How fast is a photoresistor?
How small can the obstruction be? (I guess it depends on the surface area of the LDR, itself.)


Yes, LDR tend to collect light from a larger area than photodiodes or phototransistors. A photodiode or phototransistor may include a lens that will focus incoming light to the tiny internal silicon sensor. You can roughly estimate its light-collecting area by the diameter of its lens.

2.How fast can I go? (Will the LDR be able to detect a jump frequency of 300+ jumps/minute?)

Where the blocking object is small, it will very briefly block the light source. So speed-of-response is not determined by 300+ jumps/minute, but by that very much shorter moment while light is blocked.

A LDR is slow-enough that it could miss sensing a briefly-blocked light source.

A photodiode can sense very fast-changing events. Since photo-currents are small, an amplifier is likely necessary. An amplifier would translate photo-currents to a voltage that an Arduino could detect.

A phototransistor is slower than photodiode, but is likely fast-enough to detect a small light-blocking object. Since a phototransistor internally boosts photo-currents, an external amplifier might not be necessary. A simple load resistor to your Arduino's DC supply may be all that's required. Some experimentation would be required to choose its value (a large-value resistor increases light-detecting sensitivity).


A phototransistor is a good compromise between speed/sensitivity in this application.