 Hey everyone, sorry it's a couple days late here. I know I said I would have a video up this week, but with everything that's been going on lately, I think you can forgive me for being a couple days late. So today what we're gonna do is we're gonna look at a two-pair transistor circuit and see how we can use them for switching. So what I gotta do is I gotta get the office kind of set up first off, so let's just get that all figured out. So I've got my studio set up. So just to give you a little rundown here, I've got some lights over here. I got my tripod there. I got a boom stand for the mic in a second here. I got some lights over here. And then behind me, I got an ice cream screen. I want to give a huge thanks to Flynn West Solutions. They provided me with so much gear that it allowed me to kind of up my game in the videos. If you need help with anything to do with web design, anything to do with digital marketing, anything to do with upping your game in social media, you gotta check out Flynn West Solutions. They're amazing and I'm not just saying it because my name's in the name of the company. Now I'm just gonna put the camera on the tripod and we're gonna take a look and see how all this stuff's gonna get set up and then we'll get going on it. All right, I got everything set up, got the green screen set up, got the tripod set down. So I figured instead of just using the screen screen, let's give us a nice little background. So let's take a maybe try this. So last week we talked a lot about the Darlington pair transistor and how you can use two transistors to kind of up your game. Cause typically the gain on transistors anywhere between 10 and 500. But if you need to go beyond the 500, you need to add that extra transistor in line with the other transistor to see how that goes. Now, if you don't understand what I'm talking about, make sure you just check up on the video there and click on it and you can go back and look at how the Darlington pair transistor works. This week we're gonna be talking about how we can use the transistors to switch, which is often how they're used. So what we're gonna do is look at a very simple two transistor alarm circuit. We're gonna see how having these two transistors in series, you can use them to have one switch the other one off or on. So as always, let's hop into the whiteboard and take a look. So here we have two transistors, Q1, Q2. We're gonna go ahead and we're gonna assume that they have betas of a hundred. You can basically always assume that if you're working with a transistor question and the beta or the gain isn't written down, go ahead and assume a hundred. Here we have a hundredth kilo ohm resistor. We have a 3.3 kilo ohm resistor. We have this, which is the coil of a relay. We have the normally open contacts of a relay. Then some sort of alarm so it could be a buzzer or light or however you wanna have it. And then we have a 12 volt battery that's lined up there. So we're gonna walk through this whole circuit and see how it works. Over here, this little red wire, that's our trip wire. Now it could be a trip wire, it could be a set of contacts, it could be anything. But as soon as that is tripped, something's gonna happen. And we're gonna take a look to see how that all plays out. Now, what first thing we need to do is we need to work back through just like we do with any other transistor. So we're gonna take this voltage here and we know that we have a B to E. Our base to emitter voltage is 0.7 volts. So we end up with 11.3 volts at our base resistor. So what we're gonna end up doing, remember, is we've got a base, emitter, and collector. We're gonna take our base current and multiply it by our beta here to get our collector current. So first off, in order to do that, we need to get our base current. So we're gonna go 11.3 divided by 100,000 ohms and that gets us 113 microamps. So remember, we're gonna be taking the base current now that we've worked that out. We took our voltage minus the 0.7 volt drop equals 11.3 volts, 11.3 divided by 100,000 gives us the 113 microamps. Next up, what we have to do is take that 113 microamps and we're gonna multiply it by 100 because that's our beta to get our collector current. So let's see what happens here. 113 microamps times the beta of 100 gives us 11.3 milliamps, all right? So if we have 11.3 milliamps on a resistor that is 3,300 or 3.3 kilo ohms, we can use Ohm's Law. So what we can do is we can take our current times our resistance to get our voltage. That works out to be 37.29 volts. Hopefully right there you see that there's a problem because what we're dealing with here is we've got a voltage source of 12 volts. Let's take a look here. We have a 12 volt source, but we've calculated out this collector to have 37.29 volts. That's too high. There's no way that we can make voltage out of nothing. And if you wanna learn more about how this works, you can go ahead and hit the I there and there'll be some information on how transistors in saturation works. But basically in this situation, we have a saturated circuit. That's not possible. So what ends