Photocell or LDR
A photocell or photo resistor is a Light Dependent Resistors (LDR). LDR's are sensors that detect light. They are small, inexpensive, low-power, easy to use and don't wear out.
An LDR is a resistor that changes its resistive value (in ohms Ω) depending on how much light is shining onto it. They are very low cost, easy to get in many sizes and specifications, but are very innacurate. Each LDR sensor will act a little differently than the other, even if they are from the same batch. The variations can be really large, 50% or higher! For this reason, they shouldn't be used to try to determine precise light levels. Instead, you can expect to only be able to determine basic light changes. When light falls on the sensor the resistance will drop. When it is dark the resistance can be as much as 10Mohm, when the light increases the resistance can drop to 1Kohm. The photocell is the most sensitive between the green light (400nm) and the red light (700nm). You can test the photocell with a multimeter. Below you see a graph how the resistance of the LDR changes when the light changes.
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