Light Dependent Resistors: Core Specs and Selection Data
Unlike fixed resistors, LDRs are highly non-linear and specified by their resistance at specific illumination thresholds rather than a single nominal value. The industry-standard GL55 series dominates the through-hole market. When designing a voltage divider or selecting a replacement, you must match both the light resistance (typically measured at 10 lux) and the dark resistance (measured after 5 seconds in total darkness).
| Part Number | Light Res. @ 10 Lux | Dark Res. (Min) | Max Voltage (DC/AC) | Peak Wavelength | Response Time (Rise/Fall) |
|---|---|---|---|---|---|
| GL5516 | 5 - 20 kΩ | 0.5 MΩ | 150 V | 540 nm (Green-Yellow) | 20 ms / 30 ms |
| GL5528 | 10 - 20 kΩ | 1.0 MΩ | 150 V | 540 nm (Green-Yellow) | 20 ms / 30 ms |
| GL5537-1 | 20 - 30 kΩ | 2.0 MΩ | 150 V | 540 nm (Green-Yellow) | 30 ms / 40 ms |
| GL5549 | 45 - 140 kΩ | 5.0 MΩ | 150 V | 540 nm (Green-Yellow) | 30 ms / 50 ms |
The response times listed above highlight a critical limitation of LDRs: they are inherently slow. A 20ms rise time means an LDR cannot be used for high-speed optical data transmission or RPM counting on fast-moving shafts. For those applications, active photodiodes or phototransistors are mandatory. For further reading on the physics of photoconductivity in these cells, refer to the All About Circuits semiconductor guide.
Decoding LDR Markings and Physical Identification
Unlike surface-mount devices that rely on cryptic 3-digit SMD codes, through-hole LDRs are physically large enough to print model numbers directly on the epoxy dome. However, surplus bins and salvaged boards often yield parts with faded text. Here is how to identify what you have on the bench.
Reading the Printed Code
Most commercial LDRs print a four-digit number, such as 5528. The prefix "55" designates the standard Cadmium Sulfide visible-light series. The suffix (16, 28, 37, 49) dictates the specific resistive curve, directly correlating to the rows in Table 1. If you see a prefix like "PDV" (e.g., PDV-P9203), you are looking at a premium grade cell from Advanced Photonix, which typically offers tighter tolerances and higher dark resistance.
The Color Dot System
Look closely at the rim of the epoxy dome. You will usually find a small painted dot.
- Red or Orange Dot: Standard CdS cell, peak sensitivity around 540nm (visible light). This is the most common variant for streetlights and night-lights.
- Green or Blue Dot: Often indicates a specialized spectral response, sometimes shifted toward shorter wavelengths, or denotes a specific factory bin for tighter dark-resistance tolerances.
- No Dot / Bare Epoxy: Usually a generic, un-binned import. You must characterize these manually with a multimeter and a calibrated lux source.
If the text is entirely worn off, measure the part. Cover the sensor completely with your thumb (darkness) and read the resistance; then shine a standard 60W-equivalent LED flashlight from 10cm away (roughly 10-50 lux) and read it again. Cross-reference those two data points with Table 1 to bin the component.
Material Comparison: Which Sensor Type for Which Job
While CdS is the hobbyist standard, it is not the only photoresistive material available. Furthermore, modern environmental regulations have forced a shift in commercial manufacturing. Use the comparison matrix below to select the correct sensor topology for your specific application.
| Material / Type | Construction & Spectral Peak | Typical Tolerance | Tempco (Temp. Coefficient) | Typical Use Case |
|---|---|---|---|---|
| Cadmium Sulfide (CdS) | Serpentine trace on ceramic; ~540 nm (Visible) | ±30% to ±50% (Light Res) | ~-0.5% / °C (Resistance drops as temp rises) | Hobby ambient light sensing, legacy streetlights, audio compressors. |
| Lead Sulfide (PbS) | Polycrystalline film; ~2000 nm (Near-IR) | ±20% | Highly temp-dependent; requires thermal stabilization | Flame detection, gas analysis, IR spectroscopy. |
| Silicon Phototransistor (e.g., TEPT5600) | Silicon NPN junction; ~570 nm (Visible) | ±10% (Current output) | Positive tempco for dark current (leakage doubles every ~10°C) | Modern RoHS-compliant commercial PCBs, fast optical switching. |
Which Type for Which Job?
