K.R.A.I.G. 107

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KRAIG 107

1. Sensor Circuits2. Comparators

TOPICS

One of the most widely used sensors is the Light Dependent Resistor. An LDR (Light Dependent Resistor) is a small resistor, the resistance of which depends upon the intensity of light incident on it. More the intensity of light falling on it, lesser is its resistance. The resistance usually varies in the range of about 10k ohms.

SENSOR

The LDR basically consists of a metal surface containing cadmium sulphide tracks. When light is incident on the track, electrons are released from the track which aid in the transmission of current. That is why the resistance of the LDR decreases with increase in intensity of the light.

WORKING

USING AN LDR

One of the major uses of an LDR is in LINE FOLLOWING.

• The sensors come into the picture here. For detecting a white line on a black surface the robot must be able to distinguish between the colors.• In an electric circuit distinguishing a difference in any field is achieved using difference in voltages. The sensors basically provide this voltage difference, if the correct circuit is made.• We will consider an example of a line follower using LDR sensors at the end of the class                              

COMPARATOR

• LM339, LM393 and LM311 are some integrated circuits that consist of one or more comparators (LM 311 has one comparator)• Like the motor driver ICs which had H-bridges built in them, these ICs have comparators inside• As we shall see from the pin diagrams, the inputs given consist of the voltage to be checked and the threshold voltages against which they are to be compared, besides the power supply• The outputs are taken from particular pins, and they are either 0 or 1 depending on the input given to that comparator

COMPARATOR ICS

LM393

LM339

LM339

LM339 PIN DIAGRAM

A SENSOR CIRCUIT

The end result of the above circuit is that it when the LED-LDR sensor is over a white line, the voltage across the LDR is low

due to low resistance, which gives an output of 0 if the threshold for the comparator is chosen correctly. Over a black

line, the output given by the sensor circuit is 1.

• A line follower is a robot capable of tracking a line drawn on a surface

• Optical sensors capture the line position at the front end of the robot

• The robot is steered to keep it always over the line

AN EXAMPLE – LINE FOLLOWER

BLOCK LAYOUT

Optical Sensor Array

Steering Controller

Left Motor

Right Motor

• Step 1: All the sensors are assigned some weight such as

Sensor 1 = a1 = 3Sensor 2 = a2 = 2Sensor 3 = a3 = 1Sensor 4 = a4 = -1Sensor 5 = a5 = -2Sensor 6 = a6 = -3

• Step2: Let b1 – b6 denote the state of each sensor(0 for white line and 1 for black line)

• Step3:Line Position = Σaibi

LINE FOLLOWING ALGORITHM

• Step 4: Based on the value of position, the motor will turn accordingly

A SIMPLE EXAMPLE WITH 2 SENSORS

HANDS-ON ASSIGNMENT

DESIGN AN LED-LDR SENSOR CIRCUIT ON A BREAD BOARD.

OBSERVE THE OUTPUT VOLTAGE WHEN IT IS

EXPOSED TO WHITE AND BLACK SURFACES