Light dependent resistors are known to be slow to respond. We’ll find out just how slow they are by using an LED connected to a square wave, and measuring the resistance of the LDR using the oscilloscope utility ( Test and Measurement -> Oscilloscope  ).

Response of an LDR to an LED driven by a 47Hz square wave.

The resistance is measured by connecting the LDR, and a 5K1 resistor in series between 3.3V and Ground. By monitoring the midpoint of the two, and using the known values of either ends(3.3 , 0 ) as well as 1 resistor(5K1), one can calculate the resistance of the LDR.

From the graph, it is evident that the LDR just about manages to reach the expected voltage values at either ends within 10mS.
Increase the frequency, and note that the LDR fails to reach the expected resistance before the input signal from SQR1 changes polarity

– Repeat this study using a phototransistor, and characterise its response time.

Response time of an LDR
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