To accurately define light dependent resistor (LDR) behavior, you must look past the simple textbook definition. An LDR, or photoresistor, is a passive, variable resistor whose bulk resistance decreases as incident photon energy increases. But on the workbench, knowing that 'light makes resistance drop' is not enough to design a reliable circuit. You need exact lux-resistance curves, spectral peak matching, and thermal coefficients.
Take the industry-standard GL5528 Cadmium Sulfide (CdS) cell. In total darkness (0 lux), it presents roughly 1 MΩ of resistance. Expose it to 10 lux (dim room light), and it drops to 10–20 kΩ. Hit it with 100 lux (overcast daylight), and it falls to 2–4 kΩ. This non-linear, logarithmic response is the defining characteristic of the component. If you are designing an automatic streetlight trigger or a laser-break alarm, selecting the right LDR material and decoding its physical markings will dictate whether your circuit switches cleanly or chatters erratically.
LDR Material Types and Selection Matrix
Not all photoresistors are created equal. The semiconductor material deposited in the zigzag trace on the ceramic substrate determines the spectral response (which color of light it 'sees'), the base resistance, and the temperature coefficient (tempco). Below is the definitive selection matrix for the four primary LDR chemistries available in 2026.
| Material | Spectral Peak | Typical Dark Resistance | Tempco (ppm/°C) | Response Time | Primary Application |
|---|---|---|---|---|---|
| Cadmium Sulfide (CdS) | 520 nm (Green) | 1 MΩ – 10 MΩ | -2000 to -3000 | 20 – 50 ms | Streetlights, nightlights, camera exposure |
| Cadmium Selenide (CdSe) | 720 nm (Red) | 500 kΩ – 2 MΩ | -3000 to -4000 | 2 – 10 ms | Fast optical switches, flame flicker detection |
| Lead Sulfide (PbS) | 2000 nm (Near-IR) | 1 MΩ – 5 MΩ | +3000 (Positive) | 100 – 300 µs | IR spectroscopy, missile plume tracking |
| Indium Antimonide (InSb) | 5000 nm (Mid-IR) | < 1 kΩ (Cooled) | N/A (Cryogenic) | < 1 µs | Thermal imaging, industrial gas analysis |
Which Type for Which Job?
For 95% of hobbyist and commercial DIY projects (Arduino-based sun trackers, automatic porch lights, laser tripwires), CdS is the correct choice. Its spectral peak at 520 nm closely mimics the human eye's sensitivity to visible light, making it ideal for lighting automation. However, if you are building a sensor to detect the invisible infrared heat of a flame or a remote control beam, CdS will be completely blind. You must step up to a PbS cell. Note that PbS cells have a positive tempco, meaning their dark resistance actually increases as they get hotter, requiring thermal compensation in precision analog circuits.
Decoding Physical Markings and Binning Codes
Unlike standard carbon-film resistors, LDRs do not use color bands. Because the resistance varies dynamically, manufacturers print a part number and use epoxy color dots to indicate binning. The most ubiquitous series on the market is the GL55xx family. The 'GL' stands for the original manufacturer lineage, '55' denotes the standard 5mm disc package, and the final two digits define the illuminated resistance at the 10-lux test point.
Here is how to read the GL55 series markings when sorting through your component bins:
| Part Number | Resistance @ 10 Lux | Dark Resistance (Min) | Max Voltage Rating |
|---|---|---|---|
| GL5516 | 5 kΩ – 10 kΩ | 0.5 MΩ | 150 VDC |
| GL5528 | 10 kΩ – 20 kΩ | 1.0 MΩ | 150 VDC |
| GL5537-1 | 20 kΩ – 30 kΩ | 2.0 MΩ | 150 VDC |
| GL5539 | 30 kΩ – 90 kΩ | 3.0 MΩ | 150 VDC |
| GL5549 | 45 kΩ – 140 kΩ | 5.0 MΩ | 150 VDC |
The Color Dot: If you look closely at the top of a premium LDR (like the Excelitas VT90N2 or higher-bin GL55 parts), you may see a tiny red, green, or blue paint dot. This is not a polarity marker—LDRs are non-polar. This dot indicates the manufacturer's laser-trimmed bin for dark resistance tolerance or specific gamma-curve matching for dual-cell applications (like stereo audio compressors).
