Where I can learn about 555 Astables.

ha ha you got LMGTFY'd. I suggest thatyou get a book entitled 'Practical Oscillator Circuits'. It describes the 555 and 556 ICs and how they work, plus it has many great circuits.

http://homepage.ntlworld.com/g.knott/elect388.htm
sometimes looking at the circuit diagram can help. this is the 555 configured as a monostable, so its different to what you're interested in, in terms of output; but it'll still be helpful to look at.
observe the capacitor at the bottom left and forget about the rest of the circuit. the cap charges up to 2/3 of the supply (this is a RC circuit as there are resistors from the supply to the cap). when this V is reached, the discharge transistor is switched on, so the cap starts to discharge. when the V across the cap reaches 1/3 of the supply, the discharge transistor is switched off and the cycle repeats ad infinitum.

I can't explain it that well; you should really get the book!

 

the equation for (astable) frequency is 1/(0.7 x (R1 + 2.R2) x C1)

Say that R1 is 5k, R2 is 5k, and C1 is 20n.
Freq = 4761.904761Hz.

Say that C is increased to 820n.
Freq = 116.1440186Hz.

Note that a larger capacitance decreases the frequency, and vice versa. This because a cap with a larger value takes more time to charge and discharge.

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you're welcome!

 

Try to read the datasheet, there are application notes included. Like here: http://www.alldatasheet.com/view.jsp?Searchword=LM555
You need to select the datasheet to view it. NS normally has the best list of applications.

TOK

 

You need to define the specifications of what you want to get out of the design. Like the frequency and duty cycle for an oscillator. Then you look at the timing diagrams to get a ballpark value of the capacitor, for example. From that you can calculate the resistors, using the formulaes in the datasheet.

If your ballpark value was far out, adjust it and calculate again. it is a kind of iterative process.
For final tuning the values to standard component values, you may need to add components, in series or parallel. Or add a pot for tuning.

This is the art of guesstimating

TOK

 

what is the 'spec' you speak of?
if it is, for example, 555 operating temperature, then read the datasheet. If you want the frequency equation rearranged to give, say, R1 or C, then ask me.

 

The specifications we talk about is the specifications for the end product, what you want to get out of your design. It is normal to setup the system specs when you design a new device or product. If you don't do that, how will you know how to design anything, or know that you are finished with what you wanted. You need some guidlines to start with, and to use as testcriteria underway.

TOK

 

ahhh, those sort of specifications. thanks for clearing that one up

 

But, you'll also need the datasheet's technical specs of the part in question, to find if this is the correct chosen part in this particular design.

So, we are in fact talking about several 'specs' in all cases of designing gizmos, of any kind.

TOK