According to the Monostable circuit of the 555 IC as provided in Wikipedia, shouldn't the capacitor be charged and pin 6+pin 7 in LOW ? Until the capacitor is fully charged, only pin 6 and 7 will be in HIGH, right/wrong? Having trouble understanding the 555 IC.
Hello,
The 555 connections you are talking about are never HIGH or LOW, they are analog voltages in between. The 555 is based on two COMPARATORS that decide when (at what voltage) the output should switch from HIGH to LOW..
Look at figure 3 Here: A Plethora Of NE-555 data - NE555 Tutorials Page
And there's LOTS more on that page 
shouldn't the capacitor be charged and pin 6+pin 7 in LOW ?
No.
Pin 7 and 6 are connected together, pin 7 is the discharge pin and is high impedance until the voltage on pin 6 reaches a certain analogue threshold voltage. When that point is reached pin 7 conducts to ground (low if you like) and discharges the capacitor and sets the output pin 3.
This will hold like this until pin 7 is set to high impedance again by a transition on the trigger pin 2.
So for an edge on pin 2 you get a constant width pulse on pin 3. - A monostable.
So when the 555 is connected like above (monostable mode), will the capacitor be charged when the circuit is powered on? how will the current flow? Is the voltage after the resistor smaller than 2/3 Vcc?
will the capacitor be charged when the circuit is powered on
How can it, the capacitor has to charge through R, but until triggered pin 7 is shorting it out so it won't start to charge until pin 2 makes a transition.
Does that mean that if the trigger pin is triggered, it will block pin 6 and 7 and finally the capacitor get to charge. Before, the capacitor can't be charged because pin 6 and 7 is open, the charged current and current from the Vcc is flowing through it. Is it right?
the capacitor can't be charged because pin 6 and 7 is open,
no
the capacitor can't be charged because pin 6 and 7 is closed, that is connected to ground.
Yes the current from R flows through pin 7 so the capacitor can't charge. See the diagram of the internals of a 555 in the data sheet you will see this is a flip flop.
I've seen the internal of it. How can a capacitor be charged? (I'm still basics on electronics)
How does the internal of the flip-flop work? (Internal circuit attached.) When the reset is HIGH, Qnot is HIGH, when the set is HIGH, Q is high, right? What do I still need to know?
How does the internal of the flip-flop work?
When the S input (set input) is triggered the output Q is set to one and Q bar (inverse of Q) is set to zero.
When the R input (reset input) is triggered the output Q is reset to zero and Q bar (inverse of Q) is set to one.
The flip flop also has a reset pin which will put Q at zero as well.
The output Q is connected through an inverter to the pin 3 output, it also feeds into the base of a transistor whose collector is the discharge pin 7.
The capacitor can only be charged when Q is zero and so the transistor is not conducting.
See page 2 of the data sheet
Can't a capacitor be charged like that ? attached
Yes but only if there is not a short across the capacitor, which is what happens in the case of the 555 when it is attached to pin 7.
Pin 6 triggers the short to appear and pin 2 triggers it to be removed.
So when it is removed the capacitor charges up.
When pin 6 reaches two thirds of the supply it triggers pin 7 to put the short across the capacitor again and so it discharges.
oooo, TQ, now I understand the 555, TQ very much. One more thing, is the short you mentioned short circuit?
is the short you mentioned short circuit?
Yes
In the bistable circuit above, can I trigger the threshold pin instead of the reset pin in order to put the output back to LOW?
Yes I would have thought so, you are just using the flip flop part of the 555, you could do that with half an 74LS00 or 74LS02.
Since we're talking about the 555 internals and what can be done with it, here's how to use it as a 12V 200ma driver, for power FET gates as an example. Or maybe a 12V LED strip.
It could also be used to make a Arduino to 10V PWM driver for 30 and 60 watt LED power drivers from MeanWell, etc.
Can the capacitor of the circuit be non-polarized? What is the difference between a polarized cap and a non-polarized one?
You can always use a non polarised capacitor in place of a polarised one, but not necessarily the other way round.
