This circuit powered by photovoltaic cells was found in the November 1998 issue of Electronics Now magazine. The circuit uses two motors and can follow a line thanks to its sensors.
An advanced motor control software plug-in for Microchip’s MPLAB® X Integrated Development...
This relay without lock uses a general-purpose transistor. The relay is according to the supply....
This signal follower is from a 1970's documentation. So it uses the traditional configuration at the time...