Sense the world, then move it

• How sensors work · LDR · IR
• Pull-up / pull-down
• Line-following robot
• Motors & drivers
• PWM speed · H-bridge
• Servo · ultrasonic
• ESP32 Bluetooth

Light, temperature, distance, motion → voltage, resistance, or a digital pulse the microcontroller can read.

Map each sensor we use to what it converts: LDR → resistance, IR → reflected pulse, thermistor → resistance.

A common digital sensor — it measures temperature and humidity and sends the reading to one data pin as a digital signal.

• Single-wire digital output
• A library decodes the timing for you
• DHT11 (basic) / DHT22 (more accurate)

Many sensors change resistance. A divider (sensor + fixed resistor) turns that changing resistance into a changing voltage the ADC can read.

• Vout = Vin × R2 / (R1 + R2)
• Sensor resistance shifts → Vout shifts
• The MCU reads Vout with analogRead()

• Resistance drops as light increases
• Pair with a 10 kΩ resistor to ground for a clean reading

// LDR + 10k resistor form a divider into pin 34
int light = analogRead(34);   // bright = high, dark = low
Serial.println(light);
if (light < 800) {
  digitalWrite(ledPin, HIGH); // dark → turn light on
}

That squiggly face is light-sensitive material — its resistance falls as more light hits it.

• Bright light → low resistance
• Darkness → high resistance
• No polarity — connect either way

Two facing LEDs: one emits invisible infrared, the other (black) receives what bounces back. A trim-pot sets the threshold.

• Emitter + receiver pair
• Trim-pot tunes sensitivity
• 3 pins: VCC · OUT · GND

An IR LED emits infrared; a photodiode reads what bounces back.

The same sensor reads different surfaces: a white surface bounces the beam back (detected), a black surface soaks it up (nothing returns).

• White → reflects → OUT goes HIGH/green
• Black → absorbs → OUT goes LOW/red
• This is exactly how line-following works

Watch the surface flip white↔black and the OUT indicator follow it.

int ir = digitalRead(IR_PIN);
if (ir == LOW) {           // line detected (module-dependent)
  Serial.println("On the line");
}

A trim-pot on most IR modules sets the detection threshold — calibrate it before relying on it.

An unconnected input "floats" and reads random noise. A pull-up resistor ties it HIGH when idle; a pull-down ties it LOW.

The ESP32/Arduino have built-in pull-ups: pinMode(pin, INPUT_PULLUP). Then the button connects the pin to GND.

R = (V_supply − V_LED) / I_LED

• V_supply — your source voltage
• V_LED — LED forward voltage (~2 V red, ~3 V blue)
• I_LED — desired current (~10–20 mA)

Example: (5 V − 2 V) / 0.01 A = 300 Ω → use 330 Ω. Without it the LED burns out.

Wired in reverse, it conducts once voltage reaches its rated "Zener voltage", holding the node steady. A simple way to make a stable reference or protect an input from over-voltage.

Contrast with the 7805 linear regulator coming on Day 7 — Zener is cheap/simple but low-current.

• DC motor: Spins continuously; speed by voltage/PWM, direction by polarity.
• Servo: Moves to a precise ANGLE (0–180°). Has built-in control electronics.
• Stepper: Rotates in exact steps for precise positioning — printers, CNC.

A pin supplies a few mA; a motor needs hundreds. A driver IC (L298N, L293D, DRV8833) takes your small logic signal and switches the big motor current — and isolates the MCU from electrical noise.

The moving parts and the chips that drive them.

• DC motor — spins freely
• Servo (SG90) — moves to an angle
• Stepper — exact steps
• L298 driver — switches motor power
• HC-SR04 — ultrasonic distance

• Duty cycle sets average power → speed
• The two IN pins set spin direction

// ESP32 LEDC PWM on the driver's enable pin
ledcAttach(enablePin, 1000, 8);  // 1 kHz, 8-bit
ledcWrite(enablePin, 128);        // 50% speed (0–255)

// Direction via two input pins
digitalWrite(in1, HIGH);
digitalWrite(in2, LOW);           // forward

Four switches form an "H". Closing the diagonal pairs sends current one way or the other — that flips motor direction.

Warning: never turn on both switches on the same side — that shorts the supply ("shoot-through").

• write(angle) commands a position, not a speed
• Servos use a PWM pulse width to encode the angle

#include <ESP32Servo.h>
Servo s;
void setup(){ s.attach(18); }
void loop(){
  for (int a = 0; a <= 180; a++){ s.write(a); delay(15); }
  for (int a = 180; a >= 0; a--){ s.write(a); delay(15); }
}

Unlike a DC motor that spins freely, a servo rotates to a precise commanded angle and holds it.

• write(0) → write(180) sweeps the arm
• Internal electronics hold the position
• Great for steering, arms, grippers

The sensor sends an ultrasonic pulse and times how long the echo takes to bounce back off an object.

• distance = time × 0.0343 cm/µs ÷ 2
• Divide by 2 — sound makes a round trip
• pulseIn() measures the echo time

• Send a pulse, time how long the echo takes back
• Distance = (time × speed of sound) / 2

digitalWrite(trig, HIGH); delayMicroseconds(10);
digitalWrite(trig, LOW);
long us = pulseIn(echo, HIGH);     // echo travel time
float cm = us * 0.0343 / 2;        // speed of sound
Serial.println(cm);

For multiple sensors, fire them one at a time so echoes do not cross-talk.

The whole point of IR line sensing: a robot that drives itself along a track by constantly checking which sensors see the black line.

• IR sensors point down at the floor
• Line under a side sensor → steer that way
• Loop fast → smooth following

This is the classic first-robot project — the bot follows the curve on its own.

• Mount IR sensors facing the floor — usually two or more across the line
• Calibrate the threshold — read values over black AND white, pick the midpoint
• Steer from the difference — left sensor sees line → turn left, and vice versa
• Tip — consistent lighting and sensor height matter more than code

• Pair from a phone app, send characters as commands
• SerialBT works just like the USB Serial you already know

#include "BluetoothSerial.h"
BluetoothSerial SerialBT;
void setup(){ SerialBT.begin("ElectroBot"); }
void loop(){
  if (SerialBT.available()){
    char c = SerialBT.read();
    if (c == 'F') forward();
    if (c == 'S') stop();
  }
}

• Sensors convert the world into electrical signals
• Resistive sensors need a voltage divider; pull-ups stop floating inputs
• IR reflects off white, absorbs on black — the basis of line following
• Motors always need a driver, never a bare MCU pin
• PWM duty cycle = speed; an H-bridge sets direction
• Servos take an angle; HC-SR04 measures distance by echo
• ESP32 Bluetooth Serial = wireless control with familiar code