Your negative input signal voltages are destroying the PNP input transistors of the LM358.
The datasheet says the absolute maximum allowed negative input voltage (with respect to the grounded pin #4) is -0.3V so that the forward-biased base-collector diode of a PNP input transistor is not burned out.

 

i don't know what wrong with my question and words, frankly speaking i think everybody reading this can understand the question, i am not able to write in special words for you, but i think its not your lack of understanding my words but rather childish act of insisting and i don't know why because your answer above shows you exactly know what my question is.

again, i want to center the square wave at 0, forget about purpose, i am not interested in the purpose(but if you insist assume i want high and low or whatever you wish to think), if i wanted to know about the output, i would have asked and frankly speaking i would not ask about the simple output in forum.

forget about frequency, forget about supply voltage, forget about which wire i am using, forget about source of sine wave, forget about op-amp, forget about everything unless they relate to centering the square wave output from the timer.

Well, since you don't care whether your circuit can function properly or not, what does it matter? Even if you get your 555 output to be centered at 0 V, your results are going to be meaningless garbage -- they will NOT be the result of comparing a squarewave to a sine wave.

You have been given options on how to center your square wave.

1) Put it through a summing circuit that adds the offset you want to center it.
2) AC couple it via a capacitor.
3) High pass filter it into a bias network that holds the DC point at 0 V.

In general, you need a negative supply voltage to get these to work. But since you demand that we forget about supply voltages, we can't tell you how to make it work.

Whether or not these will do what you want will depend on frequency and duty cycle, particularly when sizing components. But since you demand that we forget about frequency, we can't tell you how to make it work.

Then there's the fact that the slew rate of the LM358 is only 0.5 V/µs. So if the output has a 5 V swing, each transition will take 10 µs. The frequency that would result in the output being a triangle wave is therefore just 50 kHz and your 555 appears to be set to run at about 70 kHz. But since you demand that we forget about frequency and opamps, we can't tell you how to make it work.

On top of that, your 40 Ω resistor on the 555 is going to seriously abuse the discharge transistor. But your attitude strongly indicates that you don't care about that, either. So we'll just ignore it.

Good luck.

 

Well, since you don't care whether your circuit can function properly or not, what does it matter? Even if you get your 555 output to be centered at 0 V, your results are going to be meaningless garbage -- they will NOT be the result of comparing a squarewave to a sine wave.

You have been given options on how to center your square wave.

1) Put it through a summing circuit that adds the offset you want to center it.
2) AC couple it via a capacitor.
3) High pass filter it into a bias network that holds the DC point at 0 V.

In general, you need a negative supply voltage to get these to work. But since you demand that we forget about supply voltages, we can't tell you how to make it work.

Whether or not these will do what you want will depend on frequency and duty cycle, particularly when sizing components. But since you demand that we forget about frequency, we can't tell you how to make it work.

Then there's the fact that the slew rate of the LM358 is only 0.5 V/µs. So if the output has a 5 V swing, each transition will take 10 µs. The frequency that would result in the output being a triangle wave is therefore just 50 kHz and your 555 appears to be set to run at about 70 kHz. But since you demand that we forget about frequency and opamps, we can't tell you how to make it work.

On top of that, your 40 Ω resistor on the 555 is going to seriously abuse the discharge transistor. But your attitude strongly indicates that you don't care about that, either. So we'll just ignore it.

Good luck.

don't mind but just don't give smart and heroic answer, i mean its boring.
because:
- who said the function of the circuit doesn't matter, i don't mind the output does not necessarily mean the circuit
- the question is how to center the square wave at the op-aamp input, that's it. so why keep asking and commenting and keep commenting about result and op-amp output, makes no sense to me
- you said "result will be meaningless garbage", i mean how can you be so sure? the circuit could be part of a larger circuit, it could just be an experiment, you may have experience but not all people do, so why such smart conclusion and comment(attitude)?nobody asked about the output, i repeatedly said about this so why still hanging there?unless you mean the output will effect the centering of the square wave input.
- you said about negative supply and about slew rate, well i suggest read my posts(#18) again before you comment, it says:
" forget about frequency, forget about supply voltage, forget about which wire i am using, forget about source of sine wave, forget about op-amp, forget about everything UNLESS they relate to centering the square wave output from the timer" and i said this because i observed that the problem was being shifted from centering problem to the output from the op-amp.
- if you care to read all posts above #18 there is no mention about frequency or power supply or other factors that could have impact on the question involved. if anyone had earlier indicated about negative supply or slew rate, thats would be fine
- attitude? go through the posts again, analyze them including yours and surely you will find reasoned attitude

 

We told you how to do it already way back at post #4.

If your square wave is 0 to 5V then add -2.5V to it.
Then your square wave will be -2.5V to +2.5V.

 

Also note that the output of a 555 timer isn't guaranteed to swing rail to rail. It still isn't clear what you're using because there's no part number on the schematic.

 

don't mind but just don't give smart and heroic answer, i mean its boring.
because:
- who said the function of the circuit doesn't matter, i don't mind the output does not necessarily mean the circuit
- the question is how to center the square wave at the op-aamp input, that's it. so why keep asking and commenting and keep commenting about result and op-amp output, makes no sense to me
- you said "result will be meaningless garbage", i mean how can you be so sure? the circuit could be part of a larger circuit, it could just be an experiment, you may have experience but not all people do, so why such smart conclusion and comment(attitude)?nobody asked about the output, i repeatedly said about this so why still hanging there?unless you mean the output will effect the centering of the square wave input.
- you said about negative supply and about slew rate, well i suggest read my posts(#18) again before you comment, it says:
" forget about frequency, forget about supply voltage, forget about which wire i am using, forget about source of sine wave, forget about op-amp, forget about everything UNLESS they relate to centering the square wave output from the timer" and i said this because i observed that the problem was being shifted from centering problem to the output from the op-amp.
- if you care to read all posts above #18 there is no mention about frequency or power supply or other factors that could have impact on the question involved. if anyone had earlier indicated about negative supply or slew rate, thats would be fine
- attitude? go through the posts again, analyze them including yours and surely you will find reasoned attitude

.
After following this discussion for quite a while now ............
I think that You deserve a FULL-REFUND of ALL YOUR MONEY !!!

You can get extremely valuable,
maybe even priceless,
advice in these Forums,
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Did You learn anything ???
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yes, i will try today again

 

hi cktboy,
My LTSpice simulation, on post #17, using a +/-2.5V square source in place of the 555, does show a possible Vout signal output.

In a real component circuit, the LM358 would not survive the high voltage input levels.
E