The Photo Resistor on the Bench: Beyond the Datasheet
A photo resistor (commonly called a Light Dependent Resistor or LDR) is a passive, light-sensitive component whose electrical resistance drops as incident light intensity increases. If you grab a standard GL5528 CdS cell off your bench, you will measure 10kΩ to 20kΩ at 10 lux (dim twilight), 2kΩ to 5kΩ at 100 lux (overcast day), and >1MΩ in total darkness.
Unlike photodiodes or phototransistors, a photo resistor has no PN junction. It relies on bulk photoconductivity: photons with sufficient energy strike the semiconductor lattice, exciting electrons into the conduction band and creating electron-hole pairs that lower the material's bulk resistance. Because they are purely resistive, they pass both AC and DC equally well and have no inherent polarity. However, their slow response times (typically 20ms to 50ms) and severe temperature dependencies mean you cannot just drop them into a high-speed or precision optical circuit without understanding their physical quirks.
Material Types and Selection Criteria
Not all photo resistors are created equal. The semiconductor material dictates the spectral response (which colors of light it sees), the operating temperature range, and the dark resistance. Here is how to choose the right type for the job.
| Material | Construction & Spectral Peak | Tolerance & Tempco | Typical Use Case |
|---|---|---|---|
| CdS (Cadmium Sulfide) | Ceramic substrate, interdigitated silver traces. Peaks at ~540nm (green/yellow, matches human eye). | ±30% at 10 lux. High negative tempco (resistance drops as heat rises). | Streetlights, night-lights, camera exposure meters, dusk-to-dawn relays. |
| CdSe (Cadmium Selenide) | Similar to CdS but peaks further into the red (~720nm). | ±40%. Faster response time than CdS, but much less stable over temperature. | Red-light triggered alarms, specific industrial sorting sensors. |
| PbS (Lead Sulfide) | Evaporated thin-film on glass. Peaks in mid-IR (~2000nm). | Requires temperature stabilization. Very high dark resistance. | Flame detection, missile guidance, IR spectroscopy. |
| InSb (Indium Antimonide) | Crystal lattice. Peaks in far-IR (~5000nm+). | Requires cryogenic cooling (liquid nitrogen) to reduce thermal noise. | Astronomy, thermal imaging, advanced military optics. |
Which type for which job? For 95% of hobbyist, DIY, and commercial lighting applications, CdS is the default choice because its spectral response closely mimics the human eye's sensitivity to visible light. You only step into PbS or InSb territory when you are specifically designing for infrared thermal detection, which requires entirely different amplification and cooling architectures.
Decoding Markings and Sizing the Voltage Divider
If you look at a standard 5mm CdS photo resistor, you will rarely see an alphanumeric part number. Instead, the "marking" is the physical geometry of the interdigitated zig-zag pattern on the face. A denser, tighter zig-zag with more fingers indicates a higher dark resistance and a higher 10-lux resistance. Larger cells (12mm, 20mm, 25mm) from manufacturers like Excelitas (formerly PerkinElmer) or Advanced Photonix will have the part number (e.g., VT90N2) printed in ink on the ceramic backside.
To use an LDR in a microcontroller or comparator circuit, you must pair it with a fixed resistor to create a voltage divider. The formula is: Vout = Vcc × (R_fixed / (R_ldr + R_fixed)). Choosing the right fixed resistor depends on the specific LDR's gamma and your target lux threshold.
| Part Number | Diameter | 10 Lux (kΩ) | Dark Resistance (MΩ) | Best Fixed Resistor Pairing |
|---|---|---|---|---|
| GL5516 | 5mm | 5 - 10 | 0.8 | 4.7kΩ (for bright/dim indoor transitions) |
| GL5528 | 5mm | 10 - 20 | 1.0 | 10kΩ (standard dusk-to-dawn outdoor) |
| GL5537-1 | 5mm | 20 - 30 | 2.0 | 22kΩ (heavy shade / dense canopy detection) |
| GL5549 | 5mm | 45 - 100 | 5.0 | 47kΩ (very low light / moonlight triggers) |
Real-World Scenario: The Twilight Relay Chatter
Theory is clean; the workbench is not. Here is a real-world walkthrough of a common photo resistor failure mode in a DIY dusk-to-dawn relay circuit.