Choose CdS LDRs when you need a massive resistance swing (from 1MΩ down to 2kΩ) to directly bias a transistor or trigger a 555 timer without an op-amp. Choose PbS cells strictly for infrared thermal detection where visible light must be ignored. Choose Silicon Phototransistors when you need microsecond response times, RoHS compliance, and linear current output for precise ADC measurements.
Safe Substitution: Swapping Parts and Adjusting Bias
Because LDRs have wide manufacturing tolerances (often ±50%), substituting a missing part is rarely a simple drop-in replacement. You must adjust the surrounding biasing network to maintain your circuit's switching threshold. For a comprehensive guide on interfacing these sensors with microcontrollers, see the Adafruit Photocell Tutorial.
The Voltage Divider Math
LDRs are almost exclusively used in a voltage divider configuration feeding an analog-to-digital converter (ADC) or a comparator. The output voltage is calculated as:
V_out = V_cc × [ R_fixed / (R_LDR + R_fixed) ] (assuming the LDR is tied to V_cc and the fixed resistor is tied to Ground).
Worked Substitution Example:
Suppose your original circuit used a GL5528 (nominal 15kΩ at 10 lux) with a 10kΩ fixed resistor to ground, yielding roughly 2.0V at the ADC pin at 10 lux on a 5V rail.
If the GL5528 is unavailable and you substitute a GL5549 (nominal 60kΩ at 10 lux), keeping the 10kΩ fixed resistor will drop your V_out to 0.71V—likely failing to trigger your microcontroller's daylight threshold.
The Fix: To restore the ~2.0V threshold at 10 lux with the higher-resistance GL5549, you must swap the fixed resistor. Using the voltage divider formula algebraically, you need a fixed resistor of approximately 39kΩ. Always recalculate the fixed resistor whenever you change LDR series.
Substituting with a Phototransistor
If you are forced to replace a CdS LDR with a RoHS-compliant phototransistor (e.g., TEPT5600), you cannot simply swap them pin-for-pin. A phototransistor acts as a current source, not a variable resistor. You must remove the top half of the voltage divider, tie the phototransistor's collector to V_cc, its emitter to ground via a single pull-down resistor (typically 100kΩ to 1MΩ), and read the voltage at the emitter. The voltage swing will be much smaller, often requiring a non-inverting op-amp stage to scale the signal for a 3.3V microcontroller ADC.
Failure Modes, Visual Diagnostics, and Bench Testing
LDRs are generally robust, but their epoxy encapsulation and chemical traces are vulnerable to specific environmental stressors. When troubleshooting a failing light-sensing circuit, inspect the component for these specific visual symptoms before replacing it.
1. Moisture Ingress and Delamination
- Visual Symptom: The clear epoxy dome appears milky, cloudy, or exhibits micro-cracks. Under magnification, the internal serpentine CdS trace looks white or grey instead of its normal dark reddish-brown.
- Electrical Result: The dark resistance plummets. Instead of reading >1MΩ in the dark, a moisture-degraded LDR will "leak" and read 50kΩ to 200kΩ. This causes night-light circuits to never turn on, as the voltage divider never crosses the dark threshold.
- Bench Test: Cover the sensor completely with black electrical tape. Measure with a DMM on the highest Ohms range. If it reads below 500kΩ, the cell is compromised by humidity.
2. Thermal and UV Degradation
- Visual Symptom: The epoxy window is heavily yellowed, blistered, or physically charred. This is common in LDRs mounted too close to high-wattage incandescent bulbs or outdoor fixtures without UV-stabilized covers.
- Electrical Result: The cadmium sulfide crystal lattice degrades, resulting in a permanent loss of photoconductivity. The light resistance stays artificially high (e.g., 500kΩ even in bright sunlight).
- Bench Test: Shine a high-lumen flashlight directly on the dome. If the resistance fails to drop below 5kΩ, the photosensitive layer is burned out.
3. Lead Frame Fatigue
- Visual Symptom: A hairline crack or visible gap where the metal lead wire enters the base of the epoxy dome. Often caused by repeated mechanical stress or bending the leads too close to the body during PCB insertion.
- Electrical Result: Intermittent open circuits. The multimeter will read "O.L." (Over Limit) or the resistance will spike erratically when the lead is gently wiggled.
- Fix: The component is unrepairable. Desolder and replace. When installing the new LDR, always bend the leads using needle-nose pliers gripping the wire below the epoxy seal, never bend directly at the epoxy interface.
By understanding the specific resistive curves of the GL55 series, properly decoding physical markings, and applying rigorous substitution math, you can reliably integrate or repair light dependent resistors in any analog sensing topology.
