By Duane Careb: Query regarding LDRs

I have a circuit using LDRs to monitor lighting in a room with florescent fixtures throughout. When directly under a fixture, and the LDR is covered, a separate LED is lit. Got that, no prob.

BUT when the same LDR circuit is placed in a part of the room not directly under the florescent fixture, the LDR doesn’t “see” enough light (as if covered) and thus the LED is constantly lit – that is a problem!

So, how can I vary the LDR’s sensitivity to react to the uneven amount of florescent illumination (illuminance) as in a corner of the room, etc)?

Should I be using photodiodes instead of LDRs?

Application: I’m a model railroader. I have RR crossing signals located where roads cross or intersect RR tracks (just as in prototypical situations). I’ve used OR IC chips so that multiple LDRs can detect when they are covered by freight cars or engines as they roll over them. When that happens, light is blocked and the RR crossing signals begins alternating flashings until the cars are past the LDRs when light is, once again, detected and the flashing stops as default.