Unit E - College Practice

26. Capstone 1: Environmental Monitor

Integrate sensing, display, timing and alarms.

Estimated time 6-8 hours

Learning outcomes

Design input-process-output blocks
Integrate DHT11, LDR and I2C LCD
Handle sensor failure without corrupting the display
Calibrate and document an alarm threshold

Parts and preparation

Uno, DHT11, LDR divider, I2C LCD, RGB LED/buzzer and jumper wires. Install the same libraries as lessons 17 and 18.

Before power: inspect wiring, confirm supply voltage and ensure all connected circuits share GND.

Libraries for this lesson

In Arduino IDE 2 open Tools → Manage Libraries…. Search by the Library Manager name and install the package by the exact author below. Similar names from other authors can use different APIs and will break the example.

Include	Library Manager name	Author	Install note
`Wire.h`	Wire	Arduino	Built into the Arduino core. No Library Manager install required.
`LiquidCrystal_I2C.h`	LiquidCrystal I2C	Frank de Brabander	Same package as lesson 17. Install via Library Manager.
`DHT.h`	DHT sensor library	Adafruit	Same package as lesson 18. Install via Library Manager.
`(dependency)`	Adafruit Unified Sensor	Adafruit	Required with the DHT sensor library.

Capstone 1: Environmental Monitor instructional connection diagram — Open the diagram for a larger view. Trace every connection before building.

System design

Inputs are temperature, humidity and light. Processing validates readings and compares thresholds. Outputs are LCD information and visual/audible warnings.

Integration order

Test LCD, then DHT11, then LDR, then alarm. Combine only after each subsystem passes its own acceptance test.

Timing

DHT updates every two seconds; display and alarm logic can run more often. Use millis so the interface remains responsive.

Assessment evidence

Submit block diagram, wiring diagram, commented sketch, test table at three conditions, fault record and demonstration checklist.

Wiring and safe build sequence

DHT DATA -> D2
LDR divider junction -> A0
I2C LCD SDA/SCL -> A4/A5
Buzzer -> D6
All modules share GND

Power rule: switch off before moving wires. Arduino I/O pins are control signals; high-current loads require a driver and suitable external supply.

Worked sketch

Download .ino sketch

#include <Wire.h>
#include <LiquidCrystal_I2C.h>
#include <DHT.h>

LiquidCrystal_I2C lcd(0x27, 16, 2);
DHT dht(2, DHT11);
unsigned long lastSample = 0;

void setup() {
  lcd.init();
  lcd.backlight();
  dht.begin();
  pinMode(6, OUTPUT);
}

void loop() {
  if (millis() - lastSample >= 2000) {
    lastSample = millis();
    float t = dht.readTemperature();
    float h = dht.readHumidity();
    int light = analogRead(A0);
    lcd.setCursor(0, 0);
    if (isnan(t) || isnan(h)) {
      lcd.print("Sensor error    ");
      noTone(6);
      return;
    }
    lcd.print("T:");
    lcd.print(t, 1);
    lcd.print("C H:");
    lcd.print(h, 0);
    lcd.print("% ");
    lcd.setCursor(0, 1);
    lcd.print("Light:");
    lcd.print(light);
    lcd.print("    ");
    if (t >= 30.0) tone(6, 1000); else noTone(6);
  }
}

How the code works

Subsystems use non-conflicting pins.
Failed sensor data produces a clear error and safe alarm state.
Trailing spaces keep both LCD rows clean.

Test and record evidence

Expected result: The display updates every two seconds and the alarm activates at the documented temperature threshold.

Practical evidence checklist

Labelled wiring or block diagram Uploaded sketch and observed result At least one measured/tested value Fault or improvement recorded in logbook

Common faults and checks

Retest each subsystem separately.
Use I2C scanner and DHT failure messages.
Check the buzzer type and pin.

Extension challenge: Add maximum/minimum temperature tracking and a button to reset those values.

Check your understanding

Q1. Why integrate one subsystem at a time?

Show answer

It localises faults and confirms each interface independently.

Q2. What evidence proves calibration?

Show answer

Recorded inputs/conditions, readings, chosen threshold and observed output.