Hello all back to basics. Today, with some flashing leds, this is a transistorized, a stable multivibrator. Here's a closer look at it. We've got two npn transistors 2n3904: two white leds 1k resistors to provide a current limit for the leds and 100k resistors which are providing the transistor base current.

Now these two capacitors in order to get this to flash quite slowly, i've used 10 microfarad mlccs multi-layer ceramic capacitors. Now, let's see what happens, if i take the capacitors out so out, they come and all we've got now is two separate transistor led driver circuits which i'll draw the schematic of so npn transistor arrow, pointing away from the base that goes to ground. Oh, i've not really left myself much room for this, because i've got two components up here. One is the led which is there that goes to the transistor collector and then there's the resistor.

The current limiting resistor i've put in 1k, i'm using all e1 series components. So there's only one value 10 to the 2. That is isn't it 10 to the 3, and this one is 10 to the 5 100 k. So there are two of these circuits i'll draw the other one.

So two circuits, where the base resistor provides a small current through the base emitter junction transistor has gain so they then it allows a larger current to flow through the collector emitter junction, and that provides a current limited by this 1k to light the led - and this Up here is 5 volts. Actually it's 4.8, because i've got four nickel metal hydrides, and this is zero volts now putting the capacitors in enables the circuit to flash. So, let's put them back in between base on one side and collector on the other, and vice versa, and the circuit flashes. But what's interesting is i can also put it from base on one side to the junction between the led and the resistor and vice versa, and it still flashes, although it flashes a little bit more quickly and i'm sure there's a good reason for that.

So let me draw in the two capacitors and show the two options for where i can connect them. So here are my capacitors, 10 micro farads. So that's 106, 106., and normally you would connect the left hand side up to the collector on the right hand, side and this right hand capacitor, would connect up to the collector, on the left hand, side circuit. That's how you normally draw this, but there is the alternative that you can have it coming up here to the junction between the led and the resistor, and similarly, this one can go up to that point both of these work.

However, it seems to flash faster if i put the capacitors up to the resistor diode junction rather than the diode collector junction. So, let's put these back how they were to generate the slower flash which is between base of one transistor and collector of the other and, of course, vice versa. Now what if we want to slow this down? Well, we can either have bigger resistors here, bigger base. Resistors, these are already 100k, but we could go to say one meg in our e1 series uh or we have bigger capacitors.

Now these are 10 microfarads. So i could go to 100 microfarads um. I don't have any 100 microfarad mlcc's. Now this is my kit of mlcc's and the largest value.

Is this one, which also happens to have the longest legs which make them easy to use? And this is the 106 or 10 microfarad multi-layer ceramic capacitor, so to slow this down, i'm going to increase the value of these two base. Resistors these two resistors here 100k, i'm going to increase them to one mega ohms. Now i don't have any three band one mega ohm resistors, so i'm going to use these four band ones! Uh, let's take out these capacitors, let's take out these base resistors. Where am i here? They are so, let's take out that 100k.

The led turns off because of course, the transistor is not getting any base current. Similarly, on that side - and let's put these one mega, ohm resistors in and now something quite interesting has happened - i'm not sure if you're going to be able to see this. But let's get in really close, what's happening is that the led comes on bright to start with. Yes, i think you can see it in the area of light on the breadboard comes up bright to start with, but then it goes dimmer and that's because if i take the capacitors out these two transistorized led drivers now have so little current flowing through that one Mega ohm resistor into the base of the transistor that we're not getting a lot of collector current, so we're not getting the best brightness out of the leds.

So this is a combination of base current and the gain of the transistor. Now i could lower the value of these 1k resistors to 470 ohms or something, but then that breaks my e1 series component values. So what else can i do to get more gain from these transistors darlingtons? Yes, a darlington pair and i don't need a darlington transistor. I can actually make a darlington out of these two n3904s, so a darlington looks like this.

We've got one transistor here and another transistor here, and the emitter of the upper transistor goes to the base of the second transistor collectors connect and then the emitter of the second transistor becomes your final emitter. The base is the base of this upper transistor. So, let's add another couple of transistors in here to create a darlington, a stable multivibrator flashing led circuit, and this is not too difficult, but i'm going to have to move these um grounding wires, i'll just turn them round. I think so that they are one uh breadboard position lower.

Let's do that and now i need two more transistors kind of bent like that, because the collector this top leg has to go to collector of the other transistor. But then base and emitter have to stretch across from emitter to ground here and base goes to the emitter of the previous transistor. So by putting them in like that, i've now got darlington's and you probably saw that these leds are now brighter than they were. Oh i'm gon na struggle to put the capacitors in now i'll move this around a little bit there we are now, let's put the capacitors back in oh now, where's it gon na go base of the darlington to collector of the darlington on the other side.

Yes, that's it so base of the darlington to collector. On the other side, i think it's like this. Does it flash? No, so clearly, that's not right and there it is, and it's flashing really slowly because of those one mega resistors. Well, let's have a look and see if we get that bright pulse problem and no that seems to be a clean switch transition.

So the darlington having extra gain, allows us to use the very high value, one mega ohm resistor, in conjunction with these 10 micro farad capacitors, to get a really slow flash, but without that pulsing effect. So i call that a success. Old-School transistor led flasher with resistors capacitors and transistors cheerio.