If you are using 9V batteries make sure whenever output voltage of 7805 falls below 5V replace the battery. Small cheap 9V batteries will drain completely within 4-5 minutes. Batteries: It is advisable to use Li-ion, or NiCd or NiMh or small lead acid batteries.Motors: They revolve according to the input from L293D.If you haven't used this IC yet for any project, you can read this quick and easy tutorial of motor driver circuits using L293D L293D: They take input from the AVR and correspondingly give output to the motors.AVR: They takes input from 4 different comparators, processes it and outputs different control signals to L293D according to the sensors condition.Comparator part: They takes the input from IR LED photodiode pair, compares it with the threshold set by potentiometer and gives a digital logic output to AVR.IR LED photodiode pair: They simply gives analog output to the Comparator.Here, I have divided the circuit into 5 different modules (blocks) named IR LED photodiode pair, Comparator, AVR, L293D and motor. It is always better if you divide big circuits into small modules (Parts). Using two separate boards allows us to place the IR pairs below the robot for line sensing and the potentiometers above it, to make the adjustment of the threshold easier. Both boards are connected to each other with the help of connectors. And a Comparator is to be soldered on another general purpose board, a little bigger than one used previously (Microcontroller circuit will also be soldered on this board later). First, the IR LED and the photodiode pair is to be soldered on a small general purpose board. The complete circuit is divided into two parts. In the Forth condition, when the 3rd sensor is on the black line but the 2nd sensor is on the white surface, the robot will rotate right.In the Third condition, when the 1st and the 4th sensors are on the black line, an intersection is detected.In the Second condition, when the 2nd sensor is on the black line but the 3rd sensor is on the white surface, the robot will rotate left.In the First condition, when the 2nd and the 3rd sensors are on the black surface the robot will move forward.As described in above picture, the 2nd and the 3rd sensors are for line following and the 1st and the 4th sensors are for intersection detection.
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