This model illustrates motor encoder using an LED and LDR.
A black disk with a single hole (where light passes through) is attached to the motor shaft. Each time the motor completes a revolution, the LED light passes through the hole and LDR receives it, which then alter the resistance of the LDR and hence the signal reading at the digital input port.
Techsource Technical Team (2020). Low speed motor Encoder with LED and LDR (https://www.mathworks.com/matlabcentral/fileexchange/51939-low-speed-motor-encoder-with-led-and-ldr), MATLAB Central File Exchange. Retrieved .