Light dependent resistor (LDR)

LDR Summary

  • LDRs are light dependent resistors.
  • Resistance decreases with increasing light levels.
  • Typically > 1 MΩ when dark and < 1 kΩ in high light levels.
  • Response time of 2 – 50 ms. (Much slower than photo-transistors.)
  • They have a “light history”. Characteristics change a little as they acclimatise after long periods in darkness.
  • LDRs are more sensitive to certain wavelengths (colours).  The CdS types are most sensitive to green.

Details

LDRs, light depenedent resistors or photo-resistors are often used with LED circuits as simple photo-detectors.

Silonex LDR.
Figure 1. A Silonex NSL-19M51 LDR top view. Note that the brown zig-zag stripe forms a “border crossing” between the two terminals. The number of charge carriers that make it across the border is proportional to the amount of light hitting the border zone.

In dark conditions the resistance between the terminals is high – up to 1 MΩ or so. The resistance falls with increasing light levels down to a few hundred ohms at high brightness.

Figure 2. Typical LDR packaging.
Figure 3. Graph of resistance versus light level.
Lighting condition From (lux) To (lux) Mean value (lux)
Pitch Black 0 10 5
Very Dark 11 50 30
Dark Indoors 51 200 125
Dim Indoors 201 400 300
Normal Indoors 401 1000 700
Bright Indoors 1001 5000 3000
Dim Outdoors 5001 10,000 7500
Cloudy Outdoors 10,001 30,000 20,000
Direct Sunlight 30,001 100,000 65,000

Spectral response

LDR Sunram CdS spectral response
LDR Sunram CdS spectral response with visible light spectrum superimposed.

The common CdS (cadmium sulfide) LDRs have peak spectral response at about 550 nm which is the green region of the visible spectrum. Note that if illuminating the LDR with an LED that a red LED might give only about 30% of the response of a green LED on the same current.

Understanding the datasheet

The Luna datasheet for the NSL-19M51 gives the following figures.

LDR NSL-19M51 opto electrical specifications.
LDR NSL-19M51 opto electrical specifications.
  1. Note the wide variation in resistance at 10 lux – 20 to 100 kΩ.
  2. Note a “typical” resistance at 100 lux of 5 kΩ.
  3. Note that maximum dark resistance is specified for 10 s after removal of light. This suggests that the device response is slow.
  4. The gamma characteristic is a measure of the slope of the resistance versus lux relationship. (See below.)

LDR gamma characteristic

LDR gamma.
LDR gamma graphical calculation.

LDR gamma characteristic is a measure of the rate of change of resistance over range of light variation. Typically the gamma is specified for the range 10 to 100 lux and calculated as

$$ \gamma = \frac {log( \frac {R_{10}}{R_{100}}) }{log( \frac {100}{10}) }  = log( \frac {R_{10}}{R_{100}}) $$

since log(10) = 1.

In our case we can estimate that a is about 25k divisions up and b is about 4k divisions up so

$$ \gamma =  log( \frac {R_{10}}{R_{100}})  = log( \frac {25}{4}) = log(6.25) = 0.795$$

Gamma values of 0.6 to 0.8 are common for CdS LDRs. A gamma tolerance of ±0.1 is common.

For the digital ON-OFF applications such as flame detectors, LDRs with steep slopes to their resistance versus light intensity curves are appropriate. For analog or measurement types of applications such as exposure controls for cameras, LDRs with shallow slopes may be better suited.

Response times

LDR rise and fall times.
LDR rise and fall times.

The slow response times of LDRs makes them unsuitable for data transmission, for example, but can be an advantage in other circuits such as flame detectors or audio effects where the slow response smoothes out the signal being monitored or controlled.

Light history

A photocell which has been stored for a long time in the light will have a considerably higher light resistance than if it was stored for a long time in the dark. Also, if a cell is stored for a long period of time at a light level higher than the test level, it will have a higher light resistance than if it was stored at a light level closer to the test light level. This effect can be minimized significantly by keeping the photocell exposed to some constant low level of illumination (as opposed to having it sit in the dark). This is the reason resistance specifications are characterized after 16 hours light adept. (Source.)