A photo resistor (commonly called a Light Dependent Resistor or LDR) is a passive, light-sensitive component whose resistance drops as incident photon flux increases. In total darkness, a standard cadmium sulfide (CdS) cell exhibits >1 MΩ resistance; under bright room light (10 lux), it falls to 10 kΩ–20 kΩ. While modern commercial electronics have largely shifted to digital ambient light sensors for RoHS compliance, the classic CdS photo resistor remains a staple in hobbyist kits, educational labs, and legacy analog lighting controls due to its simplicity and low cost.
Photo Resistors at a Glance: The GL55 Series Spec Sheet
The GL55 series is the most ubiquitous through-hole photo resistor family on the market. When designing a voltage divider or selecting a part for an analog light-tracking circuit, you must match the specific dark and light resistance values to your microcontroller's ADC range. The table below provides the exact manufacturer specifications for the four most common variants.
| Part Number | Resistance @ 10 Lux (kΩ) | Dark Resistance (MΩ) | Max Voltage (V) | Max Power (mW) | Spectral Peak (nm) | Response Time (ms) |
|---|---|---|---|---|---|---|
| GL5516 | 5 – 10 | 0.5 | 50 | 100 | 540 | Rise: 20 / Fall: 30 |
| GL5528 | 10 – 20 | 1.0 | 150 | 100 | 540 | Rise: 20 / Fall: 30 |
| GL5537 | 20 – 30 | 2.0 | 150 | 100 | 540 | Rise: 30 / Fall: 40 |
| GL5549 | 45 – 140 | 5.0 | 150 | 100 | 540 | Rise: 40 / Fall: 60 |
Note: The GL5528 is the default part included in 95% of Arduino and Raspberry Pi starter kits. Its 10-20kΩ range at 10 lux pairs perfectly with a standard 10kΩ fixed resistor to create a voltage divider that sweeps cleanly across a 0-5V or 0-3.3V ADC input.
Decoding Markings and Physical Identification
Unlike SMD resistors or capacitors, through-hole photo resistors do not use complex alphanumeric marking codes. Identification relies on visual cues and simple printed numbers.
- The Printed Number: Most parts will have a 4-digit number printed directly on the epoxy dome or the ceramic substrate edge (e.g., "5528" denotes a GL5528). If the number is faded, you must measure the dark resistance with a multimeter to identify the bin.
- The Zig-Zag Trace: Looking through the clear or amber epoxy, you will see a serpentine, zig-zag pattern. This is the cadmium sulfide thin-film trace. A wider, thicker trace generally indicates a lower base resistance and higher power dissipation capability.
- Epoxy Color Coding:
- Amber/Clear: Standard visible light response, peaking around 540nm (green/yellow spectrum). Matches human eye sensitivity.
- Dark Red/Black: Contains an integrated daylight-blocking filter. These are designed to pass infrared (IR) light while rejecting visible light, often used in flame sensors or IR beam-break circuits.
Material and Type Comparison: Which Sensor for Which Job?
While the CdS photo resistor is cheap, it is not always the right tool. If you need high-speed response or linear output, you must look at active semiconductor alternatives. The following comparison matrix dictates which type to choose based on your circuit requirements.
| Criteria | CdS Photo Resistor (LDR) | Phototransistor (e.g., TEFT4300) | Photodiode (e.g., BPW34) | Digital ALS (e.g., TSL2591) |
|---|---|---|---|---|
| Construction | CdS thin-film on ceramic | Silicon NPN junction | Silicon PIN junction | CMOS IC with dual diodes |
| Tolerance / Linearity | Highly non-linear (logarithmic) | Moderate (current proportional to lux) | Highly linear (current proportional to lux) | Digital linearized (I2C output) |
| Tempco & Drift | High temp drift, severe hysteresis | Moderate temp drift | Low temp drift | Internally compensated |
| Response Time | Slow (20ms - 60ms) | Fast (Microseconds) | Ultra-fast (Nanoseconds) | Integration time dependent (100ms+) |
| Typical Use Case | Streetlights, nightlights, solar trackers | Optical encoders, pulse oximeters | Fiber optic receivers, precision lux meters | Smartphone auto-brightness, IoT weather stations |
Failure Modes and Visual Diagnostics
Photo resistors are generally robust, but they fail in predictable ways when pushed beyond their physical limits. Here is how to diagnose a dead or drifting LDR on the bench.
