A resistor LDR (Light Dependent Resistor), commonly called a photoresistor, is a passive, light-sensitive component whose electrical resistance decreases as incident light intensity increases. Unlike active semiconductor photodiodes that generate a photocurrent or require a specific reverse-bias voltage to operate, an LDR is a purely passive device. It acts as a variable resistor governed by photons. In total darkness, a typical Cadmium Sulfide (CdS) cell exhibits a 'dark resistance' of 1MΩ to 10MΩ. Under a 10-lux illumination (roughly twilight), that resistance plunges to 2kΩ–10kΩ.
While modern commercial designs often migrate to ambient light sensor ICs or phototransistors for RoHS compliance and faster response times, the resistor LDR remains unmatched for simple, low-cost, high-voltage AC/DC light switching. Because they are purely resistive, they can directly trigger TRIACs or relays in streetlamp controllers and night-lights without complex biasing networks. Here is how to select, decode, and troubleshoot them on the bench.
Resistor LDR Type Comparison: Which Photoresistor for Which Job?
Selecting the right resistor LDR requires matching the semiconductor material to your target light spectrum and the physical construction to your operating environment. CdS cells are optimized for visible light (peaking around 540nm, matching the human eye), while Lead Sulfide (PbS) and Indium Antimonide (InSb) cells are strictly for infrared applications.
| Material | Construction | Tolerance | Tempco (Temperature Coefficient) | Typical Use |
|---|---|---|---|---|
| CdS (Cadmium Sulfide) | Epoxy-coated radial leads | ±20% to ±50% (at 10 lux) | -0.5% to -2.0% / °C (in dark state) | Streetlights, night-lights, camera exposure, toy light-chasers. |
| PbS (Lead Sulfide) | Hermetic metal can (TO-18) or ceramic package | ±10% to ±20% | +0.1% to +0.5% / °C | Flame detection, IR spectrometry, missile plume tracking. |
| InSb (Indium Antimonide) | Cryogenic dewar / bare chip on sapphire | ±5% (usually sold as matched pairs) | Highly temp-dependent (requires cooling) | Thermal imaging, mid-wave IR gas analysis. |
Selection Criteria: Choose CdS when you need to detect ambient visible light and cost is the primary driver, accepting a slow response time (20-50ms). Choose PbS when you are detecting heat signatures or IR flames (1-3μm wavelength) in harsh industrial environments where hermetic sealing is mandatory. Never use a CdS cell for IR detection; its spectral sensitivity drops to near-zero past 700nm.
Decoding Physical Markings and Spec Sheet Codes
Unlike standard resistors with 4-band or 5-band color codes, resistor LDRs rely on alphanumeric silkscreen printing or simple color dots. The most ubiquitous series on the hobbyist and commercial bench is the GL55xx family. Here is how to read the physical markings on these 5mm diameter parts:
- Prefix (e.g., 'GL' or 'PDV'): Indicates the manufacturer or product family. 'GL' is a generic Asian manufacturer prefix, while 'PDV' denotes Advanced Photonix / Vishay.
- First Two Digits (e.g., '55'): Denotes the physical package size and series. '55' almost universally means a 5mm diameter epoxy-coated disc. '54' indicates a 4mm disc, and '75' indicates a 7mm disc.
- Last Two Digits (e.g., '28'): This is the critical selection code. It indicates the maximum resistance at 10 lux. For the GL5528, the '28' means the resistance at 10 lux is between 20kΩ and 30kΩ. For a GL5516, it is 10kΩ–20kΩ. For a GL5549, it is 40kΩ–70kΩ.
Some military or aerospace hermetic LDRs use a color dot system. A yellow dot typically indicates a spectral peak matched to the human eye (visible), while a red or black dot indicates an IR-optimized PbS cell. Always cross-reference the dot with the manufacturer's MIL-PRF spec sheet before soldering.
Failure Modes, Visual Symptoms, and Safe Substitution
Photoresistors are rugged but have strict thermal and environmental limits. The maximum power dissipation for a standard 5mm CdS cell is typically 100mW at 25°C. Exceeding this destroys the internal serpentine semiconductor trace.
