This week I focused on input devices: as a group we probed an existing motion-sensor board with an oscilloscope, and individually I connected a PIR sensor to my own board and read it from firmware. The goal was to see real analog/digital behavior on the scope, then close the loop with code running on my microcontroller.

Assignments

  • Group: probe an input device’s analog levels and digital signals.
  • Individual: measure something — add a sensor to a microcontroller board I designed, and read it.

Group assignment (1–3)

We used the oscilloscope to examine a motion-sensor board. Following the clock line helped us understand the timing; we also probed several random GPIO/test pads. Some pins showed noticeably higher switching frequencies than others, which suggested different sub-circuits (clocked logic vs. slow I/O or interrupts). It was a fast way to “reverse read” behavior without having the full schematic.

1. Probing the motion-sensor board while watching the clock line on the scope.
2. Zooming in on one channel to see the digital pulse train more clearly.
3. Comparing different GPIO/test pads and their very different switching frequencies.

Individual assignment (4–11)

For the individual part, I connected a PIR motion sensor to my own board and read it from firmware. I first checked the sensor on a milled test board, inspected solder joints, and then wired the signal to the MCU headers. After validating power and I/O levels, I logged the readings over serial and confirmed responsiveness by waving my hand in front of the sensor.

Sensor mounted on milled board
4. PIR sensor mounted on a milled breakout board for easier testing.
Breakout inspection
5. Inspecting the breakout pads and solder joints under good lighting.
Back side check
6. Back side check to make sure there are no accidental bridges.
Fixture and wiring
7. Fixture and wiring for connecting the PIR board to my microcontroller.
Bench wiring with laptop
8. Bench setup with laptop, USB serial, and the PIR sensor wired to D2.
Serial readout screenshot
9. Serial monitor output showing “motion / no motion” updates while I move my hand.
Sensor plus connector harness
10. Final wiring with a small connector harness to keep things tidy.
11. Live test: moving in front of the PIR sensor and watching the board respond.

Firmware — PIR motion test

I tested the PIR sensor with a simple sketch. I haven’t soldered the LED yet, so the program uses the Serial monitor (115200 baud) for feedback; the LED pin is already defined and will light once the LED is added. Wiring used: PIR signal → D2, PIR VCC → 3V3, PIR GND → GND. 

const int PIR = D2;
const int LED = D3;

void setup() {
  Serial.begin(115200);
  pinMode(PIR, INPUT);
  pinMode(LED, OUTPUT);
  Serial.println("PIR sensor test start...");
}

void loop() {
  int motion = digitalRead(PIR);
  if (motion == HIGH) {
    Serial.println("Motion detected!");
    digitalWrite(LED, HIGH);   // LED will work after it’s soldered
  } else {
    Serial.println("No motion");
    digitalWrite(LED, LOW);
  }
  delay(500);
}