9. Input Devices

This week I interfaced an HC-SR04 ultrasonic distance sensor to my Week 8 XIAO ESP32-C3 board and measured object distance in centimeters. I also participated in the group assignment to probe input signals with an oscilloscope on our populated PCBs, and individually practiced reading an NTC temperature sensor on a breadboard.

Group Assignment

As a group we used the lab oscilloscope (OWON EDS102 CV) to probe analog levels and digital waveforms on the ESP32 carrier PCBs we designed in Week 6 and fabricated in Week 8. We measured servo PWM pulses and discussed how the same probing method applies to buttons, ultrasonic echo pulses, and other inputs.

Week 9 Group Assignment — Input Devices (Chaihuo Fab Lab)

Oscilloscope — Instrument Parts & Setup

In the lab we used an OWON EDS102CV (100 MHz, 1 GS/s) digital oscilloscope to probe signals on breadboard circuits and populated PCBs. Before measuring a real input, we walked through the main parts of the front panel and how they map to what appears on screen.

Scope capture: CH1 at 2 V/div, timebase 200 ns — Measure panel shows frequency and V_max/V_min.

Front panel layout: vertical (CH1/CH2), horizontal (timebase), trigger, and BNC inputs.

Key takeaway: the oscilloscope shows how a signal changes over time, while a multimeter reports a single averaged value. For embedded debugging, I use the scope when I need to see noise, pulse width, or whether a pin is actually toggling — not just whether the voltage "looks right" on a meter.

What I learned from the group work

Individual Learn — NTC Temperature Sensor

During the same group session I individually practiced reading an NTC thermistor module (labeled NTC R1 on the breakout board) wired to a Seeed Studio XIAO on a breadboard. The module exposes SIG, NC, VCC, and GND — the thermistor forms a voltage divider whose output changes with temperature.

I verified the circuit in three steps: confirm supply voltage with the multimeter, probe the analog output with the oscilloscope to see whether the line was stable or noisy, then compare readings as I moved the sensor between my fingers and open air.

Breadboard setup: XIAO board, NTC R1 module (SIG / VCC / GND), and jumper wiring to the analog input.

Multimeter reads 16.8 (mV scale) on the sensor node while the scope probe is attached.

Following the lab probe guide on the laptop — checking coupling and termination before trusting the trace.

Supply / divider check: multimeter reads 3.27 V on the sensor output pin at room temperature.

What I learned: an NTC sensor does not output a fixed voltage — the divider ratio shifts with temperature, so firmware must either map voltage to °C with a lookup table / Steinhart–Hart equation, or calibrate against a known reference. Measuring with both tools first (meter for DC level, scope for stability) saved me from chasing a software bug when the real issue was a loose GND clip.

Individual Assignment — Measure with a Sensor

Goal: connect an input device to my microcontroller board, read it in firmware, and document the measured values.

I used an HC-SR04 ultrasonic sensor to measure distance to an object. The module sends a 40 kHz sound burst and listens for the echo. The echo pulse width is proportional to round-trip time; dividing by two and scaling by the speed of sound gives distance in centimeters.

Input test setup — HC-SR04 on 5V, GND, D1 (Trig), and D2 (Echo).

Sensor overview

HC-SR04 module — four pins: VCC, Trig, Echo, GND.

Board and wiring

The carrier PCB was designed in KiCad and manufactured in Week 8 — full documentation is on the Week 8 page. I wired the HC-SR04 to the XIAO header pins:

I chose D1 and D2 to avoid the on-board button (D7) and LEDs (D8/D9). The echo line outputs 5 V logic; on this test it worked directly on the ESP32-C3 GPIO, but a resistor voltage divider is safer for long-term use.

HC-SR04 module — four pins: VCC, Trig, Echo, GND.

XIAO ESP32-C3 pinout — D1 and D2 used for Trig and Echo.

How the Code Works

Firmware: UltrasonicRead.ino

1. Setup: starts USB serial at 115200 baud; configures D1 as output (Trig) and D2 as input (Echo).
2. Trigger: pulls Trig LOW, waits 2 µs, sends a 10 µs HIGH pulse, then returns Trig LOW.
3. Measure: uses pulseIn(echoPin, HIGH, PULSE_TIMEOUT_US) to capture the echo pulse width in microseconds (30 ms timeout ≈ 5 m range).
4. Convert: applies duration × 0.034 / 2 to get one-way distance in cm.
5. Average: takes SAMPLES (5) readings per loop and prints the mean to reduce noise.

/*
 * UltrasonicRead.ino — XIAO ESP32-C3 + HC-SR04
 * Trig -> D1, Echo -> D2
 */

const int trigPin = D1;
const int echoPin = D2;

const unsigned long PULSE_TIMEOUT_US = 30000;  // ~5 m max range
const int SAMPLES = 5;

float readDistanceCm() {
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);

  unsigned long duration = pulseIn(echoPin, HIGH, PULSE_TIMEOUT_US);
  if (duration == 0) return -1.0;
  return duration * 0.034 / 2.0;
}

void setup() {
  Serial.begin(115200);
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
}

void loop() {
  float sum = 0;
  int valid = 0;
  for (int i = 0; i < SAMPLES; i++) {
    float d = readDistanceCm();
    if (d > 0) { sum += d; valid++; }
    delay(10);
  }
  if (valid == 0) {
    Serial.println("Out of range");
  } else {
    Serial.print("Distance: ");
    Serial.print(sum / valid, 1);
    Serial.println(" cm");
  }
  delay(200);
}

Upload settings

• Board: XIAO_ESP32C3
• Enable USB CDC On Boot
• Serial Monitor: 115200 baud

Measured results

With the sensor face 10–30 cm from a flat wall, Serial Monitor printed stable lines such as Distance: 18.4 cm — each value is the mean of five samples. Moving the target closer or farther updated the reading on the next loop (~200 ms). Targets beyond ~400 cm, or surfaces too soft to reflect ultrasound, returned Out of range.

Oscilloscope — Trig and Echo Pulses

I probed the HC-SR04 on the oscilloscope while the firmware was running. The red trace (CH1) shows the Trig pulse — a short HIGH burst sent by the MCU to start each measurement. The yellow trace (CH2) shows the Echo line going HIGH for a longer interval; that pulse width is the round-trip time the firmware reads with pulseIn() and converts to distance.

Seeing both signals on the same timebase made the timing concrete: a narrow trig spike, then a wider echo pulse whose length grows when I moved the target farther away. That confirmed the Serial readings were tracking real ultrasound timing, not a software artifact.

Scope capture on the XIAO carrier board — red (CH1) = Trig pulse, yellow (CH2) = Echo duration. Echo pulse width ∝ round-trip distance.

Problems & Fixes

What I Learned

• How an ultrasonic sensor measures distance via round-trip echo time, not direct voltage.
• How to generate a precise trigger pulse and capture echo width with pulseIn().
• How to convert pulse duration to physical distance using the speed of sound.
• Why averaging multiple samples reduces noise in time-of-flight measurements.
• From group scope work: an echo pulse is a timed digital input — the scope shows width, firmware converts it to distance.

Design Files & Source Code

Hero Shot

My populated XIAO ESP32-C3 carrier board from Week 8, now used as the platform for this week's ultrasonic distance measurement.

HelloWorld board with HC-SR04 wired to 5V, GND, D1 (Trig), and D2 (Echo) — distance readout on Serial Monitor.