Line Follower circuitCybot's 'Line Follower' mode uses a pair of IR sensors aimed downwards close to the floor, these feed to the two inputs of an LM311 comparator, the output of which feeds to the Processor 1 board, using pin 11 of the chip for the actual input. The sensors themselves consist of an IR LED and an adjacent photodiode (built as a single unit), so the light from the IR LED is reflected back by a light coloured surface, and absorbed by a dark coloured surface - the photodiode responding to the amount of light reflected back. On Cybot this is done fairly crudely, for good performance more sensors are needed, as it stands with only the two sensors Cybot can only follow a black/white transition, so it actually follows the edge of the line (one sensor on black, one sensor on white) - this gives it the slow jerky response seen in the video supplied in one of the early issues. A better way to do it, though more expensive, is to use a minimum of three sensors, with the outer two spaced according to the width of the line, and the third in the middle - in this way you can follow the centre of the line, in a much smoother fashion, the middle sensor detects the line, and the outer two detect the background - even more sensors can be used to further improve the performance. As well as the sensors there are three surface mount resistors on the board, R1 is 270ohm, and limits the current to the LED's - giving about 10mA of drive. The other two, R2 and R3, are both 8.2Kohm, and provide a bias voltage for the photo diodes - the more reflected IR light the photodiodes receive, the more current they draw, and the lower the voltages at the bottom ends of R2 and R3.

Cybot graphics used by kind permission of Eaglemoss
Last Updated 18/02/02 You can reach me by email at: nigelg@lpilsley.co.uk