STMicroelectronics
0
A Light Dependent Resistor (LDR) — also known as a photoresistor or photoconductive cell — is a passive electronic component whose electrical resistance changes in response to ambient light intensity. When light strikes the active surface, resistance drops; in darkness, resistance reaches its maximum.
LDR Key Characteristics
|
Parameter |
Typical Value |
|
Dark resistance |
1MΩ to 10MΩ |
|
Illuminated resistance (at 10 lux) |
100Ω to 2kΩ |
|
Peak sensitivity wavelength (CdS) |
500–700 nm |
|
Peak sensitivity wavelength (PbS) |
1,000–3,000 nm |
|
Response time — rise |
1–10 ms |
|
Response time — fall |
10 ms to 1 s |
|
Max power dissipation |
50–500 mW |
Key Point: The LDR's resistance changes with the logarithm of light intensity — a 10× increase in light does not produce a 10× drop in resistance. This is the gamma characteristic. It makes LDRs excellent for night/day switching but unsuitable for precise lux measurement.
The LDR operates on the principle of photoconductivity. Step by step:
Step 1 — Dark state: No light, electrons are bound in the valence band. Resistance is maximum.
Step 2 — Photon absorption: Light photons with energy > bandgap release electrons to the conduction band, creating electron-hole pairs.
Step 3 — Current conduction: Free charge carriers allow current to flow when voltage is applied.
Step 4 — Recovery: When light is removed, charge carriers gradually recombine — this is why fall time (recovery) is much slower than rise time.
⚠️ Gamma Characteristic: LDR resistance follows a roughly logarithmic relationship with light intensity. A 10× increase in lux reduces resistance by only ~2.2× (gamma ≈ 0.7). This is the key reason LDRs are suited for threshold switching but not precise measurement.
Common LDR Package Sizes and Specifications
|
Package Size |
Typical Model |
Peak Wavelength |
Dark Resistance |
Illuminated (10 lux) |
|
5mm |
GL5516 |
560nm |
10–20kΩ |
5–10kΩ |
|
5mm |
GL5528 |
560nm |
1–5MΩ |
10–20kΩ |
|
5mm |
GL5537-1 |
560nm |
2–5MΩ |
1–2kΩ |
|
8mm |
GL8538 |
560nm |
5–10MΩ |
2–5kΩ |
|
12mm |
GL12528 |
560nm |
10–20MΩ |
5–10kΩ |
Key Point: The interdigital zig-zag finger pattern maximizes the area of semiconductor exposed to light while allowing both terminals to collect current across the entire surface.
Uses pure semiconductor materials with no intentional doping. Electrons jump directly from valence band to conduction band. Common materials: Silicon (Si), Germanium (Ge), Gallium Arsenide (GaAs). Sensitive primarily in the visible to near-infrared range.
Uses doped semiconductors creating intermediate energy levels within the bandgap. Extends sensitivity into far-infrared range. Examples: Doped Germanium (Ge:Si, peak 1,000–3,000nm), Lead Sulfide (PbS, peak 1,000–3,000nm), Indium Antimonide (InSb, peak 5,000–7,000nm).
LDR Materials, Spectral Ranges, and RoHS Status
|
Material |
Peak Wavelength |
Sensitive Range |
Primary Application |
RoHS Status |
|
Cadmium Sulfide (CdS) |
500–700nm (green) |
400–800nm (visible) |
Consumer light detection, night lights |
RESTRICTED |
|
Cadmium Selenide (CdSe) |
600–720nm (red) |
500–900nm (red/NIR) |
Industrial light meters, solar trackers |
RESTRICTED |
|
Lead Sulfide (PbS) |
1,000–3,000nm (NIR) |
1,000–3,000nm |
Fire detection, IR spectroscopy, gas sensing |
Restricted (Pb) |
|
Gallium Arsenide (GaAs) |
800–900nm (NIR) |
700–1,000nm |
High-speed IR communication |
Compliant |
|
Doped Germanium (Ge:Si) |
1,500nm |
800–1,800nm |
Astronomy sensors, laser alignment |
Restricted (Ge:Si variant) |
⚠️ RoHS Compliance: Traditional CdS and CdSe LDRs contain cadmium, restricted under EU RoHS (2011/65/EU). For new product designs, consider GaAs-based LDRs or silicon photodiodes as fully RoHS-compliant alternatives.
