Sensors & isolation
Light sensors (LDR & photodiode)
Parts that respond to light. An LDR is a resistor that drops from megohms in the dark to a few hundred ohms in bright light; a photodiode makes a small current proportional to the light, and reacts thousands of times faster.
Pins
- LDR
- : No polarity. Use it in a voltage divider and read the voltage.
- Photodiode
- : Usually run reverse-biased (cathode to +), feeding a resistor or an amplifier.
LDRs are flat discs with a wiggly orange track; photodiodes look like LEDs with a clear or dark-tinted body (dark ones only see infrared).
What to look for inside
- The lamp slowly brightens and dims.
- Each photon (yellow) that hits the semiconductor knocks an electron free, leaving a hole behind. The voltage across the part sweeps them to the electrodes.
- More light frees more carriers. In an LDR that means lower resistance; in a photodiode, more current.
The numbers that matter
| Spec | Typical |
|---|---|
LDR dark / light resistance Choose the divider resistor near the middle of your range. | ≈ 1 MΩ dark · 200 Ω to 10 kΩ lit |
LDR response time Too slow for data, fine for "is it dark yet?". | 10 ms to 100 ms |
Photodiode responsivity Current per watt of light. Real currents are µA, so it usually needs an amplifier. | ≈ 0.5 A/W |
Where you’ll use it
- Night lights and automatic brightness.
- Remote-control receivers and light barriers (photodiodes).
- Light meters and simple optical sensors.
Common mistakes
- Expecting an LDR to be precise or fast. It is neither.
- Some LDRs contain cadmium and are banned in new products in the EU (RoHS).
Go deeper
LDR in a divider: the output rises as light increases.
Photodiode current is proportional to light power.