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Photoresistor (LDR) sensor module

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Pin names

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VCC
Power
Positive power supply
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GND
Ground
Ground
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DO
Digital Output
Digital output
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AO
Analog Output
Analog output

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Attributes

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lux
string
default:"500"
Initial light value (lux)
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threshold
string
default:"2.5"
Digital output threshold voltage
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rl10
string
default:"50"
LDR resistance @ 10lux (in kilo-ohms)
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gamma
string
default:"0.7"
Slope of the log(R) / log(lux) graph

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Operation

The photoresistor sensor module includes a LDR (light-dependant resistor) in series with a 10K resistor. The AO pin is connected between the LDR and the 10K resistor. The voltage on the AO pin depends on the illumination - that is the amount of light that falls on the sensor. You can read this voltage by connecting the AO pin of the photoresistor sensor to an analog input pin and then using the analogRead() function. There are two parameters that control the sensitivity of the LDR: rl10 and gamma. rl10 is the resistance of the LDR at illumination level of 10 lux. The gamma value determines the slope of the log(R) / log(lux) graph. You can usually find these two values in the datasheet of the LDR. The following table shows the relationship between the illumination level (lux), resistance (R), and the voltage level on the AO pin when gamma = 0.7 and rl10 = 50 (the default values):
ConditionIllumination (lux)LDR ResistanceVoltage*analogRead() value
Full moon0.11.25MΩ4.961016
Deep twilight1250kΩ4.81985
Twilight1050kΩ4.17853
Computer monitor**5016.2kΩ3.09633
Stairway lighting1009.98kΩ2.50511
Office lighting4003.78kΩ1.37281
Overcast day1,0001.99kΩ0.83170
Full daylight10,000397Ω0.1939
Direct sunlight100,00079Ω0.048
* When VCC = 5V
** Measured one meter away from the monitor The following code to convert the return value of analogRead() into a illumination value (in lux): 
// These constants should match the photoresistor's "gamma" and "rl10" attributes
const float GAMMA = 0.7;
const float RL10 = 50;

// Convert the analog value into lux value:
int analogValue = analogRead(A0);
float voltage = analogValue / 1024. * 5;
float resistance = 2000 * voltage / (1 - voltage / 5);
float lux = pow(RL10 * 1e3 * pow(10, GAMMA) / resistance, (1 / GAMMA));
The lux variable will contain the illumination level in lux. The value of lux may be infinite (inf) when the sensor is in a very bright environment. You can use isfinite(lux) to check if the value is finite before using it, like in this example.

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Digital output

The digital output (“DO”) pin goes high when it’s dark, and low when there’s light. On the physical sensor, you tweak the small on-board potentiometer to set the threshold. In the simulator, use the “threshold” attribute to set the threshold voltage. The default threshold is 2.5 volts, or about 100 lux. The bottom LED (“DO LED”) is connected to the digital output, and lights whenever the DO pin goes low. In other words, it lights when the sensor is illuminated.

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Automation controls

The photoresistor sensor can be controlled using Automation Scenarios. The names of the controls match the names of the attributes defined above:
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lux
float
Set the lux value (lux)
The following example sets the illumination to 100 lux: 
  - set-control:
      part-id: photoresistor1
      control: lux
      value: 100

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Simulator examples

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