Unit E - College Practice

26. Capstone 1: Environmental Monitor

Integrate DHT11, LDR, I2C LCD and a temperature alarm into one monitored system.

Estimated time 6-8 hours

Learning outcomes

Draw input-process-output blocks for sensors, validation, display and alarm
Wire DHT11, LDR divider and I2C LCD on non-conflicting pins
Sample with millis every two seconds without blocking delay
Reject failed DHT readings with isnan and keep the alarm silent
Calibrate and document a temperature alarm threshold with a test table

Parts and preparation

Uno, DHT11 (bare with 4.7-10 k ohm pull-up, or 3-pin module), LDR with series resistor for a voltage divider, I2C 1602 LCD (backpack), passive buzzer, jumpers. 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

Design before you solder

This capstone combines lessons you already passed: DHT (18), LDR divider (20) and I2C LCD (17). Start on paper with Input-Process-Output so every wire and if-statement has a job.

Inputs are temperature, humidity and light. Processing validates readings and compares a threshold. Outputs are LCD information and an audible warning.

IPO diagram with DHT and LDR inputs, validate and compare processing, LCD and buzzer outputs — IPO first. Wiring and code only implement the blocks you named.

Course pin map

The breadboard photo under Wiring matches the sensing and display half of the build. DHT DATA on D2, LDR junction on A0, LCD on the Uno I2C pins (A4/SDA and A5/SCL, or the dedicated SDA/SCL headers).

Add a passive buzzer on D6 for the alarm. That part is in the worked sketch but not on the photo.

Pin map for DHT D2, LDR A0, I2C LCD A4 A5 and buzzer D6 — No shared digital pins between DHT, LCD and buzzer. Only 5 V and GND are common.

Function	Uno pin
DHT DATA	D2
LDR divider junction	A0
I2C LCD SDA / SCL	A4 / A5 (or SDA / SCL headers)
Passive buzzer	D6
Module power	5 V and GND rails

Integrate one subsystem at a time

Do not upload the full sketch on day one. Prove LCD Hello World, then DHT on Serial or LCD, then LDR cover/uncover on A0, then force the alarm with a temporary low threshold.

Combine only after each block passes its own check. A fault after the merge usually means merge order - not a new mystery component.

Four steps: test LCD, then DHT, then LDR, then alarm before combining — LCD, DHT, LDR, alarm. Tick each box before you glue the sketch together.

Sample with millis, not delay

DHT sensors need about two seconds between reads. Using delay(2000) freezes the whole sketch. Use millis so loop stays free for Serial, buttons or extra LEDs later.

Pattern: if (now - lastSample >= 2000) { lastSample = now; read and update; }

Timeline of DHT samples every two seconds using millis without blocking delay — Sample on a 2 s interval. Between samples, loop can still run other work.

LCD layout and clean rows

Row 0 shows temperature and humidity. Row 1 shows the light ADC value (0-1023 on the Uno). Print trailing spaces so shorter numbers do not leave old digits on the glass.

If your backpack address is 0x3F instead of 0x27, change the LiquidCrystal_I2C constructor (lesson 17 scanner).

Example 16x2 LCD showing T and H on row 0 and Light on row 1 — Keep both rows stable. Pad with spaces after values that change width.

Validate before you alarm

dht.readTemperature() and readHumidity() return NAN on a failed transfer. Check isnan(t) || isnan(h) before comparing thresholds. On failure show Sensor error, call noTone, and do not treat the last good temperature as still valid.

Course default alarm: t >= 30.0 C. Warm the sensor gently or lower the constant for a demo, then restore and document the value you assess against.

Decision flow from DHT read through isnan check to threshold compare and tone — Failed reads stay silent. Only a valid hot reading sounds the buzzer.

Condition	LCD	Buzzer
isnan(t) or isnan(h)	Sensor error	Off
Valid and t < threshold	T / H / Light	Off
Valid and t >= threshold	T / H / Light	On (tone)

Assessment evidence

College practicals mark process as well as a working board. Prepare:

1. IPO / block diagram 2. Wiring diagram or annotated breadboard photo 3. Commented sketch with named pins and threshold constant 4. Test table at three conditions (cool, near threshold, hot or forced) 5. One fault you found and how you fixed it 6. Short demo checklist (LCD update, light change, alarm on/off, sensor unplug error)

Wiring and safe build sequence

Breadboard wiring for lesson 26: Capstone 1: Environmental Monitor — Environment monitor breadboard: DHT11 on D2, LDR divider on A0, I2C LCD on SDA/SCL. Add a passive buzzer on D6 for the temperature alarm (not shown in the photo).

