CdS (cadmium sulfide) track · resistance drops with lightClear epoxy facePins · 3.4 mm spacing

Ceramic disc, a serpentine orange-red CdS track on top, clear coating, two long legs
Body diameter ≈ 5 mm, two long legs

Resistors

LDR (photoresistor)

In simple terms

What is this?

A gate that passes electricity easily in light and poorly in darkness.

What is it for?

Tells the circuit whether the surroundings are light or dark; a night light switches on by itself.

Where do you see it?

In night lights that switch on by themselves in the dark.

How do you spot it?

A small round surface with a winding line pattern and two legs.

Caution:

Do not connect it on its own, put a resistor next to it; it suits the question "is it bright?", not fine measurement.

Technical definition

A sensor whose resistance falls as light increases. In the cadmium sulfide layer, more charge carriers are freed as it receives light, and conductivity rises. It responds slowly; it suits threshold detection better than precise measurement.

At 10 lux8–20 kΩ

In the dark≥ 1 MΩ

Powermax. 100 mW

Schematic symbol IEC and ANSI differ

IEC 60617: the drawing style used in Türkiye and Europe.

Pins 2

1Pin 1Has no direction1

2Pin 2Has no direction2

Features typical

Resistance (10 lux): 8–20 kΩ
Dark resistance: ≥ 1 MΩ
Max. power: 100 mW
Max. voltage: 150 V DC
Peak sensitivity: ≈540 nm (green)
Response: Tens of ms (slow)
Material: Cadmium sulfide (CdS)
Example part: GL5528

Common mistakes

Connecting it on its ownAn LDR does not generate voltage itself. Build a voltage divider with a fixed resistor and read the voltage at the midpoint.

Expecting precise measurementResistor resistance varies widely even within a model; response is slow. Set the threshold with a trimpot.

Throwing it in the trashContains cadmium. Do not break it or throw it in household trash; take it to an e-waste drop-off.

Test with a multimeter

Test whether this part is good with the multimeter on the measurement screen.

Take a measurement

Where it is used

Night lightTwilight switchBrightness control by lightBeam-break alarm

Try it in the Lab