A photoresistor (or Light Dependent Resistor, LDR) decreases its electrical resistance when exposed to light. In total darkness, its resistance spikes into the megaohm range; under bright illumination, it drops to a few hundred or thousand ohms. It acts as a passive, light-activated variable resistor. Unlike photodiodes or phototransistors, an LDR has no polarity and operates on AC or DC, but it is relatively slow and sensitive to temperature.
While digital ambient light sensors (like the I2C BH1750) have taken over complex microcontroller designs, the humble analog LDR remains a staple for simple comparators, night-light circuits, and analog synth light-theremins. Below is a bench-level deep dive into LDR materials, decoding physical markings, safe substitutions, and how to spot a dead cell.
Core LDR Specifications and Material Types
The phrase "photoresistor" covers several distinct semiconductor chemistries. The material dictates the spectral response (which wavelengths it "sees"), the dark-to-light resistance ratio, and the response time. Standard 5mm visible-light LDRs are almost exclusively Cadmium Sulfide (CdS), but industrial and scientific applications use different compounds.
| Material | Spectral Peak | Dark Resistance | Light Resistance (10 lux) | Response Time | Typical Application |
|---|---|---|---|---|---|
| Cadmium Sulfide (CdS) | 540 nm (Green/Visible) | 1 MΩ to 10 MΩ | 1 kΩ to 20 kΩ | 20 ms to 50 ms | Streetlights, nightlights, camera exposure |
| Lead Sulfide (PbS) | 2000 nm (Near-IR) | 100 kΩ to 500 kΩ | 500 Ω to 2 kΩ | < 1 ms | Flame detection, IR spectroscopy |
| Indium Antimonide (InSb) | 5000 nm (Mid-IR) | N/A (Cooled sensor) | N/A (Cooled sensor) | < 1 µs | Thermal imaging, missile tracking |
| Gallium Nitride (GaN) | 365 nm (Ultraviolet) | 100 MΩ+ | 10 kΩ to 50 kΩ | 1 ms to 5 ms | UV index meters, arc welding detection |
The 10 Lux Standard: When reading datasheets from manufacturers like Vishay Optoelectronics or Advanced Photonix, light resistance is universally tested at 10 lux (roughly the illumination of twilight or a dimly lit living room) with a color temperature of 2856K. If you test a CdS cell under a 5000K LED desk lamp at 300 lux, your measured resistance will be drastically lower than the datasheet claims.
Decoding Physical Markings and Selecting the Right Type
Most hobbyist and commercial CdS photoresistors use a standardized numbering scheme, most notably the GL55 series. The physical part rarely has a brand logo; instead, it prints a four-digit code on the epoxy face. Here is how to read the GL55xx code:
- First two digits (55): Indicates the series and physical construction (typically a 5mm ceramic substrate with interdigitated carbon/cadmium tracks).
- Last two digits (xx): Indicates the specific light and dark resistance ranges. Higher numbers generally mean higher light resistance (less sensitive).
Which Type for Which Job?
| Part / Construction | Tolerance (at 10 lux) | Temperature Coefficient (Tempco) | Typical Use Case |
|---|---|---|---|
| GL5516 (Epoxy Wirelead) | ±20% | -0.6% / °C | High-sensitivity triggers, fast-switching shadow sensors. |
| GL5528 (Epoxy Wirelead) | ±20% | -0.5% / °C | General purpose: outdoor dusk-to-dawn lights, basic Arduino analog inputs. |
| GL5537 (Epoxy Wirelead) | ±15% | -0.4% / °C | Audio optocouplers (vactrols), analog synth modulation where linearity matters. |
| GL5549 (Epoxy Wirelead) | ±20% | -0.5% / °C | Low-sensitivity applications, bright indoor ambient light tracking. |
| Hermetic Metal Can (Glass sealed) | ±10% | -0.3% / °C | Harsh environments, high humidity, precision laboratory light meters. |
Understanding the Tempco: CdS cells have a negative temperature coefficient. As ambient temperature rises, the dark resistance drops. If your outdoor security light triggers at dusk in the winter but fails to trigger until pitch-black in the summer, the LDR's tempco is shifting your voltage divider threshold. Hermetically sealed metal-can variants mitigate this by buffering the die from rapid ambient thermal swings.
