Light-controlled lamp (LDR + analogRead)

Objective

Read an LDR divider on A0 and switch an LED on when the room gets dark.

Procedure

1. Press Start Simulation with the LDR at 1 lux: the LED is on.
2. Change the LDR 'lux' to 1000 and run again: the LED goes off.

Theory

The LDR and a 10 kΩ resistor form a divider: in the dark the LDR resistance is high, so A0 is low (below 300 of 1023) and the sketch lights the LED; in bright light A0 is high and the LED is off.

Questions to answer

1. How would you add hysteresis so the lamp does not flicker at dusk?

Components used

Arduino UNO R3 (clone, SMD), Breadboard 400 points, LDR GL5528, LED 5mm Red, 2 × Resistor 220 ohm 1/4W

About the model

Breadboard contacts are ideal (no contact resistance or capacitance). Arduino-style C/C++ subset interpreter (not AVR machine code): no interrupts or libraries, int is 16-bit, only delay() advances time, pin modes are fixed after setup(). Outputs are 5 V behind 25 Ω.

More Microcontroller Lab experiments

• PWM fade (analogWrite ramp)
• LED chaser (for loop over pins)
• Button press counter (Serial output)