Build an Arduino shake alarm: catch the jolt, latch it

To build an Arduino shake alarm, read an accelerometer, average a few readings to tame the noise, and compare each average with the last one. A jump bigger than a threshold is a shake: set an alarm flag that stays set until a reset button clears it. A slow tilt changes the reading too, but only a little each pass, so it never trips.

Parts and wiring

• Accelerometer on a parcel, its analog output on A2: noisy at rest, a jump when shaken, a slow drift when tilted
• Alarm lamp or buzzer on pin 9
• RESET push-button from pin 2 to GND, with INPUT_PULLUP

The plan

1. Tame the noise: average 4 readings each pass.
2. Catch the shake: if this average differs from the last by more than 100, raise the alarm and say shake once.
3. Remember it: a bool alarm, set by the shake and cleared only by RESET.
4. Show it: write the lamp from alarm every pass, with only a short delay(10) so no shake is missed.

The code

int smooth() {
  long sum = 0;
  for (int i = 0; i < 4; i++) sum = sum + analogRead(A2);
  return sum / 4;
}

void loop() {
  int reading = smooth();
  if (abs(reading - last) > 100) {
    if (!alarm) Serial.println("shake");   // say it once
    alarm = true;
  }
  last = reading;
  if (digitalRead(2) == LOW) alarm = false; // RESET
  digitalWrite(9, alarm ? HIGH : LOW);
  delay(10);
}

Set last = smooth() in setup() too, or the first pass compares a real reading with 0 and raises a false alarm.

Live experiment

Test the alarm first

A second at rest, a shake at 1 s, RESET at 2.5 s, then a slow tilt. Pick the line on the jump and whether the alarm remembers. A line at 25 trips on the noise, 450 misses the shake's jump of about 412, and an alarm that only follows the jump is off a pass later. 100 with a latch passes all four checks.

Try it yourself: free lesson

“Project: Shake alarm” is the project that ends Signal Sands, Sensors & Signal Processing World 1. Free with an account. Lessons unlock in order, so you’ll do the 6 before it first.

Why each piece is there

• The average shrinks the noise, so the delta at rest stays around 30 at most. Without it, single readings wobble enough that a sensible line would trip.
• The delta ignores where the reading is and looks only at how fast it moved. A shake jumps about 400 in one pass; a slow tilt moves it about 20 at a time.
• The latch keeps the alarm on. The jump lasts one sample, so an alarm that just follows it would flash for 10 ms and vanish.

Testing it

Leave it still for a second: no alarm. Shake it: alarm on, and still on a second later. Press RESET: off. Tilt it slowly: no alarm. Those four checks cover the noise, the jump, the latch and the drift.