's different is that it's off.
If a and B are on, um and so we can we can actually see that operation.
So right now it's off because neither of them are on.
But if we if we turn on a yeah, come, the output is on, and if we turn on B, then the output is on.
But if we turn on both at the same time, the output goes off and I kind of understand how that works.
Um, I've got the circuit diagram for for what's built here, and you can see that if we just look at this part over here, it looks a lot like the or gate.
Um, because we've got the five old source going through our resistor through the led.
And that's what's going on here for five whole source going through the resistor through the led and into either of these transistors so it could go through either of these transistors.
Um, and then the inputs for those transistors are the NBC.
It's yellow wires.
Come over here and then jump over to the to the switches.
So if either this or this is on the nettle, complete the circuit at least through to this point, um, and so in the or gate we just had this connected directly to ground, and so few this or this were on them that would complete the circuit all the way to ground.
But in this case, in order to get to ground, we've got to turn this transistor on to, um otherwise, you know, we've got a five old sort of connected back to five volts, and there's no voltage potential difference between five and five.
So the lady is off.
So in order for this or gate toe work.
Essentially, uh, this transistor has to be on, and that turns Esther is is actually normally on, Right, Because we've got this connected right to five volts.
We've got current flowing, you know, from the base to the emitter.
Eso this will normally be on.
Unless unless A and B are on a and B are on, then, uh then this.
Ah, this note here's is then gonna be connected to ground That'll turn off this transistor.
So if a and B are on, then this transistor is, uh is turned off.
And if this transistor has turned off, then this or gate essentially doesn't work, which is kind of what we want to do it right.
If a and B are on, we don't want to really do the same thing as the or gate.
We just want the output to be off.
Eso that is, that is essentially what's what's going on here.
And so that allows us to build.
You know, from, you know, takes us five transistors to do it.
But it lets us build this this other logic element, which is this this x or gate.
And so again we can see it work.
If we have a on it comes on, if we have be on it comes on.
But if we do both at the same time, it's It's not on.
And of course, if they're both often in the operatives off as well.
And so these are just a couple of the different gates that you can build with transistors, and here is actually kind of a whole list of different ones.
So we saw, you know, the and gate the or gate.
We just saw the X or gate on.
We're course looking at the inverter here.
There's also you could make a buffer, I guess is actually kind of the first thing we made with the transistor, which is the input was on the on than the output was on dimples off the apples off.
Not very exciting, but it can be useful.
Um, and there's a couple others that we that we could also make that we didn't look at, which are the knot and or nand the knot or and the not explorer of the ex, nor it's called on, and these are basically the same as the and gate, but the output is inverted on.
If actually, if you look at the symbol, you have this little little bubble on the output there, which just means that the output is inverted on.
So it's the basically the exact opposite.
You know the ex nor is the opposite of the X, or they ignore is the opposite of or and so on, so you can build any of these with with a few transistors on.
And then, of course, you can take these logic gates and then build more complex circuits that allow you to do arithmetic or store values in a memory and other things that allow you to build a computer.


Making logic gates from transistors

林宜悉 2020 年 3 月 28 日 に公開
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