I came across this sensor that can be used in trace following applications, but reading over a few applications I am not clear as to how the are used or applied exactly .
https://www.pololu.com/product/960
Just an expanded trace following pair ??
Anyone used them?
(Polulu Product)

 

reflectance sensor is similar to optocoupler but with wall between the emitter and receiver and two little windows on top.
so receiver gets to see the light not directly but from reflection from external object. basically emitter light leaves though one window, illuminates something and reflected light passes through second window and hits photo transistor. so output is analog and proportional to amount of reflected light.
https://www.vishay.com/docs/80362/vcnt2020.pdf
https://www.onsemi.com/pdf/datasheet/qre1113-d.pdf

 

they are used in close proximity to target. this also reduces interference of ambient light, this is why trace on the left is higher contrast. this is suitable for following really fat lines (like black electrical tape on a white floor) because this allows reflected light to be in wide range (0-100% white). if the line is thin, reflection would always be the same such as 99-100% which is not usable for following thin line on the drawing.
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assume the blue circle is area that reflection comes from. this is why line need to be wide ("to darken the sky"). so this is meant to follow the edge of the fat line.

this is good:



this is bad:



often this is used to make low cost custom encoder:
https://forum.arduino.cc/t/creating-an-optical-encoder-with-an-ir-sensor/140335

 

The bottom thin-line version is what I was aluding to in the trace cutting thread, https://forum.allaboutcircuits.com/threads/line-trace-plasma-cutting-machine.210268/
It managed to do it perfectly.

 

interesting, working on such small margin... i would not expect perfect in this case from this type of sensor. but congrats on making it work.

 

The link did not work for me at 10:00PM on Saturday. Maybe during regular business hours Monday it will work.