Obviously, the doors are slid using electrical power. But in the first place, how does the peculiar door inform itself of your nearby presence and decide to open the doors like a respectful and courteous doorman would?
Such a feat is the kind of functionality that a Photo-reflective sensor introduces to its device. With the sensor, one could obtain information about the distances of objects or determine whether there exist objects within a certain range of the sensor. With this, you can configure a device to react to whatever the sensor detects! Another example would be those ‘automatic’ hand dryers found mostly at well-maintained public restrooms. By placing your hands underneath the dryer at a certain distance, the Photo-reflective sensor notifies the machine to turn on and deliver an amazing draft of warm air to your hands. But besides the unexciting example above, you could perhaps (hopefully more for security than for mischief) use the sensor to detect a person becoming too close to your treasure chest and launch a tranquilizing dart from somewhere to strike the unfortunate, unsuspecting person and afterward have him or her wake up tied to a chair for interrogation.
The Photo-reflective sensor is not only limited to detection-triggering of mechanisms as it might seem. In the more subtle field of Robotics, the sensor is literally purposed to become ‘eyes’ of a robot, granting it reactions to objects around it. The e-Gizmo practices the use of Photo-reflective sensors by applying it to its Mobots. One of the demonstrations of the Mobot photo-reflective sensing is the retraction of the Mobot from markings on the floor that are shaded black. When the Mobot was placed within an area of the floor bordered with some black paint, it will remain moving only within the area enclosed by the black markings.
But to have you appreciate the Photo-reflective sensor besides the many applications it grants, we will provide information about the operations, variations and the general circuit involving the sensor.
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Basic Operation
In general, a Photo-reflective sensor detects the presence of an object within a given proximity by the interruption of a light beam. The interruption could be in the form of reflecting a light beam upon an object and subsequently receiving the reflected light beam, or it could be that a light beam that is continuously detected is blocked by some object. In either case, the general concept of detection is the same – interruption of a light beam by an object.
Nowadays, because of the tremendous amounts of advances in technology, most Photo-reflective sensors are now able to detect objects with diameters of less than 1 millimeter. Other kinds of Photo-reflective sensors have sensing ranges that have been reported to reach up to 60 meters
The simplest and basic Photo-reflective consists of only the emitter (LED) and the receiver (Photo-transistor). Example models of the different basic Photo-reflective sensors presented below are the CNY70 being an infrared emitting sensor, and a visible-ranged emitting sensor.
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| Visible Photo-reflective Sensor |
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| IR Photo-reflective Sensor |
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General Photo-reflective Sensor Circuit
The general Photo-reflective sensors circuit consist of four primary parts (total number of parts including non-primary ones exceed four). These are the light-emitting source (LED), a light-receiving component (Photo-transistor), which is sometimes simply a single basic Photo-reflective sensor, a signal converter, and an amplifier. The inclusion of the first two parts by now should be self-evident. The need for a signal converter is to verify that the received light signal is that which was emitted by the Photo-reflective sensor. The amplifier simply strengthens the received signal before it goes to the signal converter, so that the steps of a complete Photo-reflective sensor circuit operation are:
‘Light-emission > light reflection > light absorption > signal amplification > signal discrimination/verification > some output’.
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Photo-Reflective Sensor Configuration
All kinds of Photo-reflector circuits/systems, as they exist in the present time, more or less operate in the same way as the above. But the sensor owes its wide range of applicability to the different configurations it can take. There are two categories in which the sensor varies in configuration, the first is the sensor output configuration and the second is the sensor detection modes.
There are two general types of outputs a Photo-reflective sensor provides, these are the called the ‘Light-On’ and ‘Dark-On’ outputs.
1.) ‘Dark-On‘ – An output that exists and is active while the receiver (Photo-transistor) does not receive light from the emitter.
2.) ‘Light-On‘ – An output that exists and is active only when the receiver is receiving light from the emitter.
Let us imagine that we have a robot that is capable of mobility, and we program it to constantly move forward when it does not detect a specific object in front of it. If there are no objects in the way of the robot then light from the Photo-reflective sensor emitter does not return to its receiver, and the robot frolics forward with confidence. The output for the robot to move forward for this case is the ‘Dark-On’ output. The robot frolics when there is no light detected.
However, if some evil force were to place a poisoned apple in front of the robot and the robot is programmed to stop at the detection of an object by the Photo-reflective sensor. This output is seen to be a ‘Light-On’. The robot stops and stands frozen in fear because we instruct it to, and because the receiver is receiving light from the reflection of the emitted light on the apple (alternatively, you could set the robot to launch a little missile against an object it detects so that when the object is disintegrated, the robot will not be able to detect the object anymore and will resume its ‘Dark-On’ output, which is to frolic happily. This is an example of ‘Light-On’ and ‘Dark-On’ outputs operating hand-in-hand).
