A photoresistor LDR (Light Dependent Resistor) is a passive, bidirectional semiconductor component whose resistance decreases exponentially as incident light intensity increases. Unlike photodiodes or phototransistors, an LDR has no PN junction and requires no bias polarity, making it functionally identical to a standard resistor in a circuit—just one controlled by photons rather than fixed at manufacture. The core metric for any LDR is the ratio between its Dark Resistance (measured after 5 seconds in total darkness) and its Illuminated Resistance (typically measured at 10 lux or 100 lux).

While modern designs often favor active silicon sensors for speed, the Cadmium Sulfide (CdS) photoresistor LDR remains the undisputed choice for high-voltage isolation, simple AC-switching (like streetlight controllers), and low-cost ambient light tracking where microsecond response times are unnecessary.

Data-Dense Reference: Standard CdS Photoresistor LDR Specs

The GL55xx series is the industry workhorse for hobbyist and commercial ambient light sensing. When selecting a part, the physical diameter dictates the surface area of the CdS trace, which directly correlates to maximum power dissipation and base sensitivity. Below are the exact bench-measured and datasheet values for the most common through-hole variants.

Model Diameter Dark Res. (MΩ) 10 Lux Res. (kΩ) Max Voltage (VDC) Response Time (ms) Spectral Peak
GL5516 5mm 0.5 - 1.2 5 - 10 150 Rise: 20 / Fall: 30 540 nm
GL5528 5mm 1.0 - 2.0 10 - 20 150 Rise: 20 / Fall: 30 540 nm
GL5537-1 7mm 0.5 - 1.2 20 - 30 150 Rise: 30 / Fall: 40 540 nm
GL5549 12mm 5.0 - 10.0 100 - 200 250 Rise: 50 / Fall: 80 540 nm
Interfacing with an ESP32 or Arduino: To read a GL5528 with a microcontroller ADC, build a voltage divider using a 10kΩ fixed resistor. Place the LDR at the top (connected to 3.3V) and the 10kΩ resistor at the bottom (connected to GND). This specific pairing centers the voltage swing around 1.65V under typical indoor room lighting (approx. 50 lux), maximizing the usable resolution of a 12-bit ADC.

Material and Type Comparison: Which Sensor for Which Job?

Not all light-dependent components are created equal. While the CdS photoresistor LDR mimics the human eye's spectral response, other applications require infrared sensitivity or microsecond switching. Use this matrix to select the right architecture for your specific constraints.

Component Type Material / Construction Spectral Peak Linearity & Tolerance Tempco (Temp. Coefficient) Typical Use Case
CdS Photoresistor Cadmium Sulfide on Ceramic 540 nm (Visible Green) Non-linear, ±20% batch variance High (Resistance drops as temp rises) Streetlights, night-lights, camera exposure
PbS Photoresistor Lead Sulfide Thin Film 2000 nm (Near-IR) Moderate, requires thermal stabilization Extreme (Requires TEC cooling for precision) Flame detection, IR spectroscopy
Silicon Photodiode PIN or PN Junction 800 - 900 nm (Near-IR) Highly linear, ±2% tolerance Low (Predictable mV/°C drift) Fiber optics, precision lux meters, pulse oximetry
Phototransistor NPN Bipolar with exposed base 800 nm (Visible to IR) Non-linear, high gain variance Moderate (Beta shifts with temp) Optocouplers, encoder wheels, object detection

Choose a CdS LDR when: You need to switch an AC load directly via a TRIAC without a DC power supply, or when you need a sensor that naturally ignores infrared heat sources (like incandescent bulbs or heaters) and only reacts to visible ambient light.

Choose a Photodiode when: You are building a scientific instrument, a high-speed optical data link, or a circuit where temperature stability from -20°C to 80°C is mandatory. For deeper theory on semiconductor light interaction, refer to the Electronics Tutorials guide on photoconductivity.

Decoding Physical Markings and Form Factors

Unlike standard resistors that rely on color bands, or SMD capacitors that use cryptic three-digit codes, the physical markings on a photoresistor LDR are notoriously sparse. Here is how to identify what is sitting on your workbench.

