 Hi, I'm Zor. Welcome to Inizor Education. Today I would like to present, I would say, the top of the knowledge which I would like to convey to you as electronics implementation of certain things. It is related to something very useful, which we are using every day, most of us, at least. It's the memory of the computers. All these different devices, like triodes, diodes, etc., they are good by themselves, obviously, but they're supposed to serve certain purposes and they have served very important purposes in radio, for instance, or television. With development of the computers, there was a completely different kind of task. They should maintain certain state, which can be associated with memory. If something is in one state, it can be one, and if it's another state, it can be zero, and the states should be stable. So memory, computer memory, is characterized by the fact that we can set it to a certain value, and we can read from that particular value later on, and then we can set it to another value and then read it again. And obviously, ones and zeros are sufficient to basically store anything, because with the combination of ones and zeros, we can store any information, like numbers, for instance, for instance, in binary system or characters, and using some kind of codes, eight bits, ones or zeros, per character, or 16 or 24 bits, or whatever. Even the Chinese set of higher rogues, we can actually code in some ones and zeros, right? So I would like to present an implementation of one particular bit of memory in this lecture. How this bit of memory can be implemented, which means it can be set, it can be reset, it can be set to zero, it can be reset back to zero, it can be set to one, and can be reset to zero, and that is a stable state of this particular circuit, which I'm going to present to you. So this is one of the first implementation of the memory cell, which is used in the computers. There are many others, and contemporary computers probably are much more advanced as far as their design than the first implementation of the memory cell, but this is something which we can actually present to you right now using whatever the knowledge you have already acquired from the previous lectures, about diodes, about triodes, about logical operations and or not. So that's what this lecture is about. Okay, now this lecture is part of the course called Physics for Teens, presented on Unisor.com. I do suggest you to which this lecture from the website, from Unisor.com, instead of straight from YouTube, where it's physically stored. Well, primarily because the website contains a course, which means there is a menu, there is a sequence of the lectures. Every lecture has a textual notes to it, which basically is like a textbook, and I will use some pictures, but my notes are kind of a little bit better as far as the pictures are presented, and in some cases I present maybe a little bit more details or slightly different explained in the textual part than whatever I have explained verbally during the video presentation. So I do suggest you to go to this website, Unisor.com. Also it contains prerequisite course, which is called Math for Teens. Mathematics is must to properly learn physics. So, yes, and it's completely free, so there are no ads. You don't even have to log in unless you just want to take exam and remember, and you would like system to remember actually your previous marks to analyze, etc. Not necessary. Anyway, let's go. First of all, I would like to remind you what we have learned in the previous lectures. We have learned about how diodes are working, how triodes are working, and how using diodes and triodes we can implement logical operations on the signals. Now the signals we are differentiating like ones and zeros. One means some positive potential on the terminal and zero means zero potential basically is grounded or somehow connected to the ground. So we have zero and one, zero and positive potential. And logical operations are binary like and and or. So the end operation, which sometimes we are using this symbol like ampersand, and you have something which I would just symbolize as a box. You have two signals coming in and you got one signal coming out. So these signals can be in one of these four different states. Zero, zero, zero, one, one, zero, and one, one, which means zero potential, zero potential, positive potential, zero, zero positive or positive, positive. And in all these cases we have some output. The output from zero, zero will be zero, output from zero, one will be zero, this will be zero, and this will be one. This is how end is working. Okay? And that's basically it corresponds the logical definition of conjunction, operation of conjunction, on truth and false. One usually is associated with truth and zero is usually associated with false. So truth, sorry, false and false will be false. False and truth will be false, truth and false will be false, and truth and truth will be true. Then there is an operation or which we use a vertical bar. And this is also the same kind of a binary operation, two inputs and one output. And the same inputs would produce slightly different outputs. This would be zero, that would be one, one, and one. So false or false will be false, false, false or true would be true, true or false would be true, and true and true, or true would be true. Then there was a unary operation, not. It actually has only one input and one output, that's why it's unary. Binary, unary. Now, so here we have either zero or one, and not is just reversing the various. Not false is true, not true is false. So we have gone through this. Now, the next step, which is a very important implementation used in implementation of the memory cell, which I'm going to talk about today, is operation, which is a combination. It's not of or, or, or. Now, what is this operation? Well, first we implement the operation or, which means a or b. And then we implement operation not. Sometimes it's this type of character. Sometimes it's a horizontal bar about this. It doesn't really matter. This is symbolizing the operation not. So this is what we need. Now, what is the logic similar to this one? Well, let's just think about it. First, we have to implement, so 0, 0, 0, 1, 1, 0, 0, 1, 1. Okay. So what would be the result of this? 0 or 0, 0 or 0. This is operation or. And this is operation of and. So 0 or 0 would be 0, right? But then we implement the not operation, which is a reverse. So the result would be 1. 