A photo sensitive resistor (commonly called a photoresistor or Light Dependent Resistor / LDR) is a passive semiconductor component whose electrical resistance decreases as incident light intensity increases. Unlike active photodetectors, it requires no bias voltage or PN junction to operate; it simply acts as a variable resistor driven by the photoelectric effect. When photons with sufficient energy strike the semiconductor material, they excite electrons into the conduction band, increasing conductivity. For hobbyists and engineers, the 5mm Cadmium Sulfide (CdS) cell—specifically the GL5528—remains the benchmark part for visible light detection, though material science and environmental regulations are shifting the landscape.

Photo Sensitive Resistor Specifications & Material Types

Selecting the right photo sensitive resistor requires looking past the physical package and examining the semiconductor material. The substrate dictates the spectral sensitivity (which wavelengths it detects), the baseline resistance, and the thermal stability. Below is a data-dense comparison of the primary LDR constructions available on the market.

Material Type Construction Tolerance (@ 10 lux) Tempco (Temperature Coefficient) Typical Use
Cadmium Sulfide (CdS) Sintered pellet on ceramic substrate, epoxy coated ±30% to ±50% Negative (resistance drops as ambient temp rises) Streetlights, nightlights, camera exposure meters
Lead Sulfide (PbS) Chemically deposited thin film on glass/ceramic ±20% to ±30% Negative (highly sensitive to thermal noise) IR flame sensors, missile tracking, spectroscopy
Indium Antimonide (InSb) Crystalline lattice, requires cryogenic cooling ±10% Extreme negative (requires liquid nitrogen) Mid-IR astronomy, thermal imaging arrays
Amorphous Silicon (a-Si) Thin-film deposition on glass substrate ±20% to ±25% Minimal (highly stable across -20°C to 60°C) RoHS-compliant consumer electronics, LCD backlighting
RoHS Compliance Warning: Cadmium Sulfide (CdS) and Lead Sulfide (PbS) contain heavy metals restricted under the EU RoHS Directive. While CdS LDRs are still widely sold for hobbyist and prototyping use, commercial production in 2026 requires substituting them with Amorphous Silicon (a-Si) photoresistors or active phototransistors.

Decoding Markings and Physical Identification

Unlike standard through-hole resistors that use EIA color bands, photo sensitive resistors rely on printed alphanumeric codes and physical dimensions to indicate their specifications. The most ubiquitous series is the GL55xx family. The physical diameter of the serpentine trace face dictates the power rating and voltage limits, while the suffix numbers dictate the resistance profile.

For the standard GL5528:
GL = Manufacturer prefix / General LDR
55 = 5mm diameter package (Max voltage typically 150VDC, Max power dissipation 100mW)
28 = Resistance code indicating the specific dark and illuminated resistance ranges.

Here is the exact spec-sheet data for the most common 5mm CdS markings you will encounter in component kits:

Part Marking Illuminated Resistance (10 lux) Dark Resistance (0 lux) Spectral Peak
GL5506 2kΩ - 5kΩ 0.2 MΩ (min) 540 nm
GL5516 5kΩ - 11kΩ 0.5 MΩ (min) 540 nm
GL5528 10kΩ - 20kΩ 1.0 MΩ (min) 540 nm
GL5537 20kΩ - 30kΩ 2.0 MΩ (min) 540 nm
GL5549 45kΩ - 100kΩ 5.0 MΩ (min) 540 nm

Note: "10 lux" is roughly equivalent to the ambient light of a dimly lit living room or twilight. "0 lux" is measured after the part has been kept in complete darkness for at least 30 seconds to allow the resistance to stabilize.

Selection Guide: Which Type for Which Job?

Choosing the correct photo sensitive resistor depends entirely on the speed of the light change you need to detect and the ambient environment.

  • Slow Ambient Light Changes (Streetlights, Nightlights, Solar Trackers): Use standard CdS (GL5528). The slow response time (20ms to 50ms) is actually a feature here, as it acts as a natural low-pass filter, preventing the circuit from triggering during brief shadows or lightning flashes.
  • Infrared Flame Detection: Use PbS cells. They peak around 2000nm, making them highly sensitive to the specific IR emission of hydrocarbon fires, ignoring visible light entirely.
  • High-Speed Optical Communications or Encoders: Do not use an LDR. The photoconductive decay time of CdS is far too slow for signals above 10Hz. For kHz or MHz optical signals, you must use a PIN photodiode or an avalanche photodiode.
  • Commercial Consumer Products (EU/Global Markets): Use Amorphous Silicon (a-Si). They mimic the visible light response of CdS but comply with modern environmental regulations and offer better thermal stability.

