Building a Simple LDR Sensor Circuit

A photoresistor and a fixed resistor, wired as a voltage divider, turn incoming light into a voltage you can measure or act on.

You'll need: a light-dependent resistor (LDR/photoresistor), a fixed resistor (try 10kΩ as a starting point), a supply, and a multimeter (or later, an ADC input once you reach Lesson 29).

An LDR's resistance changes with light: bright light drops its resistance to a few hundred ohms, while darkness raises it into the hundreds of kilohms. On its own, that changing resistance isn't something you can easily read — but wired into a voltage divider (Lesson cb-06) alongside a fixed resistor, it becomes a changing voltage, which a multimeter or a microcontroller's ADC can read directly.

Wire the LDR and the fixed resistor in series across your supply, exactly like the two resistors in Lesson cb-06, and measure the voltage at their junction. Try both orders: LDR on top (junction voltage rises with light) or LDR on the bottom (junction voltage falls with light) — pick whichever direction makes sense for what you're building next.

This same divider, feeding a transistor's base (Lesson cb-10) instead of a meter, is the basis of a simple light-activated switch — dark triggers the transistor on, turning on an LED or a relay automatically.