Light Sensors

Light sensors let a project react to how bright or dark it is: night lamps, dusk-to-dawn switches, plant monitors, laser tripwires and UV monitors.

  • LDR (photoresistor): the simplest. Its resistance falls as light rises, so it goes in a voltage divider with a fixed resistor (10 kΩ suits indoor light) and you read the voltage on an analog pin.
    • An LDR gives relative light levels, not lux.
    • It responds in tens of milliseconds, fine for switching but too slow for data.
  • LDR modules: add an LM393 comparator with a potentiometer, so DO switches HIGH or LOW at a set brightness, while AO still gives the raw level.
  • Digital light sensors such as the BH1750 report calibrated light levels over I2C.
  • The VCNL4010: combines ambient light with proximity sensing over I2C.
  • The GUVA-S12SD: measures UV-A and UV-B, as an analog voltage you can convert to an approximate UV index.

How to choose:

  • Pick an LDR for on/off decisions.
  • Pick a digital sensor for real lux readings.
  • Pick a UV sensor for sun exposure.

On an ESP32, read analog light sensors on ADC1 pins (such as GPIO34), which keep working with Wi-Fi on.

Frequently asked questions

Does an LDR measure lux?

No. It gives relative levels. For calibrated lux use a digital sensor such as the BH1750.

What resistor goes with an LDR?

A 10 kΩ resistor in a divider suits typical indoor light. Pick one close to the LDR's resistance at your usual light level.

What is the difference between DO and AO on a module?

DO switches at the potentiometer's threshold. AO is the raw analog light level.

Is a DIY UV sensor accurate?

It is a relative indicator. The module ships uncalibrated, so keep using an official UV forecast for sun safety.

Guides

Reference pages on soldr.ai

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