When most people think of piezos, they think of the shiny, round contact microphones – the gold coins of experimental music! But, there are many different types of piezos, and they can be used as actuators as well as sensors. An assortment of these is shown below. Piezos are generally inexpensive, and are very sensitive to vibration. The only downsides are that they can only sense time-varying inputs, and can be a bit noisy.
Figure 1 – Example piezo materials and applications (click for larger image).
- 1. Flat piezo speaker.
- 2. Buzzer using common disc piezo (same as contact mic.).
- 3. MSI vibratab vibration sensor.
- 4. MSI mini-vibratab vibration sensor with proof mass.
- 5. MSI piezo multi-axis accelerometer IC.
- 6. Murata surface mount piezo shock sensor (airbag deployment).
- 7. Piezo crystal microphone.
- 8. Ultrasonic transducer.
- 9. Piezo igniter (barbecue grill lighter).
- 10. Piezo coaxial wire.
- 11. Large piezo sheet (can be used as a speaker).
How do piezos work?
Piezo comes from the Greek word “to squeeze”, which is the basis of how they function. Inside of a piezo material, there are electric charges which are fixed relative to the shape of the material. So, when the material is squeezed, the charges move with the material and create a voltage. This can also work in reverse, such that applying a voltage can cause the material to shrink or expand. A representative drawing of this is shown below.
Figure 2 – Piezo material being compressed and realigning internal dipole charges.
Piezos are very sensitive, producing large voltages from small amounts of movement. Here is an example of a piezo vibration sensor being used to turn an ordinary table into a percussive instrument. But, this also means you must be careful with piezos. If you’re using a piezo pick-up to sense a drum hit, the output could be in the kilovolt range! Since piezos produce so much voltage, you won’t need to power your sensor, and your sensor might even be able to power your circuit. Check out this project where a person lit up a tambourine with piezos!
Unfortunately, this high sensitivity is a liability when you use a piezo as a speaker or actuator. Since a small movement produces a large voltage, a large voltage will only create a small movement. This means you will need to apply anywhere from 20V to 100V to get a lot of sound out of a piezo. Piezo buzzer elements get around this by using a resonant cavity, which can produce very loud sounds from very little energy. But, despite their small movements, piezos can produce a great deal of force.
How do i use piezos in a circuit?
The schematic symbol and equivalent circuit for a piezo are shown below. Note that the symbol looks like some material sandwiched between 2 plates. This is how piezos are constructed. The plates are placed on either side of the material to conduct the electricity away. These parallel plates also form a capacitor, which is where the capacitor in the equivalent circuit comes from.
Figure 3 – Piezo schematic symbol (left), and equivalent circuit (right).
This capacitance is the main thing that makes piezos a bit tricky to work with. Since capacitors can not pass a DC voltage, piezos can only be used with time-varying signals. This means you can not use them for controllers where you would like to hold a value. If you press on a piezo, you will get a voltage spike, but it will just decay away, even if you keep pressing on it. Fortunately, all sound is time-varying, so they make for good microphones.
When using piezos in a circuit, there are 2 things to watch out for. First, make sure you are using a high-impedance amplifier, and second, be sure to protect your amplifier. The source capacitance of a piezo element can vary from 500pF to 20nF, and creates a high-pass filter with the input impedance of your amplifier. So, if you want response down to 20Hz, you’ll need an input impedance from 15Mohms to 400kohms (Fc = 1/(2*pi*R*C)). If you use too small of an input impedance, only high frequencies will get through, which partially explains the ‘tinny’ sound that most people experience with piezo contact mic’s.
This high impedance makes piezos a bit noisy, as the intrinsic noise of a 15Mohm resistor is already 70uVrms over the audible range. This sets the lower threshold of sensitivity. Fortunately, piezos generate large voltage swings, so this is usually not a problem. What is more difficult to deal with, is that the high impedance makes the circuit very prone to picking up 60Hz hum and other interference, so good shielding is essential. Use short cable runs, be sure to shield both the cable and the element, and use a differential amp if necessary.
Figure 4 – Example piezo circuits with high-impedance input, and input protection.
Examples of good amplifier circuits are shown above. You will note the 10kohm resistors in series with the amplifier inputs. These limit the in-rush of current in case of large voltage spikes, and help protect the amplifiers. The diodes on the op-amp clamp the voltage to the supply rails. The JFET has an internal diode, as do some op-amps, so external diodes are not strictly necessary. Although the voltage produced by a piezo can be quite large, the current is extremely small, so the 10kohm resistor is usually enough to protect your circuit.
Which piezo should i use?
Piezos generally come in two types of materials, a ceramic or a polymer. The ceramics have a higher ‘Q’ (i.e. are more self resonant and efficient) and are less expensive. These are used in barbecue grill lighters where high energy transfer is required, and in the gold coin contact mic’s and buzzers, where cost is the main concern. The high ‘Q’ gives both mics and speakers made of this material a very uneven frequency response. The polymer film piezos, by contrast, have a very flat frequency response, but are not as efficient, so they have a much lower output voltage.
The other thing that gives the gold coin contact mic its ‘tinny’ sound, is the metal electrodes it’s made of. These discs have a resonant frequency of their own, and constrain the piezo to move in a certain way. In contrast, the polymer film piezos are usually bonded to a thin plastic layer, which moves easily and can be mounted on curved surfaces. If you want to test the response of your piezo, connect it up to a headphone amp, and give it a quick tap. Most ceramic piezos will have multiple frequency peaks in the low kilohertz range, where as the film piezos will have one dominant frequency around 40Hz.
For applications where flat frequency response isn’t required, the ceramic piezo discs are a good choice, as they are less expensive, produce a larger signal, and are shielded better due to the larger disc on one side (this side should always be connected to ground). They also have a higher capacitance, allowing for lower impedance, and therefore lower noise, amplifiers. The piezo films (most notably from MSI), on the other hand, have a flatter frequency response, are more durable, and are flexible, so they can conform to odd surfaces. But, they are also have more noise.
Another drawback to most polymer film piezos, is that they use metal staples (see Figure 1, #3) for the electrical contacts. The problem with these staples, is that it can be difficult to solder to them without melting the plastic and hurting the electrical contact within. If you do solder to the contacts, be sure to have the wire you are soldering to pre-tinned, and only heat long enough to melt the solder and make a good joint. Other options for connecting a piezo include screw terminals or SIP sockets. These have the advantage of making sensor replacement quick and easy at the expense of a less robust connection.
Piezos can make great pick-ups, accelerometers, and shock sensors. If you are interested in learning more about piezos, check out the Piezo Film Guide from MSI.