GCSE Resistors BS1852 Colour Codes E24 >LDR< Parallel PIVR Calculations Potentiometer Puzzles Series Thermistor Timing Types Voltage Divider |
Resistors LDR |
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The LDR is an analogue, passive, resistive, input transducer used to sense light intensity.
LDR Protection
The LDR is wired up with a resistor in series.
Calculate the Safety Resistor
Assume the LDR resistance is small in very R = V / I R = 12 / 0.01 R = 1.2KΩ |
Voltage Divider
Vout = 12 x R2 / ( R1 + R2 )
As the light gets brighter, R2 |
Voltage Divider
Vout = 12 x R2 / ( R1 + R2 )
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?
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