How to Substitute Safely When the Exact Part is Missing
When repairing a vintage guitar pedal, an old synthesizer, or a discontinued appliance, you often won't find the exact OEM LDR. Substituting an LDR is not as simple as matching a fixed resistor value; you must match the illumination curve. Here is the bench-tested framework for safe substitution.
Rule 1: Match the 10-Lux Resistance, Not the Dark Resistance
Most voltage divider circuits using LDRs are designed to trigger a transistor or comparator when the light hits a specific threshold (e.g., dusk). The circuit's biasing resistors are tuned to the LDR's resistance under ambient room light (roughly 10 to 50 lux). If your original circuit called for a 10 kΩ @ 10 lux LDR, but you only have a 50 kΩ @ 10 lux LDR in your kit, the circuit will trigger too late or fail to trigger entirely.
Rule 2: The Parallel Resistor Hack
If your substitute LDR has an illuminated resistance that is too high, you can force it down using a fixed parallel resistor.
Scenario: You need a 10 kΩ response at 10 lux, but only have a GL5539 (approx. 50 kΩ at 10 lux).
Fix: Solder a 12 kΩ fixed carbon-film resistor in parallel with the LDR legs.
Using the parallel resistance formula: R_total = (R1 × R2) / (R1 + R2)
R_total = (50,000 × 12,000) / (50,000 + 12,000) = 9,677 Ω
This yields a near-perfect 9.6 kΩ under room light. In total darkness, the LDR goes to 3 MΩ, and the 12 kΩ resistor dominates, capping the maximum dark resistance at 12 kΩ. If the circuit relies on a >100 kΩ dark resistance to stay fully off, this hack will fail, and you must source the correct part or add an op-amp buffer stage.
For a deeper look into how photoresistors integrate into analog biasing networks, the semiconductor chapter on All About Circuits provides excellent foundational schematics.
Failure Modes and Visual Diagnostics
LDRs are remarkably robust, but they are not immortal. Because they rely on a thin semiconductor layer exposed to the environment (sealed only by a clear epoxy dome), they suffer from specific failure modes that you can diagnose visually and with a standard multimeter.
1. Light Fatigue and Hysteresis (The 'Memory' Effect)
Visual Symptom: None. The part looks pristine.
Bench Test: Shine a bright 1000-lumen flashlight directly on the LDR for 10 seconds, then turn it off. A healthy CdS cell will return to its >500 kΩ dark resistance within 2 to 5 seconds. A fatigued LDR will 'remember' the light, taking several minutes to climb back above 100 kΩ.
Impact: This hysteresis causes automatic lighting systems to turn on late at dusk and turn off late at dawn. The only fix is replacement.
2. Moisture Ingress and Delamination
Visual Symptom: Micro-cracking in the clear epoxy dome, or a milky, cloudy appearance around the edges of the ceramic substrate.
Bench Test: Breathe heavily on the part to raise local humidity. If the resistance erratically jumps by 20% or more, moisture has penetrated the seal and is altering the conductivity of the cadmium sulfide layer.
Fix: Discard the part. You can sometimes salvage outdoor sensors by coating the dome in a thin layer of clear conformal coating (like MG Chemicals 419D), but this alters the spectral response slightly.
3. Thermal Runaway and Trace Burnout
Visual Symptom: The zigzag semiconductor trace looks dark brown or black in the center, and the epoxy may show a slight yellow burn mark.
Cause: Exceeding the power dissipation limit. Most 5mm LDRs are rated for 100 mW continuous. If you place an LDR directly across a 5V rail without a series current-limiting resistor, and the light hits it (dropping resistance to 100 Ω), Ohm's law dictates a current of 50 mA, resulting in 250 mW of heat. The semiconductor layer literally cooks itself off the ceramic substrate.
Prevention: Always place a fixed series resistor (e.g., 1 kΩ) in the branch with the LDR to cap the maximum current, a standard practice detailed in Component101's LDR design guidelines.
By understanding the material chemistry, decoding the binning marks, and respecting the thermal limits of the semiconductor trace, you can reliably integrate LDRs into analog and microcontroller circuits without falling victim to the common pitfalls that plague off-the-cuff designs.