up happening in a saturated circuit is your voltage across your collector emitter, which is right here, is always gonna end up being 0.1 volts. Which means that the rest of it is gonna be across there. So 11.9 volts will be across there. So we've got a situation of saturation. So anytime that you calculate that VC to be greater than your source voltage, you know you're in saturation so then you just go ahead and make this 0.1. This is gonna be the source minus 0.1 right over here. And away we go. Now the way that this circuit is hooked up and this is very important so pay very close attention to what I'm about to show you. Let's hop back into the whiteboard here. If I take, it takes 0.1 volts across from this point to this point. I hopefully we could agree that this point to this point is the same as this point to this point. They're in parallel with each other. If this is 0.1 volts and we know that there's a 0.7 volt drop across the base emitter on a silicon transistor, that 0.1 volts is not enough pressure to push past the 0.7. So at this point what we do is we say that this transistor is cut off because nothing can get through here. It's not gonna take this current here and push it through and get onto this side because this 0.1 volts is not enough to overcome the 0.7 volt drop of base to emitter. So at this point, this coil stays de-energized. These contacts stay open and the alarm doesn't go off. Okay, now let's say I'm creeping around your house ready to take a picture of you as you sleep and I trip the trip wire. So what I'm doing is I'm getting rid of this. So you've set up some contacts or trip wire or something. I open a window and those contacts go away. So what we need to do here, let me just pass through here, is we see that in this situation, this part of the circuit is completely dead. This Q1 is useless to us because it's not hooked up anything. It has nothing on the other side. It's a disconnect, it's an open. If it's an open, then it can't be used. So in effect, this 100 kiloohm resistor is useless to us and Q1 is useless to us. So these guys here, let me just cross them out. This one here and this one here are no longer in play. What we do have is we have this one that is in play. So we go ahead and we start doing the math based off of just this transistor. So again, we have a 0.7 volt drop here. So we end up with 12 minus 0.7, gives us 11.3 volts on the 3300 ohm resistor. We use Ohm's law, 11.3 divided by 3300. That gets us 3.4 milliamps, 3.4 milliamps times the beta. Again, we're gonna go ahead and assume it's 100. The beta ends up being 100, so 3.4 milliamps times Q2 gets us 342 milliamps, 342 milliamps times 100 ohms gets us 34.2 volts. And just like we said a few minutes ago, if I have a voltage that is higher than the source voltage, we're in saturation. At this point, that means we have 0.1 volts across the CE and then we have the rest of it across the actual load, which in this case would be the coil. In that case, we have 11.9 volts and let's say that this is a 12 volt coil. So this thing will energize. They'll pull in at that point because it has enough current and it has the voltage required to get there. So again, it pulls in when that happens, these contacts close. If you need information as to how this all works, you can go and look at the I here and you're gonna see how motor control circuits work and basically how contactors or relays work. Some great information in that playlist. That closes, the alarm goes off and Chad goes to jail again for creeping around your house. That's it, that's all there's to it. So again, the big takeaway from this and what again I find super interesting and super cool is that this transistor here is controlling this transistor here. If this CE is not large enough, if it doesn't overcome the 0.7 volt drop required on Q2, then nothing's gonna happen. But the moment we trip that wire, we get rid of Q1 and then the circuit starts working and doing its thing. And so that is a complete walkthrough on how this transistor circuit works. Hopefully that helps you out. If you're enjoying these videos, do me a favor, hit that subscribe button, ring that bell. It'll let you know when I've got more videos coming up. I'm trying to get a video out a week. Things are extremely busy and the world's a little bit crazy right now but I'm really pushing and making sure that I can get these going. If you have anything you wanna have added, please hit something down in the description below. Has anybody worked on these kind of alarm circuits before? I had a student a few years ago actually go and buy all these components and build it and it worked, which is really cool. He showed me a video of it. So has anybody else gone through and worked on these? Maybe put a little comment in the description below and do me a favor, share this stuff out there because the more word that gets out, the more I can actually get these videos made and have a great time doing it. Have a great week, everyone. We'll see you in the next one and stay safe. Hopefully you guys are all doing well and we will see you later. Stay classy.