A photosensitive resistor—commonly called a Light Dependent Resistor (LDR) or photoresistor—is a passive semiconductor device whose electrical resistance drops as incident light intensity increases. In total darkness, a standard cadmium sulfide (CdS) cell like the ubiquitous GL5528 exhibits a resistance of roughly 1 MΩ. Under bright room light (10 lux), that resistance plummets to 2–5 kΩ, and in direct sunlight (10,000 lux), it can fall below 100 Ω. This inverse, non-linear relationship between photons and resistance makes the LDR a foundational component for analog light-sensing, dusk-to-dawn switches, and automatic gain control circuits.
While modern designs often lean toward digital I2C ambient light sensors (like the Vishay VEML7700), the analog photosensitive resistor remains unmatched for ultra-low-cost, high-voltage tolerance, and simple voltage-divider interfacing. However, selecting the right material matrix, decoding the cryptic epoxy markings, and safely substituting out-of-stock parts requires bench-level knowledge.
Material Matrix and Datasheet Specs
Not all photosensitive resistors react to the same light. The semiconductor material doped into the ceramic substrate dictates the spectral peak, response time, and baseline resistance. Before you drop a generic CdS cell into an infrared or ultraviolet application, consult the material matrix below.
| Material / Part Series | Spectral Peak | Dark Resistance (0 lux) | Resistance @ 10 lux | Response Time (Rise/Fall) | Primary Application |
|---|---|---|---|---|---|
| Cadmium Sulfide (CdS) e.g., GL5528 |
540 nm (Visible Green) | 1.0 MΩ | 2.0 - 5.0 kΩ | 20 - 50 ms | Streetlights, nightlights, camera exposure |
| Cadmium Selenide (CdSe) e.g., GL5549 |
720 nm (Near-IR/Red) | 500 kΩ | 1.0 - 3.0 kΩ | 5 - 15 ms | Faster analog switching, red-laser tripwires |
| Lead Sulfide (PbS) e.g., Hamamatsu P-series |
2000 nm (Mid-IR) | 100 kΩ (cooled) | N/A (Requires AC bias) | 100 - 300 µs | Flame detection, thermal spectroscopy |
| Gallium Nitride (GaN) UV-Specific LDRs |
365 nm (Ultraviolet) | 10 MΩ | 50 kΩ | < 1 ms | UV index monitoring, sterilization interlocks |
Note on Response Time: LDRs suffer from 'memory effect' or hysteresis. A CdS cell exposed to bright light for hours will take significantly longer to return to its maximum dark resistance than one exposed briefly. For high-speed optical encoding (like reading a spinning slot disk), a photosensitive resistor is too slow; use a photodiode instead.
Decoding Markings and Selecting the Right Type
Unlike resistors with standard color bands, photosensitive resistors rely on printed alphanumeric codes or colored dots on their epoxy dome. Understanding these markings is critical when you are scavenging parts or verifying a bulk order from overseas suppliers.
How to Read the Physical Markings
The most common commercial LDRs belong to the '55' series. The first two digits (55) indicate a standard epoxy-encapsulated CdS cell with a zig-zag interdigitated electrode pattern. The subsequent two digits define the specific illuminance-resistance curve and physical size:
- 16 (e.g., GL5516): Low dark resistance (~500 kΩ), fast response. Used where rapid analog triggering is needed.
- 28 (e.g., GL5528): Medium dark resistance (~1 MΩ), standard response. The universal workhorse for general-purpose light/dark switching.
- 39 (e.g., GL5539): High dark resistance (~3 MΩ), slow response. Used in ultra-low-light detection or high-impedance electrometer circuits.
- 49 (e.g., GL5549): Indicates a CdSe (Cadmium Selenide) blend, shifting the spectral sensitivity further into the red/near-IR spectrum.
Type Comparison and Selection Criteria
Which type should you use for your specific job? Use this comparison matrix to match the component to your circuit's environmental and electrical constraints.
| Type / Variant | Construction / Package | Tolerance (at 10 lux) | Tempco (ppm/°C) | Typical Use Case |
|---|---|---|---|---|
| Standard Bare (55xx) | Open-face epoxy dome, radial leads | ± 20% | -2000 to -4000 | Consumer toys, basic nightlights |
| Hermetic Sealed | Metal can with glass window (TO-5 style) | ± 10% | -1000 to -2000 | Outdoor industrial controls, high humidity |
| SMD / Chip LDR | Surface mount, transparent resin overcoat | ± 30% | -3000 to -5000 | High-density PCBs, automated pick-and-place |
| High-Voltage (HV) | Extended creepage distance, thick ceramic substrate | ± 25% | -2000 to -4000 | Direct mains (120/240V AC) dusk-to-dawn switches |
For a detailed breakdown of how these semiconductor junctions operate at the atomic level, the Electronics Tutorials guide on LDR physics provides excellent band-gap theory context.