Key LDR Datasheet Parameters
|
Parameter |
Definition |
Typical Values |
Why It Matters |
|
Dark Resistance (Rd) |
Resistance in total darkness |
1MΩ – 10MΩ |
Determines off-state behavior |
|
Illuminated Resistance (Rl) |
Resistance at specified light level |
100Ω – 10kΩ (10 lux) |
Determines on-state resistance |
|
Peak Wavelength (λp) |
Wavelength of maximum sensitivity |
500–700nm (CdS) |
Determines what light source triggers it |
|
Gamma |
Logarithmic slope (Rl10lux / Rl100lux ratio) |
0.6 – 0.9 |
Describes non-linearity of response |
|
Rise Time (tr) |
Time to 63% of final value (light applied) |
1–10 ms |
Speed of response to increasing light |
|
Fall Time (tf) |
Time to 37% of initial value (light removed) |
10 ms – 1 s |
Speed of response to decreasing light |
|
Max Voltage (Vmax) |
Maximum allowed voltage |
100–200 V |
Never exceed — leads to breakdown |
|
Max Power (Pmax) |
Maximum power dissipation |
50–500 mW |
Never exceed — leads to thermal damage |
Gamma = log(R1/R2) / log(L2/L1). For a GL5528 LDR with gamma = 0.7:
At 10 lux: R = 10kΩ | At 100 lux: R = 10kΩ / 2.2 ≈ 4,545Ω | At 1,000 lux: R ≈ 2,066Ω
Each 10× increase in lux reduces resistance by only ~2.2× — not 10×. This is the gamma characteristic.
LDR vs Photodiode vs Phototransistor — Complete Comparison
|
Feature |
LDR / Photoresistor |
Photodiode |
Phototransistor |
|
Active/Passive |
Passive (no bias needed) |
Active (reverse bias required) |
Active (bias required) |
|
Response speed |
Slow (ms to s) |
Very fast (ns–µs) |
Fast (µs–ms) |
|
Sensitivity range |
1 lux to 10,000 lux |
0.001 to 100,000+ lux |
1 lux to 10,000 lux |
|
Linearity |
Non-linear (gamma ~0.7) |
Linear (current ∝ lux) |
Near-linear |
|
Spectral range |
Narrow (CdS: 500–700nm) |
Wide (material dependent) |
Narrow to medium |
|
Output type |
Resistance change |
Photocurrent (µA–mA) |
Amplified current (mA) |
|
Cost |
Very low (pennies) |
Low to medium |
Low |
|
Best for |
Threshold switching (night/day) |
Precision measurement, fiber optics |
Switching circuits, presence detection |
|
Temperature stability |
Poor (high temp coefficient) |
Good |
Moderate |
|
Power consumption |
Zero in dark state |
Current when illuminated |
Current when illuminated |
LDR Materials and Their Spectral Response Ranges
|
Material |
Peak Wavelength |
Sensitive Range |
Practical Application |
|
CdS (Cadmium Sulfide) |
~560nm (green) |
400–800nm (visible) |
Human-eye-simulated light meters, automatic lighting |
|
CdSe (Cadmium Selenide) |
~680nm (red) |
500–900nm (red to NIR) |
Solar trackers, industrial process control |
|
PbS (Lead Sulfide) |
~2,000nm (NIR) |
1,000–3,000nm |
Fire detection, IR spectroscopy, gas sensing |
|
InSb (Indium Antimonide) |
~6,000nm (mid-IR) |
3,000–7,000nm |
Thermal imaging, gas analysis |
|
Ge:Si (Doped Germanium) |
~1,500nm |
800–1,800nm |
Astronomy sensors, laser alignment |
Application Tip: A standard CdS LDR peaks at ~560nm (green), matching the human eye's photopic response. This makes it ideal for lighting control (night/day switching) but poor for fire detection (needs IR sensitivity) or laser alignment (needs specific wavelength matching).
