[...] will this circuit work? because all ldrs are in parrellel and that will reduce the total resistance.specially when one sensor is on the line and others are outside. thanks
Yes, if eg. the upper trimmer (don't see a use for it) is at 1M and the lower trimmers are at say 10k, with the LDRs going to 100 Ohm, just one of them seeing light will make the common node (where the 4 x 47k trimmers connects to the 1M trimmer) go down to:
5V * (10k + 100) / (1M + 1k +10k +100) = 50mV
To remedy the situation, just use 1M (or whatever you find needed) for the lower trimmers and connect them through a 1k resistor directly to the 5V rail, then they cannot interfere with each other.
If you need a global adjustment, make a variable (fairly low impedance) voltage that you regulate (instead of the 5V rail), but there's no real need for that unless you need it to run in various dissimilar environments.
Whether you measure voltage or current, Ohms Law still stands!