Thanks for the suggestion but I am using an LM393 and it now works fine. LM358 is a dual operational amplifier, not a comparator.The reason LM555 was not triggered was due to high output voltage near your +VCC and far from 1/3 VCC that needed to trigger the LM555 Trigger Input.
I suggest that you place resistor in series to R15 to ground. Acting as voltage divider.. You can try 10K for both R!5 and R?? that I suggest. Result trigger voltage will be 1/2 VCC near the 1/3 VCC that LM555 trigger voltage required. It's now easier for the LM358 to pull down it to below 1/3 VCC and therefore trigger LM555.
Pls try to place pullup resistor 4.7K on Output pin of LM358. It's also a open collector comparator.
What are the values I should use? Should I put something in the range 10μ - 100μ?Bypass capacitors should be mounted as close to the supply pins of integrated circuits.