Making logic gates from transistors

so this little guy I have here is a transistor and the schematic symbol for it over here is is this diagram here and you can see just to kind of orient there's the three leads on the transistor and those correspond to the three things here so we've got an emitter a base and a collector and just to kind of orient ourselves this would be the emitter the middle lead is the base and this lead is the collector so that's just kind of how 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 to flow from the collector to the emitter so what we can 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 I said from the base to the emitter from the base to the emitter so if we close this this little switch push this button that establishes a circuit like this and causes current to flow from the base of the emitter that turns the transistor on once the transistor is turned on then current is then allowed to flow from the collector to the emitter in this direction and that allows this sort of second part of the circuit to 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 let's see is light up and then through the transistor and then if we aren't putting current from the base to the emitter then transit or shuts off and so this this other part of the circuit will shut off and the LED will shut off so we can see in the circuit the first part of the circuit we've got from the positive voltage rail here we've got this resistor that goes through the button and then into the base of the transistor and then that will allow us to get that first current flowing to turn the transistor on and that will go back out through the negative terminal here and once the transistor is on that will allow current flow from the positive rail here through the resistor through the LED into the transistor through the transistor now it's on and 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 red board here and in this circuit we've got something a little bit different going on we've got the 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 LED so the LED is actually already on which you can see right here so the LED is is on right now but if we turn the transistor on then we'll actually sort of allow current to flow through the the transistor here and that will lower this the voltage potential of this node and so instead of having a 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 and so that will actually have the effect of turning the LED off so if we do that we see the LED turns off so in this case we're using this transistor as a switch to kind of invert the the voltage here so if we look at the voltage here right now it's zero volts because the switch is is open and the LED is seeing a 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 the LED turns off and so this is this little circuit is called an inverter and there's there's actually a symbol for it 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 inputs is zero we kind of use that as a to represent off the the output is on if the input is on then 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 we'll look at we've got two transistors it's just kind of this 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 are kind of I haven't drawn the whole the whole circuit out but 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 that allows us to 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 and then through this LED and then you know through the first transistor and then over here and through the second transistor and so if both transit are is both transistors are turned on then the LED will turn on and this is a little piece of logic that is called a NAND gate and this is the the truth table for it this is the symbol that we would use if we were drawing a more complex logic circuit we would we would actually instead of drawing all the little transistors we would just you know kind of draw this this whole circuit is just a NAND gate using this symbols kind of rounded on this side and squared off on this side and the the truth table for it says that if a and B are 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 if B is on and a is off then the output is also often we can test that just by pushing the button and the LED doesn't come on same thing if a is off or seems me of a is on and B is off then the output is also off we can test that by you know pushing the button here but if both of them are on then the output is on and that makes sense if both of these are on then that completes the circuit all the way through and so if I push both the buttons then you can see the LED 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 I'll show you the the circuit diagram first and the circuit diagram for this is a little bit different so the we still have two transistors we still have our LED but they're wired up just a little bit different so in this case the transistors are kind of in parallel and so if either of the transistors is on then then the LED 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 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 and then so if either of these transistors is on then the LED will be on 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 and so this is something called an or gate to make sense if this or this is on then then the output is on and so this is the truth table for 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 outputs also on and also if both of them are on because then that will just complete the circuit either way and of course either way current will flow and so it'll be on so we push both buttons you can see it's on so either way or both it doesn't matter it'll it'll turn on you so you can see here we've got two different circuits they're very similar very similar but slightly different different operation one is an and operation the other is an or operation so let's try let's take a look at something a little bit more a little bit more complicated so this is a different board here a different circuit and let me just power this up so this we've got actually five transistors in this circuit and it is basically the same kind of idea here we've got two inputs that we can 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 XOR 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 and so we can we can actually kind of see that operation so right now it's off because neither of them are on but if we if we turn on a yeah 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 to kind of understand how that works I've got the 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 because we've got the five volt source going through our resistor through the LED and that's what's going on here so we go five volt source going through the resistor through the LED and into either of these transistors so it can go through either of these transistors and then the the inputs for those transistors are the a and B so these yellow wires come over here and then jumper over to the to the switches so if either this or this is on then that will complete the circuit at least through to this point and so in the or gate we just had this connected directly to ground and so if you this or this we're on then 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 otherwise you know we've got a 5 volts sort of connected back to 5 volts and there's no voltage potential difference between 5 and 5 five so the LED is off so in order for this or gate to work essentially 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 volt so we've got current flowing you know from the base to the diameter so this will normally be on unless unless a and B are on if a and B are on then then this this node here is is then going to 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 is turned off then this or gate essentially doesn't work which is kind of what we want to do 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 so 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 XOR gate and so again we can see at work if we have a on it comes on if we have B 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 then the output is off as well and so these are just a couple of the different gates that you can build with transistors and here's actually kind of a whole list of different ones so we saw you know the and gate the or gate we just saw the XOR gate and we were of course looking at the inverter here there's also you can make a buffer I guess is actually kind of the first thing we made with the transistor which is the input was on the or on then the output was on the impose off the app was off not very exciting but it can be useful and there's a couple others that we that we could also make that we didn't look at which are the the not and or NAND the not or and the not X or the X nor it's called and these are basically the same as the and gate but the output is inverted 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 and so it's the basically the exact opposite you know the X nor is the opposite of the X or nor is the opposite of the or and so on so you can build any of these with with a few transistors 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 to build a computer

Channel: Ben Eater

Views: 1,324,015

Rating: 4.9632521 out of 5

Keywords: Transistor (Invention), Logic Gate

Id: sTu3LwpF6XI

Length: 13min 1sec (781 seconds)

Published: Sun Mar 22 2015

Reddit Comments

[u/j919828]

So extreme buildapc?