IR Sensors

The Infrared emitter detector circuit is very useful if you plan to make a line following robot, or a robot with basic object or obstacle detection. Infrared emitter detector pair sensors are fairly easy to implement, although involved some level of testing and calibration to get right. They can be used for obstacle detection, motion detection, transmitters, encoders, and color detection (such as for line following).

Line sensor:

In order to detect a line, we use the principle of reflection of light over different colors. As we all know, a white color object reflects almost all light incident on it while a black colored object absorbs all. Now, by measuring the variation in the intensity of reflected light we can easily determine the presence/absence of line under the sensor.

AMPLIFIED SENSOR CIRCUIT

Description:

The circuit uses an comparator to compare the value detected and ouput logic ‘1’ or ‘0’ or ON and OFF.

We use an IR LED which will emit Infrared light and it will be reflected by the surface. The detector acts as a variable resistor whose value depends on the intensity of light falling on its surface. The higher the intensity, the lower is the resistance of the detector. The detector and R2 act as a potential divider. When the intensity is high (reflected from white surface), the resistance of detector is low and so the value of the potential is high. Similarly when the intensity is low (reflected from black surface), the resistance of the detector is high and so the potential is low.

This potential is compared with a reference potential. The reference can be varied by the potentiometer (pot). Calibration is required. You have to record potentials of the white as well as black surface and then find their ranges. According these potentials ten set the reference so that for white surface output is .high and for white black surface output is low.

An LED can also be used as a visual detector so that you can see whether the surface is black or white.

R3 resistor prevents excessive current to pass through the LED. The value of the resistor depends on the size and the colour of the LED. More the diameter of the LED , more is the current it will sink in. R1 should be chosen according to the LED. Usually for 3mm Red LED and 5V supply 330Ω is enough. R2 should be larger then the maximum resistance of the detector.

THINGS TO NOTE:

• The LED and detector have very narrow emission and detection angles, so keep them close so that the circuit functions properly.
• Metallic or glossy surfaces reflect more tend to reflect more light, so make sure you calibrate the device properly.
• Many objects are opaque to visible light (that means light doesn't pass through it, like wood, black plastic, metal), but are transparent to IR light.
• If you work in the sunlight make sure it doesn’t interfere with the function of the circuit. Usually sensors placed below the body of the robot are the least affected by it.
• Depending on resistor values, your IR circuit can be tweaked to better detect color instead of distance.