This is why it should be an MCU.

Agreed, if he's up to programming a micro.

 

As per request in post # 12.


EDIT schematic

 

That won't work well as you have the second 555 trying to brute-force pull the first 555 output low, when it is high.
You could add a resistor at the first 555 output to allow that.

 

Of course, I can use an mcu, but if I give up this method for now, is there another way?
For example, if 8 seconds is increased to 10 seconds, i have to compile and program mcu again.

 

That's what I wanted
what is R1 , D1 , C1, ... value ?
vr1 , vr2, vr3, vr4 how ?

 

That won't work well as you have the second 555 trying to brute-force pull the first 555 output low, when it is high.

Correct. I edited the schematic.

 

That's what I wanted

The component values listed will get you the timing requested.

Using a micro you only need this:

 

Of course, I can use an mcu, but if I give up this method for now, is there another way?
For example, if 8 seconds is increased to 10 seconds, I have to compile and program mcu again.

Correct, but in my opinion it would be quicker and definitely more precise then adjusting potentiometers.
I only posted the 555 circuit to show it's possible going analog.
With the pots there is a limit to the timing before having to change component values.
With the micro, unlimited timing options.