Safe Substitution Rules When the Exact LDR is Missing
You are repairing a vintage guitar pedal or a commercial lighting controller, and the original LDR is dead. The exact part is out of stock. According to Components101 and standard bench practice, you can safely substitute an LDR if you follow three rules:
- Match the Spectral Peak: Never substitute a PbS (Infrared) cell into a circuit designed for a CdS (Visible) cell. The circuit will be blind to visible light and trigger off body heat or IR remote signals.
- Match the Dark-to-Light Ratio: The absolute resistance matters less than the ratio. A circuit designed for a 1MΩ dark / 10kΩ light ratio (100:1) will behave similarly with a 2MΩ dark / 20kΩ light cell, provided the bias network is scaled.
- Recalculate the Bias Resistor: LDRs are almost always used in a voltage divider feeding a comparator (like an LM393) or a microcontroller ADC. If you substitute a lower-resistance LDR, you must lower the fixed series resistor to maintain the same switching voltage.
Your circuit uses a 5V supply, a GL5528 (10kΩ at 10 lux), and a 10kΩ fixed resistor to ground. The threshold voltage at 10 lux is 2.5V.
You only have a GL5516 (5kΩ at 10 lux).
If you keep the 10kΩ fixed resistor, your new threshold drops to 1.66V, and the circuit will trigger too early.
The Fix: Swap the fixed resistor to 5kΩ. The new threshold returns to exactly 2.5V at 10 lux.
Failure Modes and Visual Diagnostics
Photoresistors are passive, but they are not immortal. Because they rely on semiconductor physics and physical packaging, they fail in highly specific ways. Before desoldering a suspected bad LDR, inspect it for these visual symptoms:
1. Thermal Runaway and Overcurrent
Standard 5mm epoxy LDRs have a maximum power dissipation rating of roughly 100mW at 25°C. If a designer places a 100Ω LDR directly across a 5V rail (perhaps due to a wiring error or a failed series resistor), it will dissipate 250mW.
Visual Symptom: The clear epoxy dome will yellow, blister, or crack. The ceramic substrate underneath may show a dark scorch mark between the interdigitated carbon traces. A multimeter will read an open circuit (OL) or a permanently shorted low value if the tracks melted together.
2. Moisture Ingress and Leakage
The epoxy coating on cheap wirelead LDRs is semi-permeable. In high-humidity environments (like outdoor garden lights), moisture penetrates the epoxy and settles on the semiconductor surface. Water creates a parallel leakage path.
Visual Symptom: The epoxy dome looks cloudy, milky, or has a white, chalky ring around the base. Electrical Symptom: The "dark resistance" plummets. A cell that should read 5MΩ in a dark room will stubbornly cap out at 50kΩ, causing dusk-to-dawn lights to stay off entirely.
3. The Memory Effect (Light Fatigue)
This is a functional failure, not a physical one. CdS cells suffer from "light history." If an LDR is exposed to intense, direct sunlight for several days, the electron traps in the crystal lattice become saturated. When moved to darkness, the resistance will not immediately return to the megaohm range; it can take hours or even days to "recover" its full dark resistance.
Visual Symptom: None. The part looks perfect. Electrical Symptom: The circuit behaves erratically for the first 12-24 hours after deployment. To prevent this, manufacturers recommend "dark conditioning" precision LDRs for 48 hours before final calibration.
Cadmium is a toxic heavy metal. Under the EU's RoHS directive, Cadmium Sulfide photoresistors are largely banned in new consumer electronics sold in Europe, with exemptions only for specific legacy or industrial applications. If you are designing a new commercial product for global markets in 2026, avoid CdS LDRs. Substitute them with discrete phototransistors (like the TEPT5600) or digital I2C ambient light sensors (like the ROHM BH1750FVI) which offer better temperature stability, zero toxic materials, and direct microcontroller integration without the need for analog voltage dividers.
Understanding what a photoresistor does goes beyond simply noting that it reacts to light. By reading the GL-series markings, respecting the 10 lux testing standard, calculating voltage divider substitutions, and recognizing the visual signs of moisture and thermal damage, you can troubleshoot and design analog light-sensing circuits with the same precision as modern digital implementations.
