report on dark detector circuit using 555 timer IC..iI want to know whether it is monostable or astable?

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Without seeing your (the) schematic, it is impossible to tell. However, you can do a search for 555 timer tutorial or try this site. I will leave it up to you to determine the difference between Astable and Monostable (hint: look at the timing components and trigger).

A darkness detector circuit can only be a bistable IMHO. How would an astable even work like that.
It is up to the querent to provide enough information to the answer volunteers of Instructables so that they can do a thorough "job" when providing the free advice given. Without knowing whether Dhwani64 is using a standard dark detector (yes, I could Google that, but I shouldn't have to if the question was completely written, phrased, etc., and it might be the wrong one anyway) or one of her own design, you (etal) cannot conclusively answer the question. What I did do is give Dhwani64 the ability to learn about 555 timers on her own so that she can draw her own conclusions. That is how people learn, and the knowledge will stay with them a whole lot longer than if one of us were to come out and say "The answer is ...."

For a good 'ible on how to ask a question so that the answer(s) will be on target, I point toward rickharris' fine effort.

rickharris6 years ago
I will let you decide what it should be:

Bistable has 2 stable stated and goes between them when triggered.

Astable has NO stable state and flips between them without any external trigger.
iceng6 years ago
Bistable because any dark detector has to allow for periodic light events to
reset for the next dark.

Only a terrorist would want a monostable device to set off some plosive event
just one time after dark to instill terror.

One could also make an astable device that would change frequency from
darkness to lightness. Good to use for blind people with intact hearing.