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LDR Sensor (Photoresistor)

An LDR changes resistance with light. Here is where to buy one and what to check.

Updated . Edited by: Ilyass Ben Hakim.

Where to buy

Hobby shops and parts distributors sell bare LDRs and sensor modules. Bags of 10 to 50 are common and cheap.

Professional insight

Engineers choose LDRs by their resistance at 10 lux and in darkness.

  • Match the part code, such as GL5528, to your circuit.
  • Expect wide tolerance between cells in one bag.
  • Check RoHS status, since CdS cells contain cadmium.
  • Use a photodiode instead when fast response matters.

Check the label

0 of 7 checked

Specs to match

Typical GL5528 figures from datasheets
SpecTypical value
Light resistance at 10 lux8 to 20 kilohm
Dark resistance1 megohm minimum
Peak responseAbout 540 nm
Max voltage150 V DC
Max power100 mW
Operating range-30 to 70 C

Figures differ a little between makers. Read the datasheet that ships with your batch.

Types

Bare CdS cell

Two leads. Pair it with a fixed resistor as a voltage divider.

LDR module

Small board with a pot and digital output. Easy for microcontroller kits.

Assorted kit

Several part codes in one box. Handy for testing which range suits you.

Setup tips

  1. 1Pick a fixed resistor near the LDR value at your trigger light.
  2. 2Wire the LDR and resistor in series between supply and ground.
  3. 3Read the middle point with an analog input.
  4. 4Log readings in dark and bright light.
  5. 5Set your threshold between those two readings.

Common uses

  • Dusk to dawn night lights.
  • Automatic display brightness.
  • Simple light alarms and toy projects.
  • Classroom electronics demos.

Common questions

Does an LDR have polarity?
No. Either lead can go to either side.
Can an LDR measure exact lux?
Not well. It suits on and off or rough levels.
Why are LDRs hard to find in Europe?
Cadmium rules limit CdS parts in many new products.