From the two general output configurations above, many sub-specific sensing modes come about (mostly for ‘Light-On’ configurations). All of the sensing modes each possess their own strengths and weaknesses in terms of application. As such, the nature of the application determines the kind of Photo-reflective sensing mode to be used; and with the amount of sensing modes, you are entitled to a wide range of applications.
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Photo-Reflective Sensing Modes
A.) Through-Beam Mode – This mode is one of the well-known Photo-reflective modes. The mode detects the presence of an object by having a light beam that is continuously emitted towards the receiver broken by the object. Because of its simplicity in structure and manner of detection, it provides one of the longest ranges of Photo-reflective detection (25 to 60 meters, depending on emitter LED type), as well as the detection of objects as small as 3 millimeters (and smaller at closer ranges). However, the through-beam mode does not provide compactness and cost-efficiency, as the emitter and receiver systems must be housed separately.
B.) Retro-Reflective Mode – Similar to the Through-Beam mode, this mode also detects objects by the interruption of its light beam. However, the difference of this mode to that of the Through-Beam is that both the emitter and the receiver are both situated within the same housing, thereby introducing compactness and cost-reduction. The light beam in this way is sent instead to a reflective surface and reflected to the receiver. Although this mode is in every way better than the Through-Beam, it also has its own troubles nonexistent in the Through-Beam mode.
Firstly, objects with a considerable amount of reflectivity could pass through the light beam undetected because an ample amount of light could still be absorbed by the receiver to have it consider the light beam as uninterrupted (There are some models now that have fixed this problem. By using a polarization filter, the Photo-reflective sensor activates whenever the polarization of light that has been received is not the polarization of light that reflects from the mirror). Secondly, Retro-reflective modes only have detection ranges up to around 10 meters.
C.) Diffused Mode – Diffused mode of Photo-reflective detection operates oppositely with the two previous modes. While the Through-Beam and Retro-Reflective modes activate their outputs with the absence of light absorption by the receiver, Diffused modes activate their outputs upon the reception of the light beam produced by the emitter when the light beam is reflected from an object. For this case, both emitter and receiver are under a single housing.
Diffused modes operate almost the same as eyesight. When there is no light, you cannot see whether there are objects within range of your eyesight. Thus, to see in the dark one needs a light source such as a flashlight to emit a light beam upon objects so that when the light is reflected upon them, your eyes receive the reflected light and thus allows you to see the objects.
There are many factors that affect the efficiency of Diffused detection modes, and most of these factors arise from the characteristics of the reflecting object. Color, surface finish, and size all affect the detecting power of Diffused modes. In addition, because only a small percentage of light is absorbed by the receiver from reflection, sensing ranges of Diffused modes are shorter than those of the two previous modes.
The e-Gizmo has its own instance of this mode at: http://www.e-gizmo.com/KIT/Collision.HTM
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Application-Specific, Photo-Reflective Sensors
Industry today, with the nature of the demands and necessities of society, experiences the greatest need for specialized electronic technologies that fit specific industrial needs. As such, specialized electronic detectors have been developed, and Photo-reflective sensors are of no exception. Here are some of the operation-specific Photo-reflective sensors.
a.) Color-Specific Sensors – These Photo-reflective sensors detect only colors configured to it. The basic color-specific sensors are called ‘single channel units’ and detect only a single prescribed color. Many-color sensors do exist, and allow different shades and colors to be detected.
b.) Luminescent-Specific Sensors – Objects with luminescent properties such as greases, inks, paints, chalks, or glues are detected by luminescent-specific sensors through the use of ultra-violet light.
c.) Contrast-Specific Sensors – Contrast-specific sensors are used to detect the difference in two colors or media of a target. To do this, the sensor is initially configured to hold two different conditions. Upon detection of an object, the reflected light is analyzed whether the received light is closer to the first condition. If it is, then the output is not activated. On the other hand, if the received light is closer to the second condition, the output will be executed.
d.) Passive Infrared Sensors – Unlike the majority of the Photo-reflective sensing operations that utilize the absorption of its own light, passive-infrared sensors detect objects through absorption of infrared radiation emitted by the object. As such, it detects objects with a higher temperature than that of its environment.
e.) Light Grid Sensors – These sensors are simply an array of line sensors arranged in such a way as to create a light screen or sheet. In most cases these sensors are used for applications wherein the physical dimensions of the targets are in question. That is, they check for deformities in the targets or if a certain form is detected, and the output is executed.
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