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On the 555 Timer Schematics I found I need help deciding what transistors to use to build it? Answered

I am a beginner in electronic schematic reading and I want to build a full sized 555 timer with all the parts that make up the tiny little IC. But I am having trouble determining what transistors to use.
This schematic has the details on what values of resistors to use but not what transistors they are:
Which ones should I use?
same thing with this one:
This one seems pretty promising since it does list what transistors to use:
and I don't know this one is hard for me to understand:

Which one do you think is best or what transistors could I use in either of the first 2?
Is the last one actually a 555 timer or is it something else?


I like the third schematic you linked to, this one:

and the reason why I like it, is because it is composed only of resistors plus two kinds of transistors, namely BC548(NPN) and BC558(PNP).

I am also guessing that the transistors chosen for that one are complimentary, meaning their characteristics are essentially the same, but with the currents in reversed directions.   Furthermore I am guessing that almost any general purpose NPN BJT transistor, and its complimentary PNP BJT version, would work too. 

For example, I am guessing the 2N3904 (which is NPN) and its compliment 2N3906 (which is PNP),  those would work, and those particular transistors are cheap and easy to find.

If you really want to look at the minutia of transistor characteristics, there is this site:
but I think anything sold as a "general purpose" BJT transistor will work for your circuit... assuming you only want your discrete 555 to run at ordinary speeds, and ordinary voltages,  e.g. as a multivibrator with a period of just a few mS (thousandths of a second), and Vcc between 5 and 20 V.

If you want something homemade like this to run at gigahertz speeds (period measured in nanoseconds), then that will be much more challenging.

I forgot that last part.  The last circuit you linked to,
is not a 555 equivalent.  It looks like some sort of gizmo for adding distortion to an audio signal.

I think the defining characteristic of a 555-like circuit is that voltage divider, made of three resistors (traditionally 5k, 5k, and 5k, from which comes the name 555) which divides the supply into (1/3)*Vcc and (2/3)*Vcc. 

Then there are comparators, to compare the voltage on the external capacitor-thingy to these internal references of  (1/3)*Vcc and (2/3)*Vcc.

And then the outputs from the comparators drive a set-reset type flip-flop.

And there is a discharge transistor driven by the output from the flip-flop.

And all that junk together is what you get in a "general" 555-like circuit.

BTW, the discharge transistor, Q21, in that circuit. It should probably have 10K resistor in series with its base, similar others like it, like those groups labeled "not gate".  The fact it is missing one, is probably an error on the part of the author.

Also for R16, R17, R18, those are unlabeled, but those are the R-R-R voltage divider I mentioned earlier.  Some believable values for those would be all 10K, or all 5K, or even all 4.7K (which is close enough to 5K)

I built something like that about 25 years ago while attending college learning electronics. I built it exactly like the book showed in the schematic. I think the
one i built was a 741 operational amplifier IC. but my finished 741 didn't work. My instructor said the schematic was only for illustration purposes and the actual circuit likely has PRECISION TRIMMED resistors during manufacturing to make the circuit work. So it is a daunting project and will likely not work when you get done building it.

You can't really build it from discrete components, you can make a functionally similar one though.

The clever designers of ICs often use tricks, like controlling transistors with MULTIPLE collectors, or oddly connected bases

Any small BJT will work here.