What is a Reflectance Sensor?
1. What is a reflectance sensor?
Most of the line following robot contests are held on a black tape
on a white board or white tape on a black board. It is used to
differ the *contrast of two colors.
There is one infrared led and one photo-transistor pair on it. This
photo transistor is connected to a capacitor. Capacitor's discharging
time gives us the data about the reflection. If the capacitor is
discharging fast this means the sensor is on a high reflection area. If
the sensor is on the top of a black part or it is far away from its
detection distance the infrared light will not reflect to photo-
transistor. Usually the maximum detection distance of a reflectance
sensor is in between 2mm and 9mm. And the optimum detection distance is
3mm.
2. Usage Areas?
They are the main component of a line following robot. To make line
following robot one needs at least 2 reflection sensors. To track the
line smoothly one needs 3-8 reflectance sensors. The more the sensor, the
smoother the robot tracks the line.
Reflection sensors are also needed for mini sumo and sumo robots to track
the limiting line of dohyo.
3. Sensor Layout?
3.1 Two Sensors
If there are two sensors present, one sensor should be mounted with respect to
the left of the line and the other one should be mounted with respect to the
right of the line.
3.2 Three Sensors
If there are three sensors present, one of the sensor should be on the top of the
road line and the others should be mounted as mentioned in two sensors layout.
3.3 Eight Sensors
If there are eight sensors present. Two of them should be on the tracking line,
the other six sensors should be devide into two and three of the should be
placed side by side respectively to the left of the line and other three
respectively to the right of the line.
4. Options
4.1 CNY-70
The most commonly used reflectance sensor is CNY70. It's cheap and easy to find
that is the main reason it is used most commonly.
*** Pros
- Cheap
- Easy to find
*** Cons
- It only gives analog data out (only 0 and 1)
- Requires circuit components to connect it to main board (resistor...etc)
*** Physical Specifications
Weight: 0.70g
*** Connection Properties
- 5V DC
- 30mA (max)
4.2 Pololu Reflectance Sensor
Reflectance sensors that are produced by Pololu.
*** Pros
- Gives digital dat aout (a value in between 0-1000)
- It does not require any circuit
components (resistor..etc)
*** Cons
- Price
*** Physical Specifications
***Connection Properties
- 5V DC
- 25mA
4.3 Pololu Reflectance Sensor Array
Reflectance sensors produced by Pololu which has 8 reflectance sensors 
*** Pros
- Only one electrical connection needed for whole of the sensor
- It can be devided into two with 6 sensors and 2 sensors
- It does not require any circuit components
- It has its own library for programming
*** Cons
- Price
*** Physical Specifications
Weight: 3.09g
*** Connection Properties
- 3.3V – 5V DC
- 100mA
* Contrast: the difference in color and light between parts of an image
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