5 V and GND to the breadboard rails; all modules share GND
DHT DATA -> D2 (pull-up DATA to 5 V if using a bare sensor)
DHT VCC -> 5 V, DHT GND -> GND
LDR divider: one side to 5 V, other side through resistor to GND; junction -> A0
I2C LCD VCC/GND -> 5 V/GND; SDA -> A4 (or SDA); SCL -> A5 (or SCL)
Passive buzzer signal -> D6 (add this for the alarm if not on the breadboard photo)

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>

const byte dhtPin = 2;
const byte buzzerPin = 6;
const byte lightPin = A0;
const float alarmCelsius = 30.0;  // Document the value you calibrate to

LiquidCrystal_I2C lcd(0x27, 16, 2);  // Try 0x3F if the display stays blank
DHT dht(dhtPin, DHT11);
// DHT dht(dhtPin, DHT22);  // Use this instead if you fitted a DHT22

unsigned long lastSample = 0;
const unsigned long sampleMs = 2000;

void setup() {
  pinMode(buzzerPin, OUTPUT);
  noTone(buzzerPin);

  Serial.begin(9600);
  lcd.init();
  lcd.backlight();
  dht.begin();

  lcd.clear();
  lcd.print("Env monitor");
  lcd.setCursor(0, 1);
  lcd.print("Sampling...");
  Serial.println("Environmental monitor ready");
}

void loop() {
  unsigned long now = millis();
  if (now - lastSample < sampleMs) {
    return;
  }
  lastSample = now;

  float t = dht.readTemperature();
  float h = dht.readHumidity();
  int light = analogRead(lightPin);

  if (isnan(t) || isnan(h)) {
    lcd.setCursor(0, 0);
    lcd.print("Sensor error    ");
    lcd.setCursor(0, 1);
    lcd.print("Check DHT wiring");
    noTone(buzzerPin);
    Serial.println("DHT read failed");
    return;
  }

  lcd.setCursor(0, 0);
  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 >= alarmCelsius) {
    tone(buzzerPin, 1000);
  } else {
    noTone(buzzerPin);
  }

  Serial.print("T=");
  Serial.print(t, 1);
  Serial.print(" H=");
  Serial.print(h, 0);
  Serial.print(" Light=");
  Serial.println(light);
}

How the code works

Named pins and alarmCelsius make the assessment sketch easy to mark and retune.
isnan failure shows a clear LCD message and forces the buzzer off.
Trailing spaces on both rows clear leftover digits when values get shorter.
Change 0x27 to 0x3F (or your scanner result) if the LCD stays blank.

Test and record evidence

Expected result: Every two seconds the LCD shows temperature, humidity and light. Covering the LDR changes Light. At or above the documented threshold the buzzer sounds; unplugging DHT DATA shows Sensor error and silences the alarm.

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

Blank LCD: run the lesson 17 I2C scanner; try address 0x3F; check SDA/SCL and backlight jumper.
Sensor error always: confirm DHT DATA on D2, 5 V, GND, and pull-up on bare sensors.
Light stuck: check the divider to 5 V and GND and that A0 is on the junction.
No beep when hot: confirm passive buzzer on D6, or lower alarmCelsius briefly for a bench test.
Everything wrong after a big merge: retest LCD, DHT, LDR and alarm as separate sketches.

Extension challenge: Track max and min temperature on the LCD and add a button (INPUT_PULLUP) that resets those extremes.

Check your understanding

Q1. Why integrate one subsystem at a time?

Show answer

It localises faults and confirms each interface independently.

Q2. Why use millis instead of delay(2000) for DHT sampling?

Show answer

delay freezes the whole sketch; millis lets other work keep running between samples.

Q3. What must happen on an isnan DHT failure?

Show answer

Show a clear error, silence the buzzer, and do not treat the reading as a valid temperature.

Q4. What evidence proves calibration?

Show answer

Recorded conditions, readings, chosen threshold and the observed alarm output.