Piezo Vibration Sensor

Working Principle

A piezoelectric vibration sensor (often a PZT ceramic disc or PVDF film) generates a voltage when mechanically deformed — the direct piezoelectric effect. When vibration, shock, or flex causes the element to bend, the crystal lattice distortion produces a charge proportional to the applied strain. This charge appears as a voltage across the element’s electrodes.

Piezo sensors are AC-coupled by nature — they respond to changes in force, not static loads. A constant pressing force produces a transient voltage spike that decays. This makes them ideal for detecting vibration, taps, knocks, and impacts.

Electrical Characteristics

Parameter	Typical (PZT Disc)
Output Type	AC voltage (proportional to vibration)
Open-Circuit Voltage	Up to ±50 V (sharp impacts)
Frequency Response	1 Hz – 10 kHz+
Sensitivity	Varies by geometry; ~1–20 mV/g typical
Impedance	Very high (>1 MΩ at low frequency)
Static Force Response	None (AC-coupled, transient only)
Temperature Range	−20 to +70 °C (PZT), −40 to +80 °C (PVDF)
Cost	Very low (~$0.10–$0.50)

Interfacing with an MCU

Connect one electrode to an ADC input and the other to GND. Critical: Clamp the voltage to protect the ADC:

• Place a 1 MΩ resistor in parallel to bleed charge and prevent floating voltages
• Add two Schottky diodes (BAT54S) from signal to VCC and GND to clamp spikes
• Alternatively, use a 5.1 V Zener diode for simple overvoltage protection

Calibration

• Threshold tuning: Tap the sensor at your desired trigger strength and note the ADC peak; set threshold slightly below
• Debouncing: After detecting a hit, ignore readings for 20–50 ms to avoid multi-triggering from ringing
• Velocity sensitivity (MIDI): Map the peak ADC value to a 0–127 MIDI velocity range for musical applications

Code Example

/*
 * Piezo Vibration / Knock Sensor — Arduino
 * Wiring: Piezo(+) → A0, Piezo(-) → GND
 *         1M resistor A0–GND, Zener 5.1V A0–GND
 */
#define PIEZO_PIN A0
#define THRESHOLD 50      /* ADC threshold for knock detect */
#define DEBOUNCE_MS 50    /* Ignore ringing after hit */

unsigned long lastKnock = 0;

void setup() {
    Serial.begin(9600);
    Serial.println("Piezo Knock Sensor Ready");
}

void loop() {
    int val = analogRead(PIEZO_PIN);

    if (val > THRESHOLD &&
        (millis() - lastKnock) > DEBOUNCE_MS) {
        lastKnock = millis();
        Serial.print("KNOCK detected! Intensity: ");
        Serial.println(val);
    }
    delay(1);  /* Fast polling for peak capture */
}

Real-World Applications

Limitations

• No static measurement: Cannot measure constant force or weight (use FSR or load cell instead).
• High output impedance: Requires high-impedance input; loading the sensor reduces output voltage.
• Voltage spikes: Can produce dangerous voltages without clamping protection on MCU pins.
• Frequency dependent: Sensitivity varies with frequency — not a calibrated accelerometer.
• Fragile: PZT ceramic discs crack under extreme bending; PVDF film is more flexible.