Day 16: Show Sensor Values on the LCD

Hi Anish! Yesterday your LCD said “Hello Anish!” Today it becomes a live dashboard — we’re going to show real-time sensor values right on the screen. No laptop needed.

What you need today

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• Arduino Uno + USB cable
• Breadboard
• LCD with I2C module (Day 15 wiring)
• 1 potentiometer (Day 8 wiring)
• 1 LDR + 10kΩ resistor (Day 9 wiring)
• Jumper wires

You’ll have a lot going on the breadboard today. Take a deep breath, wire it slowly, test one thing at a time.

The circuit

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Three subsystems.

graph TB
    subgraph LCD["LCD (I2C)"]
      V5a["5V"] --> LV["LCD VCC"]
      Ga["GND"] --> LG["LCD GND"]
      A4["A4"] --> SDA["LCD SDA"]
      A5["A5"] --> SCL["LCD SCL"]
    end
    subgraph POT["Potentiometer"]
      V5b["5V"] --> P1["Pot left"]
      P2["Pot middle"] --> A0["A0"]
      P3["Pot right"] --> Gb["GND"]
    end
    subgraph LDR["LDR (voltage divider)"]
      V5c["5V"] --> L1["LDR"]
      L1 --> JUNC["junction"]
      JUNC --> A1["A1"]
      JUNC --> R10k["10kΩ"] --> Gc["GND"]
    end

Pin assignments:

• A4, A5 — LCD (I2C, fixed)
• A0 — potentiometer middle leg
• A1 — LDR voltage divider

This is the first time we’ve used two analog inputs at once (A0 and A1). Arduino has 6 of them (A0-A5, minus A4/A5 which we’re using for the LCD), so we can have up to 4 sensors running simultaneously.

The code

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

LiquidCrystal_I2C lcd(0x27, 16, 2);

void setup() {
  lcd.init();
  lcd.backlight();

  // Print the labels ONCE — they never change
  lcd.setCursor(0, 0);
  lcd.print("Pot:");
  lcd.setCursor(0, 1);
  lcd.print("Light:");
}

void loop() {
  int pot = analogRead(A0);
  int light = analogRead(A1);

  // Update just the number portions — labels stay
  lcd.setCursor(6, 0);
  lcd.print("    ");        // clear old value
  lcd.setCursor(6, 0);
  lcd.print(pot);

  lcd.setCursor(6, 1);
  lcd.print("    ");
  lcd.setCursor(6, 1);
  lcd.print(light);

  delay(200);
}

Upload. The LCD shows:

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Pot:   512
Light: 640

Turn the pot — the top number changes live. Cover the LDR — the bottom number drops. No flicker, no clearing the whole screen.

The no-flicker trick

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Yesterday I warned you that calling lcd.clear() every loop makes the screen flicker. Here’s the clever solution:

1. Print the labels once, in setup()

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void setup() {
  ...
  lcd.setCursor(0, 0);
  lcd.print("Pot:");        // label, doesn't change
  lcd.setCursor(0, 1);
  lcd.print("Light:");      // label, doesn't change
}

Because labels never change, we write them once in setup() and never touch them again. The LCD remembers them.

2. Only overwrite the numbers, in loop()

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lcd.setCursor(6, 0);      // jump to column 6 (just past "Pot: ")
lcd.print("    ");        // 4 spaces — wipes any leftover digits
lcd.setCursor(6, 0);      // jump back
lcd.print(pot);           // new value

Three moves:

1. Move cursor to where the number should go.
2. Print 4 spaces to wipe the old value (4 spaces covers a number from 0 to 9999 — more than enough for 0-1023).
3. Move the cursor back to the same spot and print the new number.

Without the “print spaces to wipe” step, old digits would hang around. For example, if the pot used to read 1023 and drops to 5, you’d see 5023 on the screen — the old 023 didn’t get cleared.

This is the standard trick for flicker-free updating LCDs. Every sensor dashboard uses it.

Why not just use lcd.clear() once per loop?

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You could. It’s just obviously wrong visually — you’d see the whole screen go blank for a split second on every update, 5 times per second. Looks like a broken TV.

The “overwrite only what changes” trick is smoother because the LCD keeps most of its content stable. Your brain doesn’t notice updates happening if they’re localized.

Add a unit

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Let’s show percentages instead of raw 0-1023 values for the pot:

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int potPercent = map(pot, 0, 1023, 0, 100);

lcd.setCursor(6, 0);
lcd.print("    ");
lcd.setCursor(6, 0);
lcd.print(potPercent);
lcd.print("%");

Now the top row reads like Pot: 42%. Much more human.

Remember map() from Day 8? Same trick: rescale from 0-1023 to 0-100.

Try this

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1. Three sensors. Add a second LDR on A2 (same wiring pattern) and show three live values by using row 1 for two fields, split in half.
2. Reading the time. Arduino has a function millis() that returns milliseconds since the board started. Show millis() / 1000 on row 2 so you see seconds running up. It’s a live clock.
3. Battery icon. Show a simple text “bar” that grows with the pot value: [#### ] for 50%, [########] for 100%. Use a for loop to print the right number of # characters.
4. Alert on threshold. When pot > 900, print "HIGH!" on the right side of row 0. Otherwise, print 5 spaces to clear it.

Tip: too many setCursor calls get tedious

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You’ll notice that updating even two fields takes 6 LCD calls. As your dashboards get bigger, that adds up. One trick is to write a helper function:

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void printAt(int col, int row, int value) {
  lcd.setCursor(col, row);
  lcd.print("    ");
  lcd.setCursor(col, row);
  lcd.print(value);
}

Now your loop() becomes much cleaner:

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void loop() {
  printAt(6, 0, analogRead(A0));
  printAt(6, 1, analogRead(A1));
  delay(200);
}

Helper functions are a big idea — we’ll explore them more on Day 25. For now, recognize the pattern: if you find yourself typing the same 3-4 lines of code over and over, you can group them into a named function and call that function instead.

What you learned today

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• How to read multiple analog sensors at once (A0, A1, …)
• The no-flicker LCD update pattern: labels in setup(), values in loop(), overwrite in place
• Why you shouldn’t call lcd.clear() in a fast loop
• Printing numbers with lcd.print(someVar)
• Adding units with a second lcd.print("%") after the number
• Brief preview of helper functions for cleaner code

What is next

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Day 17 — a scrollable menu on the LCD with buttons. You’ll press a button to cycle through options like “Fan ON”, “Light AUTO”, “Buzzer ON”, and select one to trigger an action. Arduino gets a user interface.

Great work, Anish. Your screen is alive.