Common Failure Modes and Visual Symptoms
- Thermal Overload (Cooked Trace): Visual Symptom: The epoxy dome turns opaque yellow or brown, and you may see a dark, scorched burn mark on the internal zig-zag trace. Cause: Exceeding 100mW. For example, if an LDR drops to 100Ω in bright sunlight and is connected directly across a 5V rail without a series resistor, P = V²/R = 25 / 100 = 250mW. The part will cook in seconds.
- Moisture Ingress (Delamination): Visual Symptom: The epoxy dome looks cloudy, milky, or has micro-cracks around the lead wires. Cause: Operating in high-humidity environments without conformal coating. Moisture creates parallel leakage paths across the high-impedance dark resistance, causing the 'dark' state to read as 50kΩ instead of 1MΩ, triggering false 'light detected' logic.
- Memory Effect (Hysteresis): Visual Symptom: None. Cause: Exposure to extreme high-intensity light (like a camera flash or direct laser). The resistance takes several minutes to return to the dark baseline. This is an intrinsic material flaw in CdS, not a permanent failure, but it ruins precision timing circuits.
How to Substitute Safely When the Exact Part is Missing
If your schematic calls for a GL5528 (20-30kΩ at 10 lux) and you only have a GL5539 (30-50kΩ at 10 lux), you cannot just drop it in without recalculating your bias network. Most LDRs are used in a simple voltage divider feeding a comparator or microcontroller ADC.
The Math: Assume a 5V supply and a target trip-point of 2.5V at 10 lux.
Formula: Vout = Vin × (R_LDR / (R_Fixed + R_LDR))
For the GL5528 (nominal 25kΩ at 10 lux): 2.5V = 5V × (25k / (R_Fixed + 25k)) → R_Fixed = 25kΩ.
If you substitute the GL5539 (nominal 40kΩ at 10 lux) but keep the 25kΩ fixed resistor, your Vout at 10 lux shifts to 3.07V, potentially missing the microcontroller's logic threshold.
The Fix: Swap the fixed bias resistor to 40kΩ (or the nearest standard value, 39kΩ). Never substitute a passive LDR with an active phototransistor (like the LTR-3208E) without completely redesigning the circuit topology. A phototransistor acts as a current source requiring a pull-up or pull-down resistor and does not function as a simple variable resistor.
Frequently Asked Questions
Can I use a resistor LDR directly in a 120V AC mains circuit?
Yes, but only with specific high-voltage rated parts. Standard 5mm epoxy CdS cells are typically rated for a maximum continuous voltage of 150V DC/AC. However, for reliable mains switching (like in a DIY night-light or outdoor dusk-to-dawn fixture), use large-diameter, hermetically sealed or heavily potted cells like the PDV-P9203 or VT90N2, which are rated for 250V+ and can handle the inrush currents of incandescent or relay coils. Always ensure the LDR is enclosed in an insulated housing; the bare leads on standard radial parts present a severe shock hazard at mains potential.
Why is my resistor LDR reacting slowly to light changes?
This is an intrinsic physical limitation of Cadmium Sulfide. When photons strike the CdS lattice, electron-hole pairs are generated, dropping the resistance. However, when the light is removed, the recombination of these charge carriers takes time. Typical 'dark decay' times for a GL55xx series part range from 20ms to 50ms, and can stretch into hundreds of milliseconds if the part was exposed to very bright light. If your application requires microsecond response times (such as optical encoders, fiber optic receivers, or high-speed barcode scanners), you must abandon the resistor LDR and use a PIN photodiode or an avalanche photodiode.
Are CdS resistor LDRs RoHS compliant for new commercial products?
No. Cadmium is a highly toxic heavy metal and is strictly restricted under the European Union's RoHS (Restriction of Hazardous Substances) Directive, with very few exemptions remaining for legacy industrial sensors. If you are designing a new consumer product for the EU or California markets, you cannot use CdS photoresistors. Instead, you must design your circuit around alternative light-sensing topologies, such as discrete phototransistors, integrated ambient light sensor (ALS) ICs like the Vishay VEML7700, or blue-enhanced silicon photodiodes paired with an op-amp transimpedance stage.
