LDRで明るさを測る

An LDR is a light-dependent resistor. Its resistance drops in bright light and climbs high in the dark.

Cover an LDR and its resistance soars; shine a lamp on it and resistance falls. That change is what you measure.

The Arduino reads voltage, not resistance. So an LDR by itself gives no voltage the board can sense directly.

Combine the LDR with a fixed resistor to form a voltage divider. Together they turn light into a readable voltage.

A 10k ohm resistor is a solid default partner for most LDRs. It keeps your readings spread across a useful range.

Connect the junction between the LDR and the resistor to an analog pin like A0. That midpoint voltage is your signal.

Call analogRead on the divider pin to get a 0 to 1023 number that rises and falls with the light.

Which side the LDR sits on decides whether bright gives a high or low number. Test it and note your result.

Print the value so you can wave your hand over the sensor and see the number react with Serial.println.

Lights flicker faster than your eye sees. Averaging a few readings gives a steadier brightness value.

The LDR is a cheap way to add light awareness. It powers night lights, plant monitors, and auto-dimming displays.

Why cant an LDR connect to an analog pin on its own?

You built an LDR voltage divider with a 10k resistor, tapped the midpoint to A0, and read brightness with analogRead.