- The Setup: I built a 12V outdoor lighting controller using a GL5528 LDR and a 10kΩ fixed resistor in a voltage divider, feeding the non-inverting input of an LM393 comparator. The inverting input was tied to a 5V zener reference. When the LDR saw daylight, its resistance dropped, pulling the non-inverting pin below 5V, and the LM393's open-collector output pulled the relay coil to ground, turning the lights off.
- The Numbers: At exactly 15 lux (dusk), the GL5528 measured roughly 8kΩ. The divider output was
12V × (10k / (8k + 10k)) = 6.66V. As light faded to 5 lux, the LDR rose to 25kΩ, dropping the divider output to12V × (10k / 35k) = 3.42V, crossing the 5V threshold and triggering the relay. - The Outcome: During autumn testing, as the sun set and the ambient temperature dropped from 22°C to 8°C, the relay began to chatter violently, clicking on and off every two seconds right at twilight.
- What Went Wrong: Two factors collided. First, the memory effect. CdS cells exhibit "light history"; the GL5528 had been baking in direct noon sunlight, and its dark resistance was artificially depressed, taking over an hour to stabilize. Second, I had designed the comparator with zero hysteresis. As the LDR's resistance hovered exactly at the 15kΩ threshold, microscopic fluctuations in ambient light (wind blowing tree branches) and the LM393's internal noise caused the output to oscillate.
- The Fix: I added a 1MΩ feedback resistor from the LM393 output back to the non-inverting input to introduce 300mV of hysteresis. I also potted the LDR in a clear, UV-stable epoxy to block moisture ingress, which was exacerbating the thermal drift. The chatter stopped completely.
Failure Modes and Visual Diagnostics
Photo resistors are rugged, but they do fail. When troubleshooting a dead or erratic LDR circuit, pull the component and inspect it under a magnifying lamp for these specific visual symptoms:
- Delamination and Trace Lifting: Look closely at the silver interdigitated fingers on the face. If you see micro-cracks or the silver lifting off the ceramic substrate, the part has suffered thermal shock (often from a soldering iron held too close to the face during assembly). This causes an open circuit or massive resistance spikes.
- Moisture Ingress (The Dark Resistance Killer): If your multimeter reads < 100kΩ in total darkness, the epoxy coating on the face has likely micro-fractured. Humidity penetrates the semiconductor layer, creating parasitic leakage paths. Visually, the orange/red CdS face may look cloudy or milky rather than vibrant.
- Spectral Fatigue (Memory Effect): Not visually detectable, but electrically obvious. If an LDR is exposed to >1000 lux for hours, its resistance will not immediately return to the 1MΩ+ dark spec when the lights go out. It can take 10 to 30 minutes for the electron-hole pairs to fully recombine. If your circuit requires instant dark-state recovery, an LDR is the wrong component.
Safe Substitution When the Exact LDR is Missing
When you are out of GL5528s and need to finish a build, you cannot simply grab any 5mm LDR from the bin. You must match the gamma value (the slope of the log-log curve of resistance vs. illuminance), not just the 10-lux rating. If you substitute a GL5516 (gamma ~0.5) for a GL5549 (gamma ~0.8), the circuit will trigger at the wrong light level and the transition curve will be too shallow, making hysteresis tuning nearly impossible.
How to substitute safely:
- Measure the substitute LDR at 10 lux and 100 lux using a calibrated lux meter (or a smartphone app in a pinch, though phone sensors are often ±20% off).
- Calculate the gamma:
γ = log(R10 / R100) / log(100 / 10). - Recalculate your fixed voltage divider resistor to center the new LDR's transition curve around your comparator's reference voltage.
Cadmium is a highly toxic heavy metal. Under the EU RoHS Directive, CdS photo resistors are restricted in new commercial electronics sold in Europe. If you are designing a product for commercial manufacture, you must substitute the LDR entirely. The standard bench pivot is to use an ambient light phototransistor like the Vishay TEMT6000 or a digital I2C sensor like the BH1750. Phototransistors respond in microseconds (eliminating the memory effect), are RoHS compliant, and interface easily with a simple bias resistor, though they require careful optical filtering to match human eye sensitivity.
For deeper theoretical grounding on how bulk photoconductivity differs from junction-based photodiodes, the Electronics Tutorials LDR guide provides excellent baseline math on the log-log illumination curves. Ultimately, treating the photo resistor not as a perfect sensor, but as a slow, temperature-sensitive, light-bleeding ceramic element will save you hours of debugging at the bench.
