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It sounds like a wiring problem. Double check each wire.What hardware and robot are you using?
What works well is black vinyl electrical tape, which is about 3/4" wide. Very narrow may be problematic. Super wide (nearly as wide as the entire sensor array) will probably be more difficult for smooth tracking with the basic code.The program is also set for a white background with dark tape. You can change the code if you want the opposite.
What robot are you using?Have you confirmed the qtr sensor is orientated correctly with the headers/wires towards the middle of the bot and sensor located at the front of the bot?When you say Left left left left, do you mean short corrections to the left or just spinning forever to the left?The difference in operation between battery vs. Computer powered, is it a difference in speed only?
Look in the follow_line() function. There are values of 55 and 200 you will see spread through that function. Try adjusting those or rewrite it to just slow down one wheel while the other runs at full.
Did you figure it out?The first sequence is a calibration. That is the rightleft repeat. After should start following if it is wired correctly and the calibration was successful.Are you using the same hardware as the instructable?