Piezo
A piezo element is a passive bidirectional transducer that converts electrical voltage into mechanical strain (and vice versa). Driven with an AC signal, the element flexes a metal diaphragm that radiates sound; squeezed mechanically, it generates voltage proportional to applied force — useful as a contact mic, vibration sensor, or impact detector.
A thin disc of polarised piezoceramic (typically lead zirconate titanate, PZT) is bonded to a brass or stainless backing diaphragm. Voltage across the ceramic causes it to expand or contract along its polarisation axis; bonded to the brass, the ceramic forces the disc to bend (the bimorph effect). Drive the element at the disc's resonant frequency for maximum acoustic output.
In plain terms
A salt-cracker that bends when you push voltage on it; bend it back and forth fast and it beeps.
Why designers use it
- Passive sound output where you want full control of frequency (musical tones).
- Contact microphones for guitars and percussion.
- Vibration sensors and knock sensors in engines.
Best for
- Music tones
- Knock sensors
- Contact mics
Key specifications
- Voltage: 3 – 30 Vp-p typ.
- Current: < 5 mA
- Resonant frequency: 1 – 6 kHz
- SPL at resonance: 75 – 95 dB / 30 cm
- Disc diameter: 12 – 35 mm
When not to use it
- Where ambient noise dominates — a haptic buzz may be a better cue.
Common mistakes
- Treating an externally driven sounder like a buzzer with its own oscillator.
- Connecting a speaker load directly to a logic output without checking the required driver and load limits.
- Assuming a microphone's output format, bias or pinout from its appearance.
Where you will find it
- A musical greeting-card melody plays through a passive piezo disc driven by a small microcontroller and a step-up driver — the resonant disc gives loud audible volume from a button-cell battery.
- A car engine's knock sensor is a piezo element bolted to the cylinder block; engine knock vibrations generate a few millivolts that the ECU monitors to retard ignition timing.
- Acoustic-guitar pickups often use a piezo element under the bridge saddle; mechanical string vibration generates a high-impedance audio signal that a buffer amplifier conditions.
A short history
The piezoelectric effect was discovered by Pierre and Jacques Curie in 1880, who showed that quartz crystals generated voltage under pressure. The reverse effect (electrostriction — quartz changing shape under voltage) was predicted by Gabriel Lippmann in 1881 and verified by the Curies. Piezo transducers became practical in WWI for sonar (Paul Langevin, 1917) and went mass-market with the discovery of synthetic piezoceramic materials — first barium titanate (1947) and then PZT (1952), which delivered 100× more strain than quartz.
Good to know
- A piezo disc has practically infinite input impedance — it presents almost a pure capacitance to the driver, which means it draws nearly no current at low frequencies.
- The Curie brothers' 1880 piezo demonstration helped establish modern crystallography and earned Pierre Curie's name in two more areas of physics.
- Quartz watches use the inverse piezo effect to make a tuning-fork crystal vibrate at exactly 32,768 Hz — 2¹⁵ ticks per second, easy to divide down to 1 Hz.