Photoresistor (LDR)
Resistor (non-linear)
A light-dependent resistor whose resistance decreases with increasing light intensity. Used to detect light levels or create light-sensitive circuits.
Schematic symbol
Photoresistor (LDR)
How photoresistor (ldr) appears in a circuit diagram.
Types & variants
Key specs
Dark resistance
kΩ (MΩ)Resistance in complete darkness (typically 1MΩ to 10MΩ).
Light resistance
Ω (kΩ)Resistance under bright light (typically 500Ω to 5kΩ).
Spectral response
nmWavelength range it's sensitive to (typically 400-700nm for CdS).
Response time
msTime to change resistance when light level changes (typically 10-100ms).
Power rating
mWMax power it can handle (typically 50-100mW).
Laws that govern it
- Laws of resistanceR = ρ × L / A
Light frees charge carriers in the cadmium sulphide track, dropping ρ — the same R = ρL/A, with light doing what temperature does in a thermistor.
- Kirchhoff's voltage law (KVL)ΣV(loop) = 0
Almost always wired in a divider with a fixed resistor, so the loop sums to the supply and the junction voltage is your light reading.
Each law is stated in full — with its diagram, variables and worked meaning — on the electrical laws page.
How it compares
- Photoresistor (LDR) vs Photodiode
Side by side with Photodiode / Phototransistor — specs, variants and which one the job calls for.
Markings
Usually unmarked. Check datasheet for exact specs. Body color may indicate type (e.g., CdS are often dark green).
Standard values
Dark resistance: 0.5-20MΩ. Light resistance: 100Ω-10kΩ. Common: 5-10MΩ dark, 1-5kΩ light.
How to choose
1) Match spectral response to your light source (e.g., CdS for visible light). 2) Consider required light/dark resistance ratio. 3) Check response time for your application. 4) Ensure power rating is sufficient.
Pinout & package
2 leads (non-polar). Packages: radial lead (common), flat SMD. Often epoxy-coated with a window to allow light in.
Example circuits
- Light-activated switch
- Light level meter
- Automatic night light
- Photographic exposure control
- Line-following robot sensor
Common failures
Becomes unresponsive (fails to change resistance) with age or exposure to high humidity. Cracking or discoloration of the epoxy coating.
How to test
1) Measure resistance in complete darkness (should be high). 2) Shine bright light on it (resistance should drop significantly). 3) Compare to datasheet values.
Substitutes
Photodiodes, phototransistors (faster response), or other photoresistors with similar specs. Digital light sensors (TSL2561, BH1750) for better accuracy.