Light Sensors#

Goal. Build a simple light-sensitive circuit where the LED turns on when it gets dark—like a tiny automatic night light.

Instructions for TinkerCAD#

LDRs change resistance based on light:
- Bright light = low resistance
- Dark = high resistance
A voltage divider turns this resistance change into a voltage signal.
A transistor acts like an electronic switch to turn the LED on or off depending on that signal.

Set Up the Voltage Divider#

This is how we “read” the LDR’s signal.
- Place the LDR across two rows of the breadboard.
- Connect one end of the 10kΩ resistor to the same row as one side of the LDR.
- Connect the other end of the resistor to ground (–).
- Connect a jumper wire from the middle point (where the LDR and 10k resistor meet) to the base of the NPN transistor. This “middle point” gives a voltage that changes with light.
Connect Power to the LDR#

Connect the other end of the LDR (not shared with the resistor) to + (positive) on the battery.
Set Up the LED and Transistor#
- Place the LED on the breadboard (long leg to one row, short leg to another).
- Connect the short leg (–) of the LED to the collector of the transistor.
- Connect the emitter of the transistor to ground (–).
- Place a 220Ω resistor between the long leg (+) of the LED and + power from the battery.
Power It On and Test#
- Insert batteries or connect the battery clip.
- Cover the LDR with your hand.
- The LED should turn on in the dark and turn off in bright light.

LDRs let your circuit react to the environment.
A transistor acts as a bridge between a weak signal and a stronger action (like lighting an LED).
You’ve now used a sensor to control a circuit—no programming needed!

Reverse the logic: can you make it turn off in the dark and on in the light?