1. Thermal Overstress (The 100mW Trap)
Visual Symptom: The epoxy dome is melted, bubbled, or darkened brown. The ceramic substrate may show scorch marks near the lead crimps.
The Physics: The GL55 series is rated for a maximum of 100mW. If you place a photo resistor directly across a 12V source without a sufficient series limiting resistor, and the sun hits it, the resistance can drop to 100Ω. Using $P = V^2 / R$, the power dissipation becomes $144 / 100 = 1.44W$. This is 14 times the maximum rating. The part will literally cook itself.
Fix: Always place a fixed current-limiting resistor in series, or use the LDR strictly in a high-impedance voltage divider configuration where current draw is limited to <1mA.
2. Moisture Ingress
Visual Symptom: The epoxy dome appears cloudy, milky, or has micro-fractures along the edges where the leads enter.
The Physics: CdS thin films are highly sensitive to humidity. If the epoxy seal fails, ambient moisture alters the surface conductivity of the ceramic substrate, creating parallel leakage paths.
Fix: Discard the part. If operating in high-humidity environments (like outdoor solar trackers), you must coat the LDR in a clear, UV-stable conformal silicone or use a hermetically sealed metal-can photodetector.
3. Light Fatigue (Memory Effect)
Visual Symptom: None. The part looks physically perfect.
The Physics: CdS cells suffer from hysteresis. If exposed to intense light (e.g., direct sunlight) for hours, the resistance will not immediately return to the >1MΩ dark state when moved to a dark room. It can take minutes or even hours for the electron-hole pairs to fully recombine.
Fix: This is a material limitation, not a defect. If your circuit requires instant dark-state recovery, you must switch to a photodiode or phototransistor.
Safe Substitution When the Exact Part is Missing
If your schematic calls for a GL5528 but you only have a GL5516 or GL5549 in your bin, you cannot simply swap them without adjusting the surrounding passive components. Because photo resistors are highly non-linear, substitution requires recalculating the voltage divider.
LDR-to-LDR Substitution Math
To get the maximum voltage swing out of an analog light sensor, the fixed series resistor ($R_{fixed}$) should be the geometric mean of the LDR's dark and light resistances. According to SparkFun's voltage divider guidelines, the formula is:
$R_{fixed} = \sqrt{R_{dark} \times R_{light}}$
Example: You are substituting a GL5549 for a GL5528.
- Original GL5528: $R_{dark} = 1M\Omega$, $R_{light} = 15k\Omega$. $R_{fixed} = \sqrt{1,000,000 \times 15,000} \approx 122k\Omega$ (Use 120kΩ standard).
- Substitute GL5549: $R_{dark} = 5M\Omega$, $R_{light} = 90k\Omega$. $R_{fixed} = \sqrt{5,000,000 \times 90,000} \approx 670k\Omega$ (Use 680kΩ standard).
If you use the original 10kΩ or 120kΩ resistor with the GL5549, your ADC will barely register a voltage change in normal room lighting.
LDR-to-Phototransistor Substitution
If you need to replace a slow LDR with a fast phototransistor (like the Vishay TEFT4300), you must change the circuit topology. A photo resistor is a variable resistor; a phototransistor is a light-controlled current source.
- The Wrong Way: Wiring the phototransistor's Collector and Emitter exactly where the LDR's two legs were. This will result in a floating pin or a shorted ADC, as phototransistors require a specific bias voltage and a pull-down resistor on the emitter to convert the photocurrent into a readable voltage.
- The Right Way: Connect the Collector to VCC (e.g., 5V). Connect the Emitter to GND through a fixed resistor (typically 10kΩ to 100kΩ, depending on ambient light levels). Tap your ADC signal from the Emitter node. As light hits the base, current flows from Collector to Emitter, raising the voltage across the bottom resistor.
For deeper analog conditioning techniques and op-amp buffering to prevent ADC loading errors, the Electronics-Tutorials LDR guide provides excellent schematics for transimpedance amplifiers. Ultimately, understanding the exact spectral and resistance characteristics of your photo resistor ensures your analog front-end delivers clean, noise-free data to your microcontroller.
