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LDR - Light dependent resistor

As name indicate a resistor which resistance is vary with light intensity

 

LDR resistor. Observe the window for light entrance.

 

The LDR is highly non linear

Typical value for resistor depends on resistance for LDR. We use 4.7kOhm

 

Schematics for measuring restistance(and thereby light)

  • Typical value for resistor depends on resistance for LDR. We use 4.7kOhm for R and use (b) variant below

  • Measure LDr resistance in a typical light setup for your experiment and select that value for R


Schematic  - using (b) from above
---------------

Rldr  == resistance of LDR


Vo                  =    Vs  * ( Rldr / (Rldr + R) )

Vo * (Rldr + R)     =    Vs * Lldr

Rldr * ( Vs  - Vo ) =    R * Vo

Rldr                =    R  * ( Vo /( Vs - Vo) )

QED

comment: you need to know Vs  and R.

Calibration

You need to establish some kind of relation between light intensity and the measured Rldr

  • Total black room

  • Light in distances like x, 2x, 4x, 8x, 16x (distance rule)

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