Safe Substitution When the Exact Part is Missing

When your exact LDR is out of stock, or you are repairing legacy equipment, blind substitution will break your circuit's triggering thresholds. Here is how to substitute safely based on the scenario.

Scenario 1: Substituting a Different LDR Code

If your schematic calls for a GL5528 but you only have a GL5516, the LDR itself is usually part of a voltage divider feeding a comparator or microcontroller ADC. Because the resistance curves differ, you must recalculate the fixed series resistor ($R_{fixed}$) to center your voltage output.

Use the geometric mean formula to find the optimal fixed resistor for your specific light/dark thresholds:

$R_{fixed} = \sqrt{R_{dark} \times R_{light}}$

For a GL5528 ($R_{dark}$ = 1MΩ, $R_{light}$ = 15kΩ), the ideal fixed resistor is $\approx 122k\Omega$ (use 120kΩ standard). If you swap to a GL5506 ($R_{dark}$ = 0.2MΩ, $R_{light}$ = 3kΩ), your new $R_{fixed}$ must be $\approx 24k\Omega$. Failing to swap the fixed resistor will result in the ADC pin never crossing the logic threshold.

Scenario 2: Substituting an LDR with a Phototransistor

If you need faster response times or are migrating to RoHS-compliant parts, a phototransistor (like the PT333-3C) is the standard replacement. However, you cannot drop a phototransistor directly into an LDR footprint.

  • An LDR is a passive, bidirectional resistor. It can be placed on either the top or bottom of a voltage divider.
  • A phototransistor is an active, directional component that acts as a current source when illuminated.

The Fix: Place the phototransistor with its Collector tied to VCC and its Emitter tied to GND through a pull-down resistor (typically 10kΩ to 100kΩ). The output voltage is taken from the Emitter pin. Unlike the LDR, the phototransistor outputs a high voltage when light hits it, which may require you to invert your logic in software or swap the comparator inputs.

Failure Modes and Visual Diagnostics

Photo sensitive resistors are remarkably durable, but they are not immune to environmental and electrical stress. When diagnosing a faulty LDR, grab your multimeter, set it to the Ohms (Ω) range, and look for these specific failure modes.

Failure Mode Visual Symptom Multimeter Reading Root Cause
Moisture Ingress / Delamination Cracked, cloudy, or peeling epoxy coating; white/green crust on edges. Erratic resistance; resistance drops when breathing on it (humidity). Humidity creates parasitic conductive paths across the high-impedance ceramic substrate.
Thermal Runaway / Overvoltage Blackened, blistered, or melted serpentine trace visible through the epoxy. Permanently shorted (near 0Ω) or open circuit (OL) in both light and dark. Exceeding the 150VDC max or 100mW power rating. The trace literally burns out like a fuse.
Light Fatigue (Memory Effect) No physical damage; component looks pristine. Resistance remains artificially low for minutes after returning to darkness. Prolonged exposure to intense light causes electron traps in the CdS lattice to fill, delaying recombination.
Mechanical Shear Leads wobble at the base; visible hairline crack where the lead enters the epoxy. Open circuit (OL) or intermittent connection when tapping the part. Improper lead bending during PCB assembly. Leads must be bent before insertion, never used as a lever.
Bench Testing Tip: When testing dark resistance, do not just cup your hand over the LDR. Your body heat will slightly alter the temperature of the CdS material, and stray photons will leak in. For a true dark resistance test, place a piece of black electrical tape directly over the sensor face, wait 30 seconds for the lattice to stabilize, and then read the multimeter.

For deeper circuit integration and voltage divider math, referencing established voltage divider tutorials is highly recommended to ensure your ADC scaling matches the non-linear resistance curve of the LDR. Additionally, reviewing the fundamental physics of light dependent resistors will help you understand why the spectral peak of your chosen material must match the light source (e.g., matching a 540nm CdS cell with a green LED, rather than an IR emitter) for maximum sensitivity.