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For the group assignment, we used an oscilloscope and a multimeter to measure and compare how different sensors behave electrically — specifically looking at the difference between a digital signal (PIR sensor) and an analog signal (LDR sensor).

A PIR (Passive Infrared) sensor detects motion by measuring changes in infrared radiation from objects such as the human body. Unlike an LDR, a PIR sensor provides a digital output:

This means the signal is not continuous — it switches abruptly between two states.

Explanation

A PIR sensor detects sudden changes in infrared radiation in its environment. When movement occurs, the sensor output changes abruptly from LOW to HIGH. Because of this behavior, the output signal is digital, meaning it only has two possible values (0 or 1), rather than a continuous range.

Oscilloscope Observations

When no motion is detected, the oscilloscope shows a constant low signal. When motion is detected, the signal suddenly changes to a high value, creating a sharp transition. These abrupt changes appear as sudden jumps in the signal rather than smooth variations — resulting in noticeable pulses as the signal switches quickly between LOW and HIGH states.

An LDR changes its resistance continuously depending on the amount of light it receives:

Because of this, when used in a voltage divider circuit, the output signal is analog — a variable value, not just 0 or 1.

Explanation

An LDR changes its resistance continuously depending on the amount of light. When used in a circuit such as a voltage divider, the output signal is analog — a variable value rather than a binary one. This allows us to detect not just presence/absence of light, but precise intensity levels.

Oscilloscope Observations

In the first observation, the LDR is connected in the circuit. The oscilloscope shows a signal with approximately one division of voltage — a constant line indicating stable ambient light conditions. The sensor is receiving room light and producing a steady output signal.

When the sensor is covered, the signal changes. The frequency (Hz) decreases, and if partially covered with a finger, fluctuating waves become visible on the oscilloscope. These waves vary within a certain range depending on how much light is blocked.

For this week I wanted to put my custom PCB — designed in Week 8: Electronics Production — to the test. I intentionally designed my board with male header extensions for 5V, 3.3V, digital pins, and analog pins so that external sensors could be connected easily.

My plan was to test two sensors that will help with my final project: the HC-SR04 ultrasonic sensor and the PIR sensor. Both will be used to manage the part of my dispenser that detects hand motion to trigger the dispensing mechanism.

At university we had used these sensors with an Arduino Uno and a breadboard — but this time it would be different. Everything would run exclusively through my custom PCB.

I reviewed my notes from the Physics course I took at Universidad del Pacífico to explain the theory and operation of this sensor.

The HC-SR04 detects objects and measures their distance using ultrasound — essentially a sound too high-pitched for humans to hear.

Everything comes down to one simple idea: sound travels at a known speed, so measuring time gives you distance.

Since the pulse travels to the object and back, divide by 2:

Operation Sequence

1. Send a 10 µs HIGH pulse to TRIG.
2. Sensor fires 8 ultrasonic bursts at 40 kHz.
3. ECHO goes HIGH — the clock starts.
4. Reflection arrives → ECHO goes LOW — the clock stops.
5. Apply: Distance (cm) = Time (µs) / 58.

Pinout

The code defines three pins: TRIG (D9), ECHO (D10), and an LED (D8). In setup, all pins are configured and the LED starts off. In the loop, the TRIG pin sends a 10µs ultrasonic pulse, and the ECHO pin measures how long it takes to bounce back. That time is converted to centimeters using the formula (duration × 0.034) / 2. If the distance is under 10 cm, the LED turns on — otherwise it stays off.

const int TRIG = D9;
const int ECHO = D10;
const int MY_LED = 8;

void setup() {
  pinMode(TRIG, OUTPUT);
  pinMode(ECHO, INPUT);
  pinMode(MY_LED, OUTPUT);
  digitalWrite(MY_LED, LOW); // LED off at start
}

void loop() {
  digitalWrite(TRIG, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG, LOW);

  long duration = pulseIn(ECHO, HIGH);
  float distanceCM = (duration * 0.034) / 2;

  if (distanceCM < 10) {
    digitalWrite(MY_LED, HIGH); // Hand close — LED ON
  } else {
    digitalWrite(MY_LED, LOW);  // No hand — LED OFF
  }
  delay(100);
}

For this sensor, I also reviewed my lab notes from the Physics course at Universidad del Pacífico to explain its theory and operation.

The PIR sensor detects motion by reading the infrared radiation naturally emitted by living beings. When a person enters its field of view, their body heat causes a change in the detected radiation — and that's what triggers the sensor. Field of view: 100°. It only needs 1 digital pin.

The sensor has two small knobs that let you tune its behavior without touching a single line of code.

First I connected the PIR sensor output to D9 and the LED to D8. The code reads the PIR signal — if motion is detected the pin goes HIGH and the LED turns on, otherwise it stays off.

const int PIR = D9;
const int MY_LED = 8;

void setup() {
  pinMode(PIR, INPUT);
  pinMode(MY_LED, OUTPUT);
  digitalWrite(MY_LED, LOW); // LED off at start
}

void loop() {
  int motion = digitalRead(PIR);

  if (motion == HIGH) {
    digitalWrite(MY_LED, HIGH); // Motion detected — LED ON
  } else {
    digitalWrite(MY_LED, LOW);  // No motion — LED OFF
  }
  delay(100);
}