Sensing Light Differences in Photo-Resistors

photoresistor

I'm planning to make a little device that detects when a path of laser light is broken. The device will have 3 photo-resistors and three corresponding lasers. Like so:

|-(PHR)    <---(Laser)
|
|-(PHR)    <---(Laser)
|
|-(PHR)    <---(Laser)

I know that the environment that this will be in has variable non-laser light, which may throw off the sensors. So I want to design the circuit so that a trigger is set off when one of the three resistors is substantially more resistive than the others. I can't seem to figure out how to implement this circuit though. Does anyone have any useful information for this? I understand basic DC and AC circuits, but I guess I'm a little rusty.

Edit: I'm assuming this can be done with passive components. Not that it's a strict requirement, but it's the reason I'm asking. If this can't be done with passive components, then it's suffice to say so or explain why.

Best Answer

You might consider connecting the LDR's in a bridge arrangement.

enter image description here

As you are only interested in the event of one of the LDRs going 'high resistance' this arrangement would compensate for variation in background light.

Under normal conditions VR1 would be adjusted so V1 and V2 where the same. If LDR1 went high V2 would go LOW. If LDR3 went high V2 would go high. If LDR2 went high V1 would go low.

An op amp comparator, such as as lm339 could be used to detect the change.

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