Photocell sensor can be regarded as a transducer that is used to detect the light intensity. At full intensity beam with a photocell resistance could be worth about 10 Ohm and the low intensity light (in darkness) could be worth several hundred Ohm. This is the figure circuit of the basic photocell sensor circuit;
This circuit using a Cadmium Sulfide (Cds) photocell sensor. The workings of this photocell sensor circuit, when the light falling on the photocell (PC1) is blocked, the resistance will increase and the voltage on the PC1 will rise. When the voltage rises above half of the supply voltage, the output of the comparator will turn on and the LED will light. Because of wide variation in CdS photocells is usually best to install the cell and then measure the resistance under normal lighting conditions. A resistor with a value of approximately three to five times the cell resistance measured is then selected for R1. For example, if the cell resistance was measured at 400 ohms then from 1200 to 2200 ohm resistor is used.
Increasing the value of R1 will cause the sensitivity of the photocell sensor decreases. This may be required when the light that falls on these cells is not too strong or shadows can affect the photocell.