  • The Backstamp Code: Most modern Asian-manufactured CdS cells (like the GL series) print a 4-digit number on the rear epoxy or ceramic. A stamp reading 5528 indicates a GL5528. A stamp reading 5516 is a GL5516. If you see a small RoHS or Pb-Free logo, it means the manufacturer has substituted the traditional lead-based solder dip on the leads with a tin-bismuth or SAC alloy, which requires slightly lower soldering iron temperatures (max 350°C vs 380°C).
  • The Face Dot (Vintage/Premium): Premium Western-manufactured LDRs (such as the Excelitas/PerkinElmer VT935G series or vintage Clairex CL cells) often feature a tiny colored dot painted on the front face next to the zigzag trace. This dot is a dark resistance bin code. For example, a red dot might indicate a dark resistance of 10MΩ minimum, while a yellow dot indicates 1MΩ. Always check the specific manufacturer's binning sheet, as these colors are not standardized across brands.
  • Diameter as a Primary Code: If the backstamp is rubbed off, measure the active ceramic substrate diameter with calipers. A 5mm substrate is almost universally a mid-range sensitivity part (10-20kΩ at 10 lux). A 7mm or 12mm substrate indicates a high-resistance, high-sensitivity part designed for ultra-low light detection.

Failure Modes and Visual Diagnostics

Photoresistors are generally robust, but they are highly susceptible to environmental and electrical abuse. When an LDR fails in the field, it rarely fails open; it usually fails short or drifts out of spec. Here are the primary failure modes and how to spot them on the bench.

1. Silver Migration (Dendrite Growth)

The Physics: The interdigitated fingers on an LDR are often printed with silver-palladium paste. Under high humidity and a continuous DC voltage bias, silver ions migrate across the ceramic substrate, forming conductive dendrites.

Visual Symptom: Look closely at the zigzag trace under a magnifying glass. You will see microscopic silver or greenish-white webbing bridging the gaps between the dark trace lines. The dark resistance will plummet from 1MΩ to a few hundred ohms, even in total darkness.

Prevention: Never use unsealed CdS LDRs in outdoor or high-humidity environments without a conformal coating or a sealed glass enclosure. For DC-biased high-humidity applications, switch to a phototransistor.

2. Thermal Substrate Cracking

The Physics: The ceramic substrate is brittle. If a technician dwells too long with a soldering iron on the leads, or if the circuit pushes more than the rated power (typically 100mW for a 5mm part), the thermal expansion mismatch between the CdS layer and the ceramic causes micro-fractures.

Visual Symptom: Hairline cracks radiating from the lead entry points into the center of the substrate. The epoxy encapsulation may also appear yellowed or lifted. Electrically, this manifests as intermittent 'open' readings when the board is flexed.

3. Light Fatigue and Hysteresis

The Physics: Prolonged exposure to intense UV or direct sunlight can create deep-level traps in the CdS crystal lattice.

Visual Symptom: There are no physical marks. The symptom is electrical: after being exposed to bright sunlight, the LDR takes several minutes (instead of milliseconds) to return to its rated dark resistance when moved to a dark room. This is a common point of failure in outdoor solar tracking sensors that lack UV filters.

Safety Note on Cadmium: CdS photoresistors contain Cadmium, a heavy metal. While perfectly safe when encapsulated in epoxy, you must never sand, grind, or burn the face of an LDR. If an LDR's ceramic substrate is physically shattered, dispose of it as hazardous e-waste according to your local RoHS/WEEE regulations, and wash your hands.

Safe Substitution and Modern Alternatives

When the exact BOM part is out of stock, substituting an LDR requires matching three parameters: dark resistance, illuminated resistance at your operating lux, and physical diameter.

Substituting LDR for LDR: If your circuit calls for a GL5528 (10-20kΩ at 10 lux) and you only have a GL5516 (5-10kΩ at 10 lux), the substitution is generally safe for simple analog dividers. The GL5516 will simply output a slightly higher voltage under the same lighting. However, if the LDR is part of an RC timing circuit (like a 555 timer astable oscillator), swapping to a lower resistance part will increase the oscillator frequency. You must proportionally increase the timing capacitor to compensate.

Substituting LDR with a Phototransistor (e.g., TEPT5600): If you need to replace an LDR due to RoHS compliance mandates or require a faster response time, a visible-light phototransistor is the modern alternative. Warning: This is not a drop-in replacement. An LDR is passive and bidirectional; a phototransistor is active and polarized. You must identify the collector and emitter (usually the shorter lead or flat edge is the emitter), connect the collector to VCC via a pull-up resistor (e.g., 100kΩ), and tie the emitter to GND. The analog voltage is read at the collector node. Furthermore, phototransistors are highly sensitive to infrared light, meaning a nearby TV remote or heat source will trigger false readings that a CdS LDR would naturally ignore.