Hi Wayne and welcome to Electronics Point
This is a bit more complicated than you expect.
First, the 555 is probably not the best monostable to use, because if the trigger signal remains active, the monostable will not time out
Second, if you have a single fixed threshold on the light level, as the average light level passes near the threshold you will get multiple triggerings, because the light level picked up by the LDR will vary constantly and will cross the threshold repeatedly.
The answer to this problem is to use a circuit called a Schmitt trigger, which has two separate thresholds - a rising threshold, and a falling threshold which is lower than the rising theshold. When the input quantity (ambient light level) rises above the risng threshold, the Schmitt trigger output goes active. Then, the input quantity has to go below the falling threshold before the output will go back OFF again.
The output of the Schmitt trigger can be used to trigger a monostable.
A 555 can actually be used as a Schmitt trigger, though you can't control the thresholds independently.
I can draw you up a circuit using two 555s (or a 556, which is a dual 555) that will do what you want, if you tell me what you want, exactly.
Do you want an output pulse when the ambient light goes higher than a certain level, or when it goes lower than a certain level?
