GL5528 CdS Photoresistor (LDR)
The most common small (5mm) cadmium-sulfide light-dependent resistor used in beginner electronics kits, whose resistance drops sharply as ambient light increases but which reacts far too slowly for anything beyond simple ambient-light sensing.
How this component works
A photoresistor (also known as a Light Dependent Resistor or LDR) is a passive component whose electrical resistance changes based on the amount of light striking its surface. In complete darkness, its resistance is extremely high (often several megaohms). When exposed to bright light, its resistance drops dramatically (often to a few hundred ohms).
Photoresistors are commonly used in voltage divider circuits to create a light-sensing voltage signal. This allows microcontrollers to detect day/night cycles, measure ambient light levels for automatic screen brightness, or trigger security alarms when a laser beam is broken.
Deep Dive: GL5528 LDR
The GL5528's defining behavior is a photoconductive response that is both non-linear and slow. Its resistance spans roughly three orders of magnitude — from well over 1MΩ in darkness down to single-digit kΩ under bright illumination — following a logarithmic relationship with light intensity (lux) rather than a linear one, which is why LDR-based light meters are typically calibrated in software or read as relative thresholds rather than converted directly to a precise lux value. This large dynamic range makes it very good at the simple job of asking 'is it dark or light,' which is exactly the job it's given in night-light and streetlight-trigger circuits.
What rules the GL5528 out of more demanding roles is its response speed: cadmium sulfide's photoconductive mechanism reacts on the order of tens of milliseconds, both turning on and (notably slower) turning off after light is removed. That's imperceptible in a slowly-changing ambient-light application, but would completely fail to track anything like a modulated light source or optical data signal — those jobs go to a photodiode or phototransistor instead, which react in microseconds or less by using a fundamentally different (junction-based, not bulk-resistance) sensing mechanism.
Common Use Cases
- Night-light or streetlight circuits that switch on automatically once ambient light drops below a threshold
- Beginner Arduino/breadboard light-level sensing projects using a voltage-divider with a fixed resistor
- Simple light-triggered alarm or counter circuits where sub-second response time is not required
Frequently Asked Questions
Why is the GL5528 unsuitable for anything like detecting a fast light pulse or an optical data link?
Cadmium-sulfide photoresistors are inherently slow, with rise/fall response times typically in the tens of milliseconds — orders of magnitude slower than a photodiode or phototransistor. That is fine for ambient-light sensing where light levels change gradually, but far too slow for anything involving rapidly changing light signals.
How does its resistance actually change with light?
In darkness, the GL5528 presents a very high resistance (typically over 1MΩ), and that resistance drops sharply as illumination increases — often down to just a few kΩ under bright light, and non-linearly (logarithmically with lux) rather than proportionally — which is why it's almost always used in a voltage-divider with a fixed resistor rather than measured directly.
What is the difference between this and the GL5537-1 entry in this category?
The GL5537-1 uses a larger (7mm) sensing element than the GL5528's 5mm, giving it a different current-handling and sensitivity profile suited to somewhat different applications — see that entry's comparison notes.
Comparison Notes
Compared to the larger GL5537-1 in this category, the GL5528's smaller 5mm sensing surface makes it the more compact, lower-cost default for breadboard and kit-based light-sensing projects, at the cost of somewhat lower maximum current-handling than the larger part.