Light Dependent Resistor 

To gain access to revision questions, please sign up and log in.
The LDR Resistance decreases with increasing light intensity.
LDR ProtectionThe LDR is wired up with a resistor in series.
Calculate the Safety ResistorAssume the LDR resistance is small in very R = V / I R = 12 / 0.01 R = 1.2KΩ 
Voltage DividerV_{out} = 12 x R_{2} / ( R_{1} + R_{2} ) As the light gets brighter, R2 
Voltage DividerV_{out} = 12 x R_{2} / ( R_{1} + R_{2} ) As the light gets brighter, R1 
Light Dependent Resistors (LDR) are made from a semiconductor. In the dark, this semiconductor has very few free carriers so the LDR has a high resistance. When photons of light hit the semiconductor, they add enough energy to dislodge carriers which can then flow. This reduces the resistance. The carriers are electrons.
As the light level changes, the resistance of the LDR changes over a huge range. This is difficult to plot onto normal graph paper so logarithmic graphs are used.
If the LDR is in a 12 volt circuit, the series resistor must be at least ...
R = V / I
R = 12 / 50x10^{3}
R = 240Ω
240Ω is the minimum safe resistance.
If the goal is to detect a light level less than 10 Lux, a more suitable resistor would be about 800Ω.
This resistor satisfies both the design requirements mentioned at the top of this page.
At what light level does it become impossible to meet both design requirements?
reviseOmatic V3 Contacts, ©, Cookies, Data Protection and Disclaimers Hosted at linode.com, London