Common LDR Color Code and Resistance Values
|
Color Dot |
Model Series |
Dark Resistance |
Illuminated at 10 lux |
|
Green |
GL5516 |
10–20 kΩ |
5–10 kΩ |
|
Blue |
GL5528 |
1–5 MΩ |
10–20 kΩ |
|
Red |
GL5537-1 |
2–5 MΩ |
1–2 kΩ |
|
White |
GL5537-2 |
5–10 MΩ |
2–5 kΩ |
|
Yellow |
GL5549 |
1–3 MΩ |
3–6 kΩ |
LDR Multimeter Testing Procedure
|
Step |
Action |
Expected Result |
|
1. Power OFF |
Remove LDR from circuit |
No voltage applied during testing |
|
2. Set DMM |
Resistance mode (Ω or MΩ) |
Choose appropriate range for expected values |
|
3. Dark test |
Cover LDR completely (opaque box/tape) |
High resistance: 1MΩ to 10MΩ (varies by model) |
|
4. Light test |
Shine bright torch/flashlight at 10–30cm |
Resistance drops significantly: 100Ω to 10kΩ |
|
5. Calculate ratio |
Dark resistance ÷ Light resistance |
Good LDR: ratio > 100:1 | Marginal: 10–100:1 | Failed: < 10:1 |
Arduino LDR Circuit — Component List
|
Component |
Value |
Purpose |
|
Fixed resistor (R1) |
10kΩ – 100kΩ |
Forms voltage divider with LDR |
|
LDR |
GL5528 or equivalent |
Light sensor — variable resistance |
|
Arduino analog input |
A0 (or any ADC pin) |
Reads Vout from voltage divider |
|
LED + 220Ω resistor |
Optional |
Visual output (LED on when dark) |
|
Arduino 5V/GND |
Power supply |
Powers the voltage divider circuit |
Optimal resistor calculation: R1_optimal = √(R_dark × R_light)
Example: √(1MΩ × 10kΩ) = √10,000,000,000 ≈ 100kΩ. Use nearest standard value + trimmer potentiometer to fine-tune threshold.
Arduino Code: See full code in Blog-LDR.md — includes ADC reading, serial output, and LED night-light threshold switching.
Eight Major LDR Application Areas
|
Application |
How LDR Is Used |
LDR Type Preferred |
Why LDR Is Right |
|
Automatic Street Lighting |
Night/day switching for outdoor lights |
CdS, GL5528 |
Zero-power sensing, simple threshold, outdoor compatible |
|
Night Lamps & Garden Lights |
Turn on at dusk, off at dawn |
CdS, GL5516 |
Low cost, direct relay switching |
|
Solar Garden Lights |
Daytime charging + darkness detection |
CdS |
No power consumption in daylight, cheap |
|
Camera Light Meters |
Ambient light → exposure setting |
CdS (green peak) |
Matches photopic response of human eye |
|
Smoke & Fire Alarms |
IR detection (smoke blocks IR) |
PbS (IR peak) |
Responds to IR radiation from fire |
|
Solar Trackers |
Directional light sensing for panel alignment |
CdSe or dual-LDR array |
Low cost vs photodiodes |
|
Laser Security Systems |
Break-beam detection |
CdS (fast response variant) |
Simple, reliable beam interruption detection |
|
Musical Instruments |
Audio compressor/limiter (VU meter driven) |
CdS |
Natural response time matches audio dynamics |
Environmental Factors Affecting LDR Performance and Recommended Mitigations
|
Factor |
Effect on LDR |
Mitigation Strategy |
|
High Temperature |
Resistance drops even in darkness (NTC characteristic) |
Temperature compensation circuit or switch to photodiode |
|
Humidity / Moisture |
Oxidation of contacts, material degradation, leakage |
Use sealed LDR, IP65+ enclosure, protective window |
|
UV Exposure |
Reduced sensitivity over time, shifted spectral response |
UV-blocking transparent cover, shade the LDR from direct sunlight |
|
Mechanical Shock |
Cracked ceramic substrate, broken internal connections |
Handle gently during assembly; use sockets for frequent replacement |
|
Exceeding Vmax/Pmax |
Immediate thermal damage, permanent failure |
Always calculate worst-case power; use proper voltage limiting |
|
Long-term Aging |
Gradual sensitivity loss, increased dark resistance |
Specify higher-quality sealed LDRs for long-life applications |
⚠️ RoHS Compliance: CdS and CdSe LDRs contain cadmium — restricted under EU RoHS 2011/65/EU. For new EU-market products, source RoHS-compliant GaAs-based alternatives or use silicon photodiodes as a compliant replacement. Always request RoHS test reports from your supplier.
