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Infrared reflective sensor with...
IR-TCRT5000L
Infrared reflective sensor with photo-electric sensor (as output)
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CRT5000 sensor work with an Infrared LED (950nm) and a photo-electric transistor. The whole things included within a box allowing the light to get inside.
This sensor is used to detect contrast changes on the surface. A white surface reflect a lot (infrared) light to the photo-electric sensor (which activates the transistor). On the other hand, a dark/opaque surface return very few of the infrared light and the photo-electric sensor is not activated.
We can use this type of sensor for:
Measure distance from 3 to 6mm.
The first time we meet this sensor was in a gaz counter decoder. As the white digits have different shape, the quantity of light reflected to the photo-electric sensor will vary from digit to digit. For each digit, the reflectance scheme will be different. It is possible to detect the 9th digit (so when the counter did make a complete turn :-) ).
To make this sensor really efficient, it is very important to have surfaces with different reflectifve factor (significant difference). By example, black mate vs white mate. The black mate have a reflectance ratio from 0 to 10% whereas the white mate has a reflectance ratio from 90 to 100%
The datasheet mention an optimal current of 20 mA for the infrared LED (and also a current of 40mA). For long term usage, you should use a 20mA to avoid degradation the LED.
On the other hand, higher is the current, higher is the infrared illumination, higher illumination improves the efficiency of detection.
For a 40mA current, you will use a 125 Ohms resistor (for a 5V power supply).
Using a 250 Ohms resistor will limit the current to 20mA (also for a 5V power supply).