A photoresistor, formally known as a Light Dependent Resistor (LDR), is a passive, light-sensitive variable resistor. Its resistance drops non-linearly as incident photons increase. For 90% of hobbyist and DIY ambient light sensing, the GL5528 CdS (Cadmium Sulfide) cell is the default standard, offering roughly 10kΩ at 10 lux and exceeding 1MΩ in total darkness. While increasingly replaced by active silicon sensors in commercial manufacturing due to environmental regulations, LDRs remain a staple on the workbench for their simplicity, high voltage tolerance, and zero-requirement for biasing ICs.
The Physics and Anatomy of Light-Dependent Resistors
Unlike photodiodes or phototransistors that generate a current or require a bias voltage to operate, a photoresistor is purely passive. It relies on the internal photoelectric effect within a semiconductor lattice. When a photon with sufficient energy strikes the semiconductor material, it excites an electron from the valence band across the bandgap into the conduction band, creating an electron-hole pair. This increases the material's conductivity, thereby lowering its electrical resistance.
If you look closely at the face of a standard 5mm CdS cell, you will see a distinct zig-zag or interdigitated pattern. This geometry is not arbitrary. It is engineered to maximize the length of the semiconductor path (increasing dark resistance) while minimizing the distance between the metal electrodes (allowing for a drastic drop in resistance when illuminated). The spectral response of the material dictates which photons it can absorb; Cadmium Sulfide has a bandgap of roughly 2.4 eV, making it highly sensitive to green and yellow visible light, closely mimicking the human eye's photopic response curve.
Decoding the Datasheet: Markings, Codes, and Binning
Unlike standard carbon film resistors, photoresistors do not use color bands to indicate value. Because their resistance is entirely dependent on ambient light, a static 'value' marking would be meaningless. Instead, manufacturers use model numbers and binning dots printed on the rear or rim of the component.
- The Model Number: Printed on the back epoxy. A code like '5528' breaks down into two parts. The '55' typically denotes the series and physical diameter (5mm). The '28' designates the specific resistance curve and illumination profile. A '5516' will have a completely different resistance at 10 lux than a '5537'.
- Batch and Date Codes: Usually a 4-digit alphanumeric string below the model number, used for factory traceability. Ignore this for circuit design.
- Binning Dots: Many manufacturers paint a small colored dot on the rim of the LDR. This is a factory binning code indicating the exact tolerance of the cell at a specific test lux level (usually 10 lux). For example, a green dot might guarantee 8kΩ–12kΩ at 10 lux, while a blue dot might bin it at 15kΩ–25kΩ.
| Model | Max Light Res (kΩ) | Min Dark Res (MΩ) | Response Time (ms) | Best Application |
|---|---|---|---|---|
| GL5516 | 5 - 10 | 0.5 | Rise: 20 / Decay: 30 | Bright indoor light switches |
| GL5528 | 10 - 20 | 1.0 | Rise: 20 / Decay: 30 | General purpose dusk-to-dawn |
| GL5537-1 | 20 - 30 | 2.0 | Rise: 30 / Decay: 40 | Low-light streetlamp triggers |
| GL5549 | 45 - 100 | 5.0 | Rise: 60 / Decay: 100 | Camera exposure metering |
Material Comparison: Which Photoresistor for Which Job?
Selecting the right photoresistor requires matching the semiconductor material to the spectral output of your light source and the environmental constraints of your enclosure. While CdS dominates the hobbyist market, other materials serve critical niche roles in industrial and scientific instrumentation.
| Material | Spectral Peak | Tempco | RoHS Status | Typical Use Case |
|---|---|---|---|---|
| Cadmium Sulfide (CdS) | 540 nm (Visible Green) | Negative (Res drops as temp rises) | Restricted / Banned in EU commercial | Nightlights, solar garden lamps, streetlights |
| Lead Sulfide (PbS) | 2000 nm (Short-wave IR) | Highly temperature dependent | Restricted (Lead) | Flame detection, missile tracking, IR spectroscopy |
| Cadmium Selenide (CdSe) | 720 nm (Near IR / Red) | Negative | Restricted | High-speed optical switches, red-laser triggering |
| Organic / Polymer | 500-600 nm (Visible) | Low | Compliant | Modern commercial wearable light sensors |
Selection Criteria: Choose CdS when you need to mimic human eye response for visible lighting controls and have a simple, low-cost analog circuit. Choose PbS only when designing thermal or flame-sensing equipment that relies on infrared emission. If you are designing a commercial product for the EU or US market in 2026, you must avoid Cadmium and Lead entirely, pivoting to RoHS-compliant organic LDRs or active silicon alternatives.