Troubleshooting Table: Common LDR Problems and Solutions
|
Symptom |
Most Likely Cause |
Solution |
|
LDR always shows low resistance |
Permanently damaged (light or thermal) |
Replace LDR; never exceed Vmax or Pmax |
|
LDR always shows very high resistance |
Open circuit (broken internal connection) |
Replace LDR; check for physical damage |
|
Works in darkness but not bright light |
Semiconductor exhausted / aged LDR |
Replace LDR; UV degradation may have occurred |
|
Circuit activates in wrong conditions |
Incorrect threshold resistor value |
Adjust fixed resistor; use trimmer potentiometer to fine-tune |
|
Sensitivity degraded over time |
UV degradation or moisture contamination |
Replace LDR; add UV-blocking cover or sealed enclosure |
|
Readings fluctuate wildly |
Loose connection or intermittent open circuit |
Re-solder the LDR; check for cracked ceramic substrate |
|
LDR gets hot during operation |
Voltage too high — exceeding Pmax |
Reduce voltage; use higher-resistance LDR; check Vmax rating |
|
LED doesn't turn on at night |
Transistor/relay circuit issue, not LDR |
Test transistor separately; check relay coil resistance; check wiring |
|
Works on multimeter but not in circuit |
Wrong resistor value in divider |
Calculate optimal R1; remove LDR from circuit for independent testing |
Selection Decision Tree: Choosing the Right LDR
|
Step |
Question |
Answer → LDR Type |
|
Step 1 |
What wavelength do you need? |
Visible (400–700nm) → CdS | Near-IR (700–1500nm) → CdSe/GaAs | Mid-IR (>1500nm) → PbS/InSb |
|
Step 2 |
Simple switching or precise measurement? |
Threshold switching → Standard LDR | Precise measurement → Photodiode or digital light sensor IC |
|
Step 3 |
What resistance range? |
Higher dark resistance needed → larger package (12mm) | Lower dark resistance → smaller package (5mm) |
|
Step 4 |
How fast must it respond? |
Slow OK (lights, lamps) → Standard LDR | Fast response needed → Phototransistor |
|
Step 5 |
RoHS compliance required? |
Yes → GaAs LDR or silicon photodiode | No → Standard CdS LDR acceptable |
|
Step 6 |
What environment? |
Indoor/controlled → Standard | Outdoor/humid → Sealed LDR or IP65+ enclosure |
|
Step 7 |
What price point? |
Consumer/toys (<$0.20) → Standard CdS | Industrial/military → Quality sealed ($1–5) |
Q1: What is an LDR and how does it work?
An LDR (Light Dependent Resistor) is a passive component whose resistance decreases when exposed to light. When photons strike the semiconductor material (typically cadmium sulfide), they release electrons that become charge carriers, reducing resistance. This is called photoconductivity.
Q2: What is the difference between an LDR and a photoresistor?
There is no difference — LDR and photoresistor are two names for the same component. LDR stands for Light Dependent Resistor; photoresistor emphasizes the light-responsive (photo) nature of the component.
Q3: What is the difference between LDR and photodiode?
An LDR is passive — no external power is needed for the light-sensing mechanism, and it works by changing resistance. A photodiode is active — it requires reverse bias voltage and produces a photocurrent proportional to light intensity. Photodiodes are faster (ns vs ms), more linear, and more precise.
Q4: What is the difference between LDR and phototransistor?
A phototransistor is essentially a photodiode with an integrated transistor amplifier — it produces higher output current than a photodiode and is still much faster and more linear than an LDR. Use an LDR for simple threshold switching; use a phototransistor when you need speed or current amplification.
Q5: How do I test an LDR?
