hello friends....check here 555 testing circuit....they r saying that both led will glow when o/p comes ....but i m agree with this becz o/p of 555 is working in asatble mode in that diagram...
http://www.uoguelph.ca/~antoon/gadgets/555/555.html

i m saying this becz in another site it is shown quite practically that one led will glow at once....http://delabs-circuits.com/cirdir/theory/gates/doc00019.html

or i tried also make resistance equals in second link to first link diagram...then led starts blinking....

then may be first diagram site means blinking of led????
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If you see the leds switch on/off alternatively or if you see the leds both on all the time it depends on the frequency of oscillation.
Check on google for the effect of persistence of vision.

If the frequency is below about 30Hz then you see the leds turn on/off alternatively. If the frequency is over about 30Hz then you start to see the leds on at the same time but in reality this is not happening. What happens is that the leds turn on/off even at higher frequencies than 30Hz but your brain thinks that they are turned on all the time.

 

If you see the leds switch on/off alternatively or if you see the leds both on all the time it depends on the frequency of oscillation.
Check on google for the effect of persistence of vision.

If the frequency is below about 30Hz then you see the leds turn on/off alternatively. If the frequency is over about 30Hz then you start to see the leds on at the same time but in reality this is not happening. What happens is that the leds turn on/off even at higher frequencies than 30Hz but your brain thinks that they are turned on all the time.

thanks sir i understand .......then acc to in first link i hav posted both leds will not glow at same time ....rather looks like this due to high frequency....????

 

thanks sir i understand .......then acc to in first link i hav posted both leds will not glow at same time ....rather looks like this due to high frequency....????

if both leds dont 'light' the same time it means the frequency is low.