This description depends on the idea that the circuit is constructed correctly, the transistor is in its survivable range for voltage, current, and power dissipation, and such as that.

Good description... now this is my super-simplified analogy: a transistor is simply a valve, and depending of how much current one applies to its gate (or how much you twist the valve's stem), the gate allows much more current to pass between the collector and the emitter. This valve can be completely closed, or completely open, or something in between...
After you've understood this, then the rest is nuance... but important nuance, of course

 

That looks like more than 2 seconds per half cycle. (My bad.)
What's your LED voltage?

 

Here's the voltage between R7 and the LED




and the voltage at the transistor's emmiter

 

That looks like Spice interpreted the LED as a 1N4001. True?

 

That looks like Spice interpreted the LED as a 1N4001. True?

mmmhhhhh... I dunno, man.... lemme check....

 

Well I'll be.... I DID select LED when I placed it on the board, I just didn't select which LED... it seems that the software just simmed with an ordinary rectifier.... here's the voltage between R7 and the LED, which is now an Osram LO E67B



The thing is that this osram LED is no ordinary LED... it's a much higher performance one, with a forward voltage of 2.2V, and consuming around 50mA

 

Lots of people love LTspice, but I haven't made friends with it yet.

 

Lots of people love LTspice, but I haven't made friends with it yet.

Well... I'm not in an intimate-serious-monogamous-up-and-close-personal-relationship with it, either... but it's what I can afford... which is free

 

Anyway... here's the updated .asc file for aac52, in case he wants to take it from here...

 

Well there has been a lot to take in today so I will read over the posts and see how far I get.

Thanks

 

The reason that so many sites get hung up on a duty cycle under 50% with a 555 is because the simplest way of getting the 555 to run as an AMV (astably multivibrator -- basically a free-running rectangular wave) results in the constraint that it will have less than a 50% duty cycle. Hence the caution is really for those that need a square wave -- a rectangular wave with a 50% duty cycle -- or even a rectangular waveform form with greater than 50% duty cycle. Some sites will claim that these can't be achieved with a 555. They can, but the circuit topology is just a bit more involved.



Each application has various concerns and constraints and those generally can be used to rule out topologies that, for one reason or another, are not good choices for that particular application. At the end of the day you may have a few acceptable choices left and the decision of which to use may be based on technical merits or it might be based on which one is easier to route on a circuit board or it might be based on the personal preference of the designer.



Many designers have preferred transistors for certain jobs and those often start out as a transistor that they got a really good price in bulk for many years ago. For lots of applications there are dozens, if not hundreds, of transistors that will work just fine and you can't really tell the difference between them in terms of performance in those applications. As you gain experience with transistor circuits you will start encountering circuits that need transistors that meet certain specs that some, if not all, of the common general purpose transistors can't cut. That's when you learn what some of the myriad parameters on the data sheet mean and how to tell if a given transistor is good enough for your purposes. Don't be surprised if you never learn what all of the parameters mean, because you probably will never encounter a reason to deal with the majority of them.

Makes totally no difference.

high voltage / current
high frequency
hFE

it's like kindof a 2n3904 wont work for 80 volts.
it will be on the margins for a FM transmitter.
it cant control a small motor

Otherwise for most circuits you can use just any transistors.
The large ones or the high voltage ones normally have a much lower hFE, could be 20 to 50.

While small signal circuits often need 100 at least.

Why there are so many small signal transistors? Often they are just derivates, optimized for some mass produced application, to shave off a few cent, substitute 5 components with 3 by maxing out the parameters, give a lower noise or some particular hFE at some frequency.

 

But then I get kind of confused as to what is happening with the transistor.

What it is acting as - a switch? If so, the 9v source is going from the collector to the emitter through a current limiting resistor to the LED. This seems right as the 470ohm resistor would be far too limiting for the ~6V from the capacitor. So how is the LED being faded?

The transistor is primarily acting as a buffer for the capacitor. The timing is based on all of the current that goes through R1 also going through C1 (i.e., that they are in series). But if the capacitor was connected to the resistor and LED then a lot of current would bypass the capacitor and really mess up the timing. Plus, there isn't enough current making it through R1 in the first place. The capacitor voltage will swing between 3V and 6V while the 555 output will swing from 0V to 9V. The peak charging/discharging current will be 6V/33kΩ≈180μA, which is about two orders of magnitude less than is needed by the LED -- and this is the peak current. It will decay down to half of that just before it switches direction. That concerns me because I think the transistor base current is going to noticeably affect the timing.

