Ronald Besinga offers this terrific little line follower robot built around the LM324 quad op-amp. It uses two of the amp circuits in a triangle generator. The other two amps operate as hi/lo comparators. They compare the voltage across the photoresistors with the triangle wave, and thus determine if the robot is moving over the line or the whitespace.
This is an easy to understand, fun, and inexpensive project that could be done in a weekend with the right tools.
More than that, though, this is an excellent example of how to document a project: clearly written, well illustrated, and thorough. It’s obvious he spent a lot of time on it. Even if you’re not into robots, you might read his post just to see how it’s documented — it’s really impressive!
Ronald posted a follow-up to this project offering this neat idea for using transformer wire in perfboard prototypes. Not only does it look cleaner than PTFE-jacket hookup wire, but it takes up significantly less space, reducing bulk. Also good if you’re in a jam and need some wire ASAP.
NOTE: this won’t really work with breadboarding, as the wire gauge used in wall-warts is usually north of 25AWG and won’t make a sturdy connection with the board contacts.