LDR (Light Dependent Resistor) — Working Principle and Applications

LDR stands for Light Dependent Resistor. Also known as: Photoresistor, Photo-conductive cell, Photo-cell

Structure: • Made of a semiconductor material — most commonly Cadmium Sulphide (CdS) • The semiconductor is deposited in a zigzag pattern on a ceramic substrate to increase the effective path length (and thus resistance) • Covered with a transparent window to allow light to fall on the semiconductor • Two ohmic contacts (metal electrodes) at either end • Encapsulated in a transparent resin casing

Symbol (circuit symbol): An LDR is represented as a resistor (rectangle or zigzag) with two arrows pointing toward it (indicating incoming light). The arrows distinguish it from a regular resistor in circuit diagrams.

Physical appearance: • Small disc-shaped or rectangular component • Diameter typically 5–20 mm • Light-sensitive area has a characteristic spiral or zigzag pattern visible through the window • Common colours: reddish-orange (CdS) or dark grey (other materials)

Common LDR materials: • Cadmium Sulphide (CdS) — sensitive to visible light (peak ~540 nm, green-yellow) • Cadmium Selenide (CdSe) — sensitive to red/infrared light • Lead Sulphide (PbS) — sensitive to infrared radiation • Germanium (Ge) — infrared applications

An LDR works on the principle of photoconductivity.

Photoconductive Effect: When photons of light fall on the semiconductor material (CdS), they provide energy to electrons in the valence band. If the photon energy (hν) is greater than or equal to the bandgap energy of the semiconductor, electrons are excited from the valence band to the conduction band, creating electron-hole pairs.

In Darkness: • No photons → no additional electron-hole pairs generated • Only thermally generated charge carriers present (very few) • Very high resistance: 1 MΩ to 10 MΩ

In Light: • Photons excite electrons across the bandgap • More free electrons and holes → increased conductivity • Resistance drops significantly: 100 Ω to 1 kΩ in bright light

Mathematical relationship: R ∝ E^(-γ) (approximately) Where: • R = resistance of LDR • E = illuminance (light intensity in lux) • γ = slope constant (typically 0.7 to 0.9 for CdS)

Key property: Resistance and light intensity are inversely related. More light → More electron-hole pairs → Lower resistance → More current

Response time: • Rise time (dark to light): ~10 ms • Decay time (light to dark): ~100 ms (slower) This lag makes LDRs unsuitable for high-frequency switching applications.

Resistance vs. Illuminance: • Dark resistance (0 lux): ~1 MΩ or higher • Dim light (10 lux): ~10 kΩ • Indoor lighting (100 lux): ~1–2 kΩ • Bright sunlight (10,000 lux): 100–200 Ω

Resistance-Illuminance curve: The R-E curve is approximately linear on a log-log scale. log(R) = log(K) − γ·log(E) Where K and γ are constants specific to the LDR material.

Spectral Response: CdS LDRs are most sensitive to green-yellow light (peak ~540 nm), matching human eye sensitivity. CdSe LDRs respond to red light and near-infrared.

Temperature effect: LDR resistance also decreases with increasing temperature (negative temperature coefficient) — similar to other semiconductors.

Power dissipation: Typical maximum power: 100 mW to 300 mW Do not exceed maximum power rating.

Operating voltage: Typically 10V to 150V DC.

Operating temperature range: −30°C to +70°C (standard types)

Basic LDR Circuit (Voltage Divider): LDR and a fixed resistor R are connected in series between supply voltage V_cc and ground. Output voltage is taken across R: V_out = V_cc × R / (R_LDR + R)

In dark: R_LDR >> R → V_out ≈ 0 (low) In light: R_LDR << R → V_out ≈ V_cc (high)

This voltage can trigger transistors, op-amps, or microcontroller inputs.

LDR with transistor switch: • Light falls on LDR → resistance drops → base current to transistor increases → transistor turns ON → relay/load activates • Used to automatically turn OFF street lights during day

LDR with op-amp comparator: • V_out of LDR divider compared with a reference voltage • Op-amp output goes HIGH or LOW depending on light level • Allows precise light threshold triggering

LDR with Arduino/microcontroller: • A0 (analog input) reads voltage across fixed resistor • Analog value maps to 0–1023 range (10-bit ADC) • Used in robotics: line followers, light-following robots

1. Automatic Street Lights: LDR detects darkness → resistance rises → triggers relay/transistor → street lamp switches ON automatically at dusk. Reverses at dawn.

2. Burglar Alarm Systems: A light beam (visible or infrared) falls continuously on LDR. When an intruder breaks the beam → LDR resistance rises suddenly → alarm is triggered.

3. Cameras (Automatic Exposure Control): LDR in camera circuitry measures scene brightness → adjusts shutter speed or aperture for correct exposure. Used in older film cameras and simple digital cameras.

4. Light Meters (Lux Meters): LDR converts light intensity to resistance → voltage → display reading in lux.

5. Automatic Night Lamps: Small household lamps (plug-in nightlights) use LDR to turn on automatically when room gets dark.

6. Solar Trackers: Two LDRs on either side of a solar panel; compare outputs → servo motor turns panel toward brighter side → maximizes solar energy capture.

7. Smoke Detectors (Optical type): Smoke scatters a light beam onto LDR → increased light reading triggers alarm.

8. Display Brightness Control: Smartphone screens use light sensors (advanced photoresistors/photodiodes) to auto-adjust brightness.