I have checked -ve voltage generator by 555, here
http://www.eleccircuit.com/12vdc-negative-voltage-dc-converter-by-555/

But I don't understand why cap C4 whose +ve side is positively charged & -ve plate is grounded during positive cycle.
Then why during 0V cycle, +ve plate is at zero & -ve side is at -vcc potential.
Do charge gets transferred

 

Read this
http://forum.allaboutcircuits.com/showthread.php?t=58371

 

Or you can cover the section about voltage doublers in our AAC eBook.

http://www.allaboutcircuits.com/vol_3/chpt_3/8.html

 

If I understand it correctly (I might be wrong, I am a beginner) during the 0V cycle the +ve plate is brought to 0V potential by the 555 timer. But the +ve plate is still at a higher potential than the -ve plate. the capacitor is still charged. You can think of it as a change of reference for the voltage measurement.
So if -ve is at a lower potential than +ve then when +ve is brought to 0V it means that -ve must be below 0. This is why it is at -vcc.

Something similar happens with the capacitors in a astable multivibrator.
Maybe somebody with more experience can shed light on this for us

Thank you

 

If I understand it correctly (I might be wrong, I am a beginner) during the 0V cycle the +ve plate is brought to 0V potential by the 555 timer. But the +ve plate is still at a higher potential than the -ve plate. the capacitor is still charged. You can think of it as a change of reference for the voltage measurement.
So if -ve is at a lower potential than +ve then when +ve is brought to 0V it means that -ve must be below 0. This is why it is at -vcc.

Something similar happens with the capacitors in a astable multivibrator.
Maybe somebody with more experience can shed light on this for us

Yes you are right
http://forum.allaboutcircuits.com/showpost.php?p=200183&postcount=12