I'm currently making an advanced line follower from scratch, which will be following a 7.5mm black line with 'intersections', as a part of the RoboRave Line Follow Sprint competition. Earlier this year, I decided to go custom for the first time with a line follow robot, and lost due to many design errors and inexperience. Before, I was just using LEGO mindstorms, and spike prime, and suddenly, I found myself needing to learn C++, Fusion 360 and Fritzing. So, I wanted to come back to next year's World Championships prepared a lot more well. I decided to start the design process with the line sensor (a 5-sensor array), which uses 5 pairs of LEDs and phototransistors arranged in a line. They output a high voltage over black, and a low voltage over white, so measuring them with an MCU's ADC will help me (hopefully) make a really fast PID algorithm. It was all designed in KiCad, and based off of Pololu's QTR reflectance sensor series. In short, I chose 5 sensors because too many sensors makes the coding algorithm too complicated, and too little sensors can easily miss sharp turns. You only need one QRE1113GR reflectance sensor, and two resistors per 'sensor', which means you could even make this array on a breadboard (with through-hole parts, obviously). There is tons more documentation available on my YouTube video:

https://www.youtube.com/watch?v=jIgIj2hZbxc

and GitHub page:

https://github.com/hxelec/custom-line-sensor