Circuit Integration and Safe Substitution
Because a photosensitive resistor is entirely passive, it requires an external bias network to produce a readable voltage. The standard topology is a simple voltage divider. If your exact LDR is out of stock, you must recalculate the fixed resistor to maintain your desired switching threshold.
The Voltage Divider Math
Assume a 5V microcontroller supply ($V_{cc}$) and a target trigger voltage ($V_{out}$) of 2.5V at dusk (defined here as 5 lux). If your GL5528 measures 15 kΩ at 5 lux, your fixed pull-down resistor ($R_{fixed}$) must also be 15 kΩ to center the divider.
$$V_{out} = V_{cc} \times \frac{R_{fixed}}{R_{LDR} + R_{fixed}}$$
If you substitute a GL5516 (which might measure only 4 kΩ at 5 lux), using the original 15 kΩ fixed resistor will shift your $V_{out}$ to roughly 1.05V—potentially failing to cross the microcontroller's digital HIGH threshold. Substitution Rule: Always measure the replacement LDR's resistance at your target lux level with a multimeter, then swap the fixed resistor to match that value.
Substituting an LDR with a Phototransistor
When CdS parts become unavailable due to supply chain or RoHS constraints, makers often attempt to swap in an NPN phototransistor (like the LTR-3208). You cannot drop a phototransistor directly into an LDR footprint.
- The LDR is a variable resistor. It works in a standard divider with a fixed resistor to ground.
- The Phototransistor acts as a light-controlled current source. The collector must tie to $V_{cc}$, the emitter to ground via a high-value pull-down resistor (typically 100 kΩ to 1 MΩ), and the voltage is read at the emitter.
If you force a phototransistor into a standard LDR divider, it will remain permanently off or short the supply depending on orientation. For safe substitution, redesign the bias network to accommodate the active nature of the phototransistor, referencing Components101's LDR vs Photodiode comparison for schematic adjustments.
Failure Modes and Visual Diagnostics
Photosensitive resistors are generally robust, but they fail in predictable ways when subjected to environmental abuse or electrical overstress. When debugging a malfunctioning light-sensing circuit, pull the component and inspect it under a magnifying lamp for these specific visual symptoms.
1. Silver Migration (Dendrite Growth)
The Cause: The interdigitated comb electrodes on the ceramic substrate are typically made of silver paste. In high-humidity environments combined with a continuous DC voltage bias, silver ions migrate across the gap, forming conductive bridges.
Visual Symptom: Look closely at the zig-zag dark gaps on the face of the LDR. You will see dark, tree-like metallic dendrites bridging the gaps. The component will read as a dead short or a very low resistance (e.g., 50 Ω) even in total darkness.
The Fix: Replace the LDR and switch to a hermetically sealed metal-can variant, or apply a conformal coating (like MG Chemicals 419D acrylic) over the bare epoxy dome to block moisture ingress.
2. Epoxy Yellowing and UV Delamination
The Cause: Standard clear epoxy domes degrade under prolonged, direct ultraviolet exposure (outdoor sun exposure). The polymer chains break down, causing the epoxy to yellow and micro-crack.
Visual Symptom: The dome appears cloudy, yellowed, or opaque. You may see hairline cracks around the copper lead exits. The circuit will behave as if it is constantly in darkness because the degraded epoxy is physically blocking photons from reaching the CdS layer.
The Fix: For outdoor applications, never use bare epoxy LDRs. Specify a UV-stabilized package or mount the LDR behind a dedicated UV-transparent quartz window.
3. Thermal Runaway and Substrate Cracking
The Cause: LDRs have a negative temperature coefficient (NTC). As they heat up, their resistance drops. If placed in a high-current circuit without a current-limiting resistor, the $I^2R$ heating drops the resistance, which draws more current, creating a runaway thermal loop.
Visual Symptom: The ceramic substrate will show a distinct scorch mark or a physical crack down the center. The PCB pads beneath the component may show melted solder mask or lifted copper. The multimeter will read an open circuit (OL).
The Fix: Never place an LDR directly across a voltage source. Always ensure a series current-limiting resistor is present in the branch, sized to keep maximum power dissipation below the LDR's 100 mW rating. For direct mains (120V AC) switching, always use an LDR specifically rated for high voltage (e.g., 250V AC isolation rating) and pair it with an opto-isolator or triac rather than driving a relay coil directly.
