THink of it this way:
First you provide power to the sensor-
Brown gets 12V
Blue gets Ground
Now, you have two choices for output on the remaining 2 wires-
(These are called complementary outputs, like NO and NC on the mechanical limit switches you got in the kit)
In this example let’s say the sensor is set up so it can “see” it’s own emitted light coming back from some reflective tape - i.e., Light Operate
One of the output wires wire will be a path to ground and the other wire will will “float” above Ground.
When the tape is removed, the conditions reverse (the Dark Operate wire becomes a path to ground)
The Robot Controller is looking for that “path the ground” at an input pin.
The reason it is built that way is, like you partially guessed, because good ole’ mechanical switches worked that way. It also gives you flexibility in setting up a system.
Imagine that you are using the sensor to track a line - using the Light Operate wire makes the most sense (pun intended) “logically” - the condition is true when the tape is visible to the sensor.
Now imagine you are using the sensor in a factory to detect when someone walks through a door or when a box comes down a conveyor - Using Dark Operate makes themost sense “logically” the condition is true every time I get my beam blocked. . . .
The LEDs on the top of the sensor are to indicate that power is present and to indicate when the sensor “sees” its own light being reflected back.
Keep reading if still interested -
When the sensor has just enough light coming back into itself to be able to turn the LO output on, it is said to have an excess gain of (1). If you set the sensor up one foot away from a good target (like a bicycle reflector) you will have 200 times (excess gain) the light required coming back into the sensor.
You can find these datasheets (for range, beam pattern, and excess gain) at http://www.bannerengineering.com/literature_resources/product_literature/literature_worldbeam.html
Why would you want that? Well some factories are very dirty environments - setting up your photoeyes in this fashion (with lots o’excess gain) is akin to designing something structurally “like a tank” - Designing your photoelectric set up to an excess gain of (1) is like building a bridge that can only hold itself up - it can’t carry any cars or it would collapse . . .That lens better stay very clean and the alignment must remain PERFECT!
When you see the LED on the sensor that indicates LO “Flashing” the sensor is telling you that it now sees (1.5) times excess gain (Hey buddy, that reflected light is getting pretty faint!) However, even though that LED is flickering, the output wire for LO is NOT FLICKERING- it is ON “steadily” Keep that in mind.
Here is some info on the adjustment pot. on the sensor
Two examples of where you would use the 3/4 turn sensitivity adjustment on the sensor:
- You need to see an object pass between the sensor and its retro-target that is smaller than the beam of the sensor (the beam is roughly the size of the sensor’s 18mm “snout”. In this case you want to see the DO wire switch when only some of the light get’s blocked. If you left it turned all the way up, the output wire would never change state.
- You are using the sensor as an encoder by sensing tape on on your robot’s wheel. Gray or white surfaces may reflect enough light that the sensor is still above an excess gain of 1, even though the tape you put on the wheel reflects back an excess gain of 50. By turning the sensitivity down, you may be able to ignore the gray and white areas and only see the tape.
I hope this helps
PS - if you test the sensor by moving a target towards it from a distance (say a bin with tape on it) you will see that at “X” feet the sensor sees its light and the indicator light comes on - Now start backing away and it will be “X + Y” feet when the light goes off.
This is known as hysteresis
It is setup so you always get positive transitions ON and OFF - without it, you’d be able to get the box in a position that would cause the output to chatter on the edge of ON and OFF -
Imagine how you could screw up without hysteresis-
For example: if using the sensor as an encoder to sense wheel rotation and the robot was stopped, but the tape that the sensor was supposed to be seeing was right on the edge of the beam and the output was chattering -your controller would think that you are still in motion. . . (that is an important little note for any teams that build their own photoelectric encoder using parts from digi-key - be sure that the circuit has some hysteresis)