Strange analogRead() behavior on ESP32 with vibration sensor

Other Hardware Sensors
1

I'm seeing a behavior I don't understand using an Adafruit fast vibration sensor ( Fast Vibration Sensor Switch (Easy to trigger) : ID 1766 : $0.95 : Adafruit Industries, Unique & fun DIY electronics and kits ) wired to an analog input pin on an ESP32. When I use a delay of 10ms between reads, I get a stream of unexpected zero or near-zero readings. If I up the delay to 20ms, everything is fine. I've tried this on several input pins (25, 26, 27) with the same result. When I run the same code on a Nano, the extra delay is not needed. Can someone give me some insight into what might be happening here? Thanks!

#define VIBRATION_PIN 27    // For ESP32

void setup() {
  Serial.begin(9600);
  delay(1000);
  Serial.println("Test vibration sensor");
}

void loop() {
  int val = analogRead(VIBRATION_PIN);
  if (val < 15)  // threshold determined through trial and error
  {
    Serial.println(val);
    delay(300);           // prevent re-triggers
  }
  delay(10);
}
2

Why do you read that sensor with an analogue pin.
The sensor is just a switch.
Leo..

3

Use a pullup resistor with a switch, otherwise the input is floating.

4

Ohhhhh ... that makes perfect sense! So, I treat it as a button with appropriate debouncing code. Duh! Thanks very much!

5

No debouncing. The code should react instantly to a closure of the switch.
If you debounce, you might miss the short activation of these switches.
Leo..

6

Perhaps we're talking about different usage scenarios. I'm wanting to get the initial impulse when the vibration sensor is shaken or moved. The sensor will be attached to a plastic cable, which is to be plucked like a guitar string - I'm looking for that pluck event, and wanting to avoid retriggering, hence I'm thinking debouncing. Certainly if I wanted to sense vibration rate or something of that sort, I would look for those short activations you're talking about. Hope that makes sense.

7

Debouncing is a delayed action or no action at all after an event, which is the opposite of what you describe. What you want is a lock-out time after an instant event.
Untested example code.
Leo..

const int sensorPin = 27;
unsigned long prevMillis, interval = 300; // lockout time in ms

void setup() {
  Serial.begin(115200);
  pinMode(sensorPin, INPUT_PULLUP); // sensor between pin and ground
  Serial.println("Test vibration sensor");
}

void loop() {
  if (millis() - prevMillis > interval) {
    if (!digitalRead(sensorPin)) { // if LOW
      Serial.println("Bingo!");
      prevMillis = millis(); // reset
    }
  }
}
1 Like
8

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