The key to understanding what the LED current is doing is to note that in the active region the voltage at the emitter of the transistor will be about 0.6V to 0.7V below the voltage at the base. So, assuming it is in the active region, the voltage at the emitter will ramp up and down between about 2.4V and 5.4V. Depending on the color of the LED, it will have about 2.2V or so across it when it is on, so the voltage across R3 will vary between about 0.1V and 3.2V, which means that the current will ramp between about 0.2mA and 7mA. Assuming a transistor β of about 200, the base current will vary between about 1μA and 35μA. That means that the current being drawn away from the capacitor to feed the transistor base is almost half of the timing resistor current near the transition from charging to discharging. The circuit should still work, but I suspect that the time to complete one cycle with the transistor connected will be considerably longer than with it not connected, perhaps 50% longer give or take, but that's just a wild guess.

 

if you dont use a base resistor, the base current will be much higher depending on the collector/emitter potentials towards the base. I found that when I made a transistor tester myself.

Its not like the nominal hFE would reversely limit the base current. Current can flow into (or out of) the base freely.

 

if you dont use a base resistor, the base current will be much higher depending on the collector/emitter potentials towards the base. I found that when I made a transistor tester myself.

Its not like the nominal hFE would reversely limit the base current. Current can flow into (or out of) the base freely.

I don't see any need for a base resistor in this circuit -- it is an emitter-follower and since the base voltage should never rise above 6V it is in no danger of saturating. Why would the base current be much higher without a base resistor?

 

Here you have an alternative version of a cmartinez circuit from post 20.

I build this circuit long time ago.
And the voltage across C1 capacitor changes from 1.2V to 4.8V. And at T4 emitter from 0.6V to 4.2V

 

So I've read over this thread a few times now, and the input from all of you has been really great. I appreciate the effort you have all put in to help answer my questions, it truly is inspiring.

Thank you for all of the help so far.

I downloaded the LTspice program and created a circuit, honestly I did not find it too appealing to use. It actually reminded me of my college lessons where we would have to simulate the circuits before building them, but I couldn't remember the name of the software. After a bit of digging I found out it was Multisim. I've since installed that and had another play around with a few circuits, changing components to see what difference it makes. I guess since I have used that software before I am just more familiar with using it.

It definitely helps me to understand what is going on, but not to a point where I am confident I know exactly why something is doing what it does. I do believe I have a very basic understanding of the individual components, but my knowledge of how components work together, and their "inner workings" so to speak is very much lacking, I am sure some of this comes with experience though. Perhaps I am asking too much of myself, and such a deep knowledge really just isn't required, especially for the occasional circuit I am going to be wanting to make?

Either way, having done various simulations I think the next step is to take it to the breadboard! The simulations show that a 5v source is not optimal (perhaps a more complicated circuit would solve this), but for my requirements of simply fading an LED in and out I don't think it is critical, and without making it from physical components I cannot see what it is like. I've connected the LED I want to use up to a variable power supply, and it seems to light up enough for my purpose at the voltage levels provided by the circuit.

I'm going to order a few 555 timers, transistors, diodes, capacitors, and pots so I have some flexibility. Shouldn't need any resistors as I already have a load of those.

I will post back here with my schematic and testing results.

 

...........

 

After a bit of digging I found out it was Multisim. I've since installed that and had another play around with a few circuits, changing components to see what difference it makes. I guess since I have used that software before I am just more familiar with using it.

That software looks rather nice... but at $1,600 dlls for the basic version, and $4,000 dlls for the professional one... I think I'll stick to LTspice for a while more...

 

...........

Thank you for that, it seems a lot simpler than my attempts!

I can certainly see the logic in it, but I'm not sure if I built it right, or maybe I'm measuring it wrong? Basically the voltage over the capacitor goes between ~1.6 to ~3.3v as you indicated, but the voltage over the LED is fairly constant, as is the ~8mA current.

That software looks rather nice... but at $1,600 dlls for the basic version, and $4,000 dlls for the professional one... I think I'll stick to LTspice for a while more...

If you get it when you are a student then the price is a little easier on the wallet .

 

sometimes I'm wondering if new controllers with integrated D/A converter will cause these circuits to become less and eventually to disapear.

These days, plain light bulbs, neons, audio transformers, 2n3904, and NE555 are still selling easily, besides they are mostly no longer much used in commercial mass produced appliances.

I saw a toaster with a couple 40xx chips and discrete components (at least 30) inside 10 years ago! But more and more chinese are using epoxy glue ICs.

I guess you make these kinds of circuits for study purpose, or just as a once-off DIY. I was never thinking NE555 is still so much used, sold hundreds of them already. As 10pcs or 20pcs. Now I have a 3L container just with NE555 ICs, and two kinds of CD4017 PCBs containing NE555.