Bench Scenario: When a Dusk-to-Dawn Circuit Fails at Twilight
Theory is clean; the workbench is not. Here is a real-world failure involving a GL5528 that highlights the hidden traps of passive light sensing.
The Numbers: According to the datasheet, the GL5528 yields roughly 10kΩ at 10 lux (deep twilight). With R1 at 10kΩ and R2 at 10kΩ, the voltage divider outputs exactly 2.5V. As darkness falls and the LDR's resistance climbs toward 1MΩ, the junction voltage approaches 5V, crossing the 2.5V threshold and triggering the LM393 to pull the relay low, turning on the exterior floodlights.
The Outcome: During field testing in late summer, the relay chattered violently for 15 minutes every evening. The rapid switching eventually welded the mechanical relay contacts closed, leaving the floodlights on permanently until the breaker was manually tripped.
What Went Wrong: Two compounding physical realities of CdS cells caused this failure. First, the Temperature Coefficient. The outdoor enclosure reached 45°C in the afternoon sun. CdS has a negative tempco; heat artificially lowers the resistance. At 45°C, the 'dark' resistance of the LDR was severely depressed, shifting the twilight trip point. Second, the Decay Time. CdS cells suffer from 'memory effect' and slow decay—it can take up to 1 second for the resistance to climb after light is removed. As tree shadows flickered across the sensor in the evening breeze, the slow-decaying LDR hovered in the LM393's linear region, causing the comparator to oscillate.
The Fix: The circuit lacked hysteresis. Adding a 1.5MΩ positive feedback resistor from the LM393 output to the non-inverting pin created a deadband. The relay now requires the voltage to drop below 1.8V to turn off, and rise above 2.8V to turn on, completely eliminating the twilight chatter regardless of temperature shifts or slow decay times.
Failure Modes and Visual Diagnostics
Photoresistors are rugged, but they are not immune to environmental and electrical abuse. When troubleshooting a dead LDR circuit, pull the component and inspect it under a magnifying lamp. Here is what to look for:
- Moisture Ingress (The Milky Face): The top epoxy coating must remain perfectly clear. If the dome looks milky, cloudy, or has micro-fractures, humidity has breached the seal. Bench Test: Cover the cell completely with your hand in a dark room. If your multimeter reads anything less than 500kΩ, the moisture has created a parallel leakage path. Discard it.
- Overcurrent Track Burnout: LDRs have a low power dissipation limit, typically 100mW. If a designer places a 5mm CdS cell directly across a 12V rail without a sufficient series limiting resistor, the internal carbon track will overheat. Visual Symptom: A scorched, blistered, or blackened center on the zig-zag pattern. The part will read as an infinite open circuit.
- UV Epoxy Degradation: Continuous exposure to direct, unfiltered sunlight degrades the clear epoxy binder over several years. Visual Symptom: The epoxy turns brittle and deep yellow. While the semiconductor still functions, the yellowing acts as an unintended optical filter, shifting the spectral sensitivity and drastically reducing the cell's responsiveness to blue/green light.
Safe Substitution and Modern Design Alternatives
What happens when you need to repair a legacy nightlight, but you only have a GL5516 in your parts bin instead of the specified GL5528? You can substitute safely, but you must recalculate the voltage divider.
Because the GL5516 has a lower resistance profile (~5kΩ at 10 lux compared to the 5528's ~10kΩ), keeping the original 10kΩ fixed resistor will shift your trip point to a much brighter ambient light level. To restore the original 2.5V trip point at 10 lux, swap the fixed resistor to a 4.7kΩ or 5.1kΩ standard value. Always verify the new trip point with a lux meter app on your smartphone and a multimeter probing the divider junction.
For modern, compliant designs, substitute the passive LDR with an active Phototransistor (like the Vishay BPW85B) or a dedicated Ambient Light Sensor IC (like the Rohm BH1750). When substituting a phototransistor for an LDR in an existing analog circuit, remember that phototransistors output current (µA), not variable resistance. You must wire the phototransistor in a common-emitter configuration with a pull-down resistor (typically 100kΩ to 1MΩ) to convert the photocurrent back into a usable voltage for your comparator or microcontroller ADC.
For deeper theoretical modeling of semiconductor bandgaps and photoconductivity, refer to the Georgia State University HyperPhysics database. For practical circuit topologies and load-line analysis of LDRs, the Electronics Tutorials LDR guide remains a definitive bench reference.