Set your digital multimeter to resistance mode. Cover the LDR completely — it should read high resistance (1MΩ to 10MΩ for standard CdS LDRs). Shine a bright light — resistance should drop significantly (100Ω to 10kΩ). A good LDR should show a dark/light resistance ratio of at least 100:1.
Q6: What is the typical resistance of an LDR?
In total darkness: 1MΩ to 10MΩ. Under normal room lighting (~100 lux): 10kΩ to 100kΩ. In bright direct light (~10,000 lux): 100Ω to 2kΩ. Always check the specific model's datasheet for accurate values.
Q7: Can an LDR be used to measure light intensity (lux)?
Technically yes, but with significant limitations. LDR resistance has a logarithmic relationship with lux, and there is substantial unit-to-unit variation and temperature drift. For accurate lux measurement, use a calibrated photodiode or a digital light sensor IC (BH1750, TSL2561). Reserve LDRs for simple on/off threshold applications.
Q8: What causes an LDR to fail?
The most common causes are: (1) exceeding maximum voltage or power ratings — thermal destruction; (2) prolonged exposure to high-intensity light — permanent sensitivity reduction; (3) mechanical damage from shock or vibration; (4) moisture ingress causing corrosion.
Q9: Does an LDR have polarity?
No. LDRs have two terminals with no polarity — they can be connected in either direction. This is unlike LEDs and photodiodes which have polarity.
Q10: Can I use an LDR outdoors?
Yes, but the LDR must be protected. Use a weatherproof enclosure with a clear window, or use an LDR specifically rated for outdoor use. Standard through-hole LDRs are not sealed and will degrade quickly in rain or high humidity.
Q11: Why does my LDR circuit turn on too early or too late?
The threshold is set by the ratio of the fixed resistor (R1) to the LDR's resistance at the trigger light level. Increase R1 to make it less sensitive (turns on in darker conditions); decrease R1 to make it more sensitive. Use a potentiometer in series with R1 for easy threshold adjustment.
Q12: What is the peak wavelength of a standard CdS LDR?
A standard CdS (cadmium sulfide) LDR peaks at approximately 560nm (green light), close to the human eye's peak photopic sensitivity (~555nm). This makes CdS LDRs ideal for applications where human perception of light/dark is the relevant metric.
Q13: Are LDRs affected by temperature?
Yes — LDRs have a negative temperature coefficient (resistance decreases as temperature increases, even at the same light level). At high temperatures, an LDR may show lower resistance even in darkness, causing false triggering. Compensate in software using a thermistor or switch to a temperature-stable light sensor.
Q14: What are the alternatives to cadmium-based LDRs?
For RoHS-compliant applications: gallium arsenide (GaAs) LDRs, silicon photodiodes, or digital light sensors (BH1750, TSL2561, MAX44009). These offer similar functionality without restricted materials, though at slightly higher cost.
Q15: Can an LDR detect fire or heat?
Standard CdS LDRs cannot detect fires — they are insensitive to IR above ~800nm. However, PbS (Lead Sulfide) LDRs have peak sensitivity at 1,000–3,000nm, covering the IR signature of fires. PbS-based fire alarm modules are widely available for this purpose.
· LDR = Light Dependent Resistor = Photoresistor — passive component, resistance decreases with light
· Gamma characteristic: resistance follows logarithm of light intensity — ideal for threshold switching, not precision lux measurement
· Two main types: intrinsic (visible light, pure semiconductor) and extrinsic (infrared, doped semiconductor)
· CdS LDRs are cheapest/most common but restricted under RoHS due to cadmium content
· LDRs are slow (ms to s response time) — photodiodes and phototransistors are faster alternatives
· Voltage divider circuit is the standard interface between LDR and microcontroller ADC
· LDRs are affected by temperature, humidity, UV, and mechanical shock — protect accordingly
· For simple night/day threshold switching, LDRs are the simplest and cheapest solution
· For precise measurement, use a calibrated photodiode or digital light sensor IC (BH1750, TSL2561)
· Always protect outdoor LDRs with sealed enclosures; always stay below Vmax and Pmax ratings
WELL LINK CHIPS GROUP CO.,LIMITED FLAT/RM 1802 BEVERLY HOUSE 93-107 LOCKHART ROAD WANCHAI HK