0 or 1, in this case, would be 1. But if we operate the not on it, we will have 0. Similar here. And 1 or 1, truth or truth, gives me truth. But then we are inverting and we will get 0. So we'll get false. So this is so-called a truth table for operation nor. And operation nor would be in the heart of our implementation of the memory cell. So let's just recall this. Remember this. And this is a little bit new, but it's just nothing but a combination of these. We just introduce a combination of two elementary operation and call it nor. Basically, it's a not or, a combination of these two. So recall this. And now we will go to implementation of the memory cell. But the name of the device, which serves as a memory cell, it's either flip-flop or latch. Well, flip-flop is called, because it can be either in one position, like 1, or in another position, 0. Memory cell should be able to store and stably store. So it should be stable in such a way that whenever we send a signal, we can, let's say, set it to 1. But then when the signal is gone, it should stay in the state 1. If we reset it, for instance, and set it in state 0, and then again the signal is gone, then it should maintain the state 0. So that's exactly the essence of the memory cell. Because before when we were talking about triodes or diodes, we were talking about, okay, if this potential goes this way, then the result will be that. But if potential changes, the result will change. Here we are talking about signal processing. So we are sending a signal. Now, what is a signal? For instance, we are raising potential, and then it goes down back to 0. From 0 to something to plus 5 volts, and then it goes to 0. If we are sending that signal, we will set certain potentials at certain output terminals. But when the signal is gone, the potential should stay exactly where we left it. That's what means memory cell, and that's what differs in a memory cell versus some other plain triodes, for instance, or plain circuit which implements N signal. With N signal, whenever we change something on the input, the output would change, generally speaking. Here we are going up with changing something on the output, then signal disappears, and the output stays. That's what the memory means, memory cell. So that's why it's called flip-flop. It can be flipped or flopped in this or that position, and sometimes it's called glitch. One more terminology. The last one which I was talking about, nor thing. Sometimes it's called gate, nor gate. And let me just repeat exactly what nor does again. Zero and zero. Or of them would be zero, and not of zero would be one. I'll put some kind of a star here. Zero and one. Or between zero and one would be one, and negation would be zero. Same thing. Or between one and one would be one, and negation would be zero. So that's the truth table, and we will use this. So this is how nor gate is working. And I'll use a symbol for this. The symbol for this is, these are two inputs. This is output, and this is my nor gate, implementing this. So schematically, it's a combination of the or logic, which we know what it is. The or logic has two inputs and one output. And then we implement a not logic on the output of the or, and the output from the not would be this one. So inside of this box, we have two different circuits. One circuit which implements or, and how it's implemented is in another lecture, by the way, the preceding one, one of the preceding ones, on electronics. And actually in my textual part of this lecture, I present the schema to implement or, again, just for repetition. And then on the output of that schema, there is a schema which implements a not operation. Again, I present it in my textual part. And then the final is a combination of both. So the output from the or is connected to input to not, and the output from the not gives me the output from the entire circuit. It should be very easy to understood. So it's two different kind of circuits which we had before. Or circuit and the output of the or is input to not. They're just connected. And again, if you would like to complete picture, complete picture is in the textual part for this lecture. Both of them are in one picture. You have to really enlarge it, click it and enlarge it to see all the details. So this is done. We have this particular device which is called NOR gate. And that's how it works. Using the NOR gate, I'm going to present you a circuit and explain why it can serve as a flip flop or the latch or memory cell. Okay. So here is the scheme. I have two NOR gates. Now I have two inputs here. Two inputs here. And I have connection here. And I have connection here. This is reset R. This is S set. This is Q. And this is not Q. These are output. Well, that's it basically. Now let's talk about now. So this is a NOR gate. And this is a NOR gate. So let me talk about output again. These are two terminal output terminals from this circuit. And these are and S are two input terminals. So R is called reset. S is called set. Obviously, if we send an impulse to set, I expect that my Q and NOT Q would be in some particular state like 1 and 0. If I stop the signal, so it goes like to the positive and then back to 0, it should stay this way. Q still should maintain positive. And Q and NOT Q should maintain negative. Now if I send a signal to this terminal again, it was 0. Then it was some positive potential. And then again 0. When I set the signal to this reset terminal, my Q should be 0 and NOT Q should be 1. And again, when the signal stops, again it should maintain the state where it was before. That's the memory implementation. That's sense of the whole thing. So as soon as we send an impulse here, this is 0 and this is let's say plus 5 volts. Whatever state it was before doesn't really matter. But as soon as I raise, this would be positive and this would be negative. As soon as it goes down, it still maintains the positive potential and still maintains negative. That's the sense of it. And if I send signal to this, again this is plus 5 volts. So it was 0, doesn't matter what it was before. But as soon as I raise the potential to certain voltage on this terminal, this will become 0 and this will become 1. That's how we can set it. So the state of the flip flop, state of