Datasheet
Ambient light like…
Ambient
light (lux)
Photocell
resistance (Ω)
LDR + R
(Ω)
Current thru
LDR +R
Voltage
across R
Dim hallway 0.1 lux 600KΩ 610 KΩ 0.008 mA 0.1 V
Moonlit night 1 lux 70 KΩ 80 KΩ 0.07 mA 0.6 V
Dark room 10 lux 10 KΩ 20 KΩ 0.25 mA 2.5 V
Dark overcast day /
Bright room
100 lux 1.5 KΩ 11.5 KΩ 0.43 mA 4.3 V
Overcast day 1000 lux 300 Ω
10.03
KΩ
0.5 mA 5V
This table indicates the approximate analog voltage based on the sensor light/resistance w/a 5V supply and 10KΩ
pulldown resistor.
If you're planning to have the sensor in a bright area and use a 10KΩ pulldown, it will quickly
saturate
. That means that
it will hit the 'ceiling' of 5V and not be able to differentiate between kinda bright and really bright. In that case, you
should replace the 10KΩ pulldown with a 1KΩ pulldown. In that case, it will not be able to detect dark level differences
as well but it will be able to detect bright light differences better. This is a tradeoff that you will have to decide upon!
You can also use the "Axel Benz" formula by first measuring the minimum and maximum resistance value with the
multimeter and then finding the resistor value with: Pull-Down-Resistor = squareroot(Rmin * Rmax), this will give you
slightly better range calculations
Ambient light like…
Ambient
light (lux)
Photocell
resistance (?)
LDR +
R (?)
Current thru
LDR+R
Voltage
across R
Moonlit night 1 lux 70 KΩ 71 KΩ 0.07 mA 0.1 V
Dark room 10 lux 10 KΩ 11 KΩ 0.45 mA 0.5 V
Dark overcast day /
Bright room
100 lux 1.5 KΩ 2.5 KΩ 2 mA 2.0 V
Overcast day 1000 lux 300 Ω 1.3 KΩ 3.8 mA 3.8 V
Full daylight 10,000 lux 100 Ω 1.1 KΩ 4.5 mA 4.5 V
This table indicates the approximate analog voltage based on the sensor light/resistance w/a 5V supply and 1K
pulldown resistor.
Note that our method does not provide linear voltage with respect to brightness! Also, each sensor will be different. As
the light level increases, the analog voltage goes up even though the resistance goes down:
Vo = Vcc ( R / (R + Photocell) )
That is, the voltage is proportional to the inverse of the photocell resistance which is, in turn, inversely proportional to
light levels.
© Adafruit Industries https://learn.adafruit.com/photocells Page 12 of 25