Photo resistance is the physical property where a semiconductor material's electrical resistance drops as the intensity of incident light increases. In practical electronics, we harness this property using a Light Dependent Resistor (LDR), also known as a photoresistor, which acts as a passive, light-controlled variable resistor to translate optical energy into measurable electrical changes.
The Specs: Choosing the Right Photo Resistance Component
Most hobbyist and legacy commercial LDRs are manufactured using Cadmium Sulfide (CdS). The semiconductor lattice is doped to create a specific bandgap that responds to visible light, peaking around 540nm (green-yellow), which closely mimics the human eye's sensitivity. Think of the semiconductor lattice as a multi-lane highway with toll booths: in the dark, most booths are closed, causing a massive traffic backup (high resistance). When photons strike the material, they act like operators throwing open the booths, allowing electron traffic to flow freely (low resistance).
Because CdS cells are non-linear, their resistance doesn't drop in a straight line relative to lux. Instead, they follow a logarithmic curve defined by a gamma value, typically between 0.6 and 0.8. This means a 10x increase in light intensity doesn't yield a 10x drop in resistance, which is critical to remember when designing analog thresholds.
| Model | Dark Resistance (MΩ) | Light Resistance @ 10 Lux (kΩ) | Rise/Decay Time (ms) | Peak Wavelength (nm) |
|---|---|---|---|---|
| GL5516 | 0.5 | 5 - 10 | 20 / 30 | 540 |
| GL5528 (Standard) | 1.0 | 10 - 20 | 30 / 40 | 540 |
| GL5537-1 | 3.0 | 20 - 30 | 40 / 50 | 540 |
| GL5549 (High Res) | 5.0 | 40 - 70 | 50 / 80 | 540 |
When selecting a part, the GL5528 is the undisputed workhorse for general-purpose 5V and 3.3V microcontroller circuits. Its 1MΩ dark resistance ensures near-zero standby current draw, while its 10-20kΩ illuminated resistance pairs perfectly with standard 10kΩ fixed resistors for voltage dividers.
What Photo Resistance Changes in a Real Circuit
An LDR by itself just changes resistance; to make it useful, it must change a voltage that a microcontroller's ADC (Analog-to-Digital Converter) or an analog comparator can read. We do this by placing the LDR in a voltage divider network.
Let's look at a concrete numeric example using a GL5528 and a 10kΩ fixed resistor connected to a 5V supply (like an Arduino Uno). We wire the 5V to the fixed resistor, the other side of the fixed resistor to the LDR, and the other side of the LDR to Ground. Our V_out is measured at the junction between the two resistors.
The formula for the output voltage is: V_out = V_in × (R_LDR / (R_fixed + R_LDR))
- Scenario A: Pitch Dark (0 Lux)
The GL5528 dark resistance is roughly 1,000,000Ω (1MΩ).
V_out = 5V × (1,000,000 / (10,000 + 1,000,000)) = 4.95V
On a 10-bit Arduino ADC (0-1023 range), this reads as 1013. - Scenario B: Dim Room Light (10 Lux)
The LDR drops to its nominal 10,000Ω (10kΩ).
V_out = 5V × (10,000 / (10,000 + 10,000)) = 2.50V
The ADC reads exactly 512. This is why 10kΩ is the magic pairing value for a GL5528—it centers your analog read at mid-scale under typical indoor lighting. - Scenario C: Bright Flashlight (100 Lux)
The LDR resistance drops further to roughly 2,000Ω (2kΩ).
V_out = 5V × (2,000 / (10,000 + 2,000)) = 0.83V
The ADC reads 170.
Notice that as light increases, the output voltage decreases. If your application requires the voltage to rise with increasing light (for example, to trigger a high-threshold comparator), you simply swap the physical positions of the LDR and the fixed resistor in the circuit.
Where You Meet This in Practice
Photo resistance components are everywhere in low-cost, low-speed optical sensing. You will find them in:
- Legacy Streetlights and Dusk-to-Dawn Fixtures: A simple LDR paired with a TRIAC or relay to switch high-voltage loads when ambient light drops below a set threshold.
- Solar Tracker Coarse Alignment: Using two LDRs separated by a physical baffle; the microcontroller drives a motor until the voltage readings from both LDRs equalize, indicating the panel is perpendicular to the sun.
- Automatic Nightlights: Plug-in units that use the LDR to keep the LED off during the day to save power, turning on only when room illumination falls below ~5 Lux.
Cadmium Sulfide LDRs suffer from 'hysteresis' or a memory effect. If you move a GL5528 from a brightly lit room into a dark closet, the resistance will not instantly hit 1MΩ; it can take several seconds to fully stabilize. Never use CdS LDRs for high-speed optical encoding or RPM counting. Furthermore, because cadmium is highly toxic, CdS LDRs are restricted under RoHS directives for new commercial manufacturing in the EU and many other regions, though they remain widely available for hobbyist and repair markets.
Common Confusions: LDRs vs. Photodiodes and Phototransistors
When browsing a supplier like Digi-Key or Mouser, it is easy to confuse photo resistance components with other light sensors. Here is how to tell them apart on the bench:
1. LDR (Photoresistor) vs. Photodiode
An LDR is a purely passive component. It has no polarity (you can wire it backward and it works exactly the same) and requires an external voltage source to create a measurable signal. A photodiode, conversely, is an active semiconductor junction. It generates a tiny micro-ampere current when struck by light, even with zero external bias (photovoltaic mode), or operates in reverse-bias (photoconductive mode) for high-speed detection. Photodiodes respond in nanoseconds; LDRs respond in tens of milliseconds.
2. LDR vs. Phototransistor
A phototransistor is essentially a standard bipolar junction transistor (BJT) where the base current is generated by light rather than an electrical pin. It offers built-in current gain (amplification), making it much more sensitive to low light levels than a raw photodiode, and it switches in microseconds. However, unlike the bidirectional LDR, a phototransistor is strictly polarized (Collector to Emitter) and requires correct biasing to function.
Summary Decision Framework:
Choose an LDR when you need cheap, human-eye-matched ambient light sensing for slow-reacting circuits (like turning on a porch light). Choose a photodiode for high-speed data transmission (like IR remote receivers or fiber optics). Choose a phototransistor when you need faster switching than an LDR but higher sensitivity than a raw photodiode, such as in optical limit switches or slot sensors.