this memory cell, it's either 1 0 and it's a stable state or 0 1. And this is also a stable one. So whenever we send signal to this terminal becomes 1 0, whenever we send to this becomes 0 1. And that's how the memory is basically working. We store one bit of information. It's in the either state 1 or state 0. Now let me explain why it works. Okay. Okay, so let's consider we maintain r at 0 and we send a simple impulse to s terminal. So we have r terminal and s terminal. r is maintained at 0 and s has this impulse. So what happens? Okay. As soon as we raise the potential here s becomes 1, right? Plus means 1 and 0 means 0. So as soon as we raise the potential to certain voltage, this becomes 1. Now first, if you remember, we go to the OR logic and then to a NOT logic. So first my OR logic works between this one and this. Now OR logic works, if this is 1, then independently of this one the result from the OR logic would be 1, right? Okay, I have to write it down more. So 0 and 0 gives me 0, 0 and 1 gives me 1, 1 and 0 gives me 1, 1 and 1 gives me 1. So if one of my terminals is at 1, the result will be 1 regardless of the other terminal. Okay. So if I raise the potential here, this is 1. Regardless of this one, I don't really care how the state of this and I don't care about the state of this. So it actually is an input, but again if this is 1, regardless of the input result of the OR part would be 1, then NOT works. NOT of 1 is 0. So the result of this would be solid 0 immediately. So as soon as I raise this to 1, this goes to 0. Okay. This is what happens when I raise this issue. Whenever this is 0, it goes here and R is only on 0. We are raising only one of these two inputs, if we want to set, and another if we want to reset. So whenever we raise potential on this one, this is maintained as 0. These are controlling terminals. We control them. If we want to set something, we send the signal positive potential impulse to this one and we maintain this at 0. So this is 0 coming here as an input and another 0. 0 and 0 gives me OR 0 and then NOT of 0 gives me 1. So this becomes 1. So as soon as I set potential to plus 5 or whatever voltage is, my this immediately goes to 1 and then to 0. Sorry, this goes to 0 and this goes to 1. What happens next? Next my potential drops down to 0. So now we have this 0 and this 0. Both are 0s. What happens in this case? Well, let's see. This is 1. It goes here and this is 0. So the result of the OR would be 1. Then NOT works and NOT of 1 is 0. So it's still 0 and it was 0. So whenever we drop my voltage down to 0, there is no change. It's still 0. Now, this 0 goes to here. Again, this is 0 and this is 0. The result of the OR would be 0 and then NOT would be still 1 and it is already 1. So basically when I'm dropping from plus to 0, my variance of potential on these two output terminals doesn't really change. So this voltage, when it goes up from 0 to 1, it sets 1 0 and then when it goes down from 1 back to 0, it remains in this state. And that's what's very important. That's how we set this particular flip-flop, this particular memory cell, this ledge we are setting to a certain state. State is 1 0. So whenever we are gauging, for instance, potential from this terminal, it would be always 1 if we will send an impulse here. So we send an impulse. After this, this becomes solid 1. It doesn't change. And this becomes solid 0. It doesn't change until we want to change it. Now, how do we want to change it? Well, we want to reset it. We want to set it back to 0. I mean, if we want to set it again to 1, we again send the impulse to this one. But if we want to set it to 0, we send a signal to this terminal. Now, what happens in this particular case? Well, instead of explaining everything from the very beginning, look at this. It's exactly symmetrical kind of thing. So if you will flip it this way, it would be R at the bottom and S at the top, which means this Q would be there and this Q would be there. So it would be exactly the same thing. So whenever we are setting this to 1, this becomes 1 and this becomes 0. Which means we are actually resetting. Q would be 0. So if this particular terminal is a main output terminal, so it's either in the state 1 or in the state 0. Don't even have to think about this terminal. So the picture is completely symmetrical. If we set this one with an impulse, this one would be 1 and this one would be 0. If we send an impulse to this, it would be corresponding to this one and this 0. So completely symmetrical thing, you can just go through a logical exercise yourself, the same one as I did right now and you will get exactly the same result, obviously. So this is how the first memory cells were invented. One bit, it's only one bit. Now, obviously, contemporary computers had trillions of information, bits of information and obviously everything is somehow implemented in silicon, etc. So we're not talking about this how my purpose was to explain how using the regular electronic components, whatever you can even purchase if you wish, you can create your own memory cell. Just one bit, obviously. But that's how computers started. They started with one bit of memory. Then they combined it into a byte, which is 8 bits and then they introduced some logical operations, etc. But basically this is the beginning. This is how the thing is built from inside. Now, there are many other implementations of this particular device, which we call flip flop or latch, or you can call it memory cell, if you wish. One bit memory cell. So implementations are different, but that's not the point which I would like to make. My point is that you can make an electronic device which stores information. Stores means that we can send some impulse to some terminal and it will be in one state. Let's say you will have a positive potential on this particular terminal and it will stay that way as long as we still maintain the whole device intact. And if you want to set it to another state, let's say you want to have zero potential here, you have to send a signal to another terminal, another input terminal, this one in this case, and it will become one. Basically that's it. I do suggest you to read the notes for this lecture on Unizor.com. Well, basically that's it. It ends my electronics component of this course. Then we will start something new. That's it. Thank you very much and good luck.