light detecting resistor

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Direct answer to the question • A “light-detecting resistor” is the common name for a Light-Dependent Resistor LDR, also called a photoresistor. • It is a passive semiconductor device whose electrical resistance falls as incident light intensity rises and rises again in darkness. • LDRs are inexpensive, simple to use, and well-suited to slow or medium-speed light sensing e.g. duskdawn switches, display dimming. Key points – Operating principle: photoconductivity in CdS, CdSe, PbS, etc. – Typical dark resistance: 0.1–20 MΩ; bright-light resistance: 10 Ω–2 kΩ. – Response time: tens to hundreds of ms slow compared with photodiodes. – Often used in a voltage-divider or Wheatstone-bridge configuration. Detailed problem analysis 1. Physical construction & materials • Semiconductor film zig-zag track on ceramic substrate; two electrodes. • Main photoconductive compounds – CdS 400–700 nm, visible – most widespread but RoHS-restricted cadmium. – CdSe extended into near-IR, ~350–900 nm. – PbS InSb infra-red sensing. – Emerging RoHS-compliant alternatives: amorphous silicon, InGaAs thin films, organic photoresistors – still niche and costlier. 2. Photoconductive mechanism • Photon energy hnu ≥ band-gap excites e⁻ from valence → conduction band. • Created e⁻h⁺ pairs increase carrier density → conductivity σ = qnμn+pμp rises → resistance R = frac{L}{Aσ} drops. • Relation to illumination lux is empirical: R = K , E^{-γ} with γ ≈ 0.7…1.2 depending on device; not linear. 3. Electrical characteristics typical Ø 12 mm CdS cell • Dark 0 lux: 5 MΩ @25 °C • 10 lux: 30–50 kΩ • 100 lux: 5–8 kΩ • 1000 lux: 500–800 Ω • Max power: 90 mW @25 °C; derate 0.8 mW °C above. • Max operating voltage: 150 Vdc rarely used so high. • Rise time τr: 20 ms; fall time τf: 50–150 ms trap-level controlled. 4. Circuit usage a. Voltage divider to create a light-dependent voltage: VOUT = VCC · Rfixed RLDR + Rfixed – Choose Rfixed ≈ √Rdark · Rlight for max sensitivity around mid-lux. b. Comparator Schmitt trigger for clean switching hysteresis. c. Interface to MCU ADC: use RC time or direct ADC sampling. d. Wheatstone bridge for higher rejection of supply drift. 5. Design considerations • Temperature coefficient −0.5 %°C … −1 %°C → add thermistor or software compensation for precision. • Ageing: CdS devices drift upward in dark resistance; perform periodic re-calibration in meters. • Optical filtering: if spectral selectivity is required, add coloured glassgel or IR-cut filter. • Protection: UV can permanently shift response; encapsulate or shield if outdoors. 6. Comparison with alternatives • Photodiode biased, µs response, linear current vs lux, small. • Phototransistor higher gain, slower than photodiodes but faster than LDR. • Integrated Ambient-Light Sensors OPT3001, TSL2591: I²CSPI, calibrated lux output, negligible temp drift, RoHS-friendly. • Camera image sensors for 2-D light mapping. Current information and trends • Regulatory: Since July 2024 the EU RoHS exemption for CdS LDRs expired for most new designs; Cd-based parts may still be sold for servicerepair but new products in EU must use cadmium-free sensors. Ref: EU RoHS Directive 201165EU, Annex III exemption 36 sunset date 6 July 2024. • Market shift: Consumer electronics, smartphones and LED lighting now preferentially use silicon ALS ICs with digital output, eliminating calibration effort. • Emerging tech: Printable organic photoresistors on flexible substrates for wearable light sensing; response slower but mechanically robust. • Supply-chain: Some mainstream distributors e.g., Mouser, Digi-Key list CdS LDRs as “Not for new designs” or EU-restricted stock. Design engineers should document RoHS compliance path early. Supporting explanations and details Example: Street-light controller - Place LDR on top of enclosure; series resistor 100 kΩ forms divider into 2 V reference comparator at 5 V supply. At dawn ≈200 lux, RLDR ~5 kΩ → Vdiv ≈0.24 V > hysteresis lower threshold turns lamp OFF. At dusk ≈20 lux, RLDR ~50 kΩ → Vdiv ≈1.7 V, comparator flips lamp ON. Add 100 nF across LDR to suppress fast headlight flashes. Analogy: Think of LDR as a valve whose opening widens gradually in sunlight but closes sluggishly in darkness – good for average daylight decisions, not for Morse-code reception. Ethical and legal aspects • Cadmium and lead compounds are toxic; disposal must follow WEEE and local hazardous-waste rules. • Designers selling in EUUK must meet RoHS and CE-mark technical file requirements; using CdS photoresistors after 2024 requires documented exemption e.g., medical lineage or a substitute technology. • Safety: High-voltage AC mains night-light circuits must respect creepage between LDR side and user-accessible parts; double-insulate or use opto-isolator. • Privacy: Using LDRs in smart-home presence detection may trigger data-protection requirements GDPR if combined with personal data. Practical guidelines • Prototype: measure real Rlight Rdark with your light source and operating temperature; datasheet curves vary ±50 %. • Choose package size: larger disks 12–20 mm offer higher peak photocurrent and lower noise; 5 mm “mini-LDR” fine for hobby. • Shield leads to avoid 5060 Hz hum on high-impedance node; keep divider impedance < 100 kΩ when feeding ADC. • Over-illumination: continuous >10 klx may heat device; ensure derating. • If fast events >1 kHz must be captured, replace LDR with photodiode + transimpedance amp. Potential challenges & mitigation – Slow decay: add shunt resistor across LDR to speed fall time at cost of reduced dynamic range. – Temperature drift: sense board temperature with NTC and correct in firmware. – RoHS compliance: migrate to OPT3002 Texas Instruments or BH1750 ROHM digital ALS. Possible disclaimers or additional notes • Absolute accuracy of LDR lux estimation seldom exceeds ±15–20 % without individual calibration. • Figures given are typical; consult the specific manufacturer’s datasheet e.g., Selco, Advanced Photonix for guaranteed minmax. • Supply chain volatility for CdS parts expected after RoHS sunset – plan second-source strategy. Suggestions for further research • Investigate thin-film amorphous-Si photoresistors as drop-in CdS replacements. • Explore logarithmic light-to-digital converters TI OPT3004 for >160 dB dynamic range. • Study temperature-compensated bridge techniques for precision lux meters. • Evaluate machine-learning approaches to fuse LDR data with PIR sensors for occupancy analytics. Resources – TI Application Report “Ambient Light Sensing Basics” SLOA190. – IEEE Sensors Journal, Jan 2023 issue on flexible photonic sensors. – EU JRC “Impact Assessment of RoHS Exemptions for Cadmium in Photoresistors”, 2022. Brief summary A Light-Dependent Resistor is a simple, low-cost sensor whose resistance inversely tracks ambient light. It is ideal for slow, wide-range light detection but limited by non-linearity, temperature drift, and new environmental regulations on cadmium. Modern designs increasingly replace LDRs with silicon ambient-light sensor ICs offering faster, calibrated, and RoHS-compliant performance. When an LDR remains the best choice—because of cost, simplicity, or legacy compatibility—engineers should carefully select bias resistors, allow for temperature and ageing drift, and confirm regulatory compliance.

Light-Dependent Resistor (LDR) vs. Photoresistor: CdS, CdSe, RoHS, Models, Circuit Use, Faults

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