A buzzer or beeper is an audio signaling device. It generates sound in a frequency range of 1 to 7 kHz as an audio indication. In this frequency range, the hearing threshold is maximum. Therefore, the sound of a buzzer is so piercing that it is noticeable even in a highly noisy environment. The buzzers/beepers are generally used as sound alarms. In many applications, they are used to give an audio indication in response to some action or event. A buzzer can be used to produce the sound of a click, beep, or ringing.
Types of buzzers
Buzzers come in a variety of construction, size, and specifications. Different types and sizes of buzzers are used for different applications. Based on construction, there are the following kinds of buzzers:
- Piezoelectric buzzers
- Magnetic buzzers
- Electromagnetic buzzers
- Mechanical buzzers
- Electromechanical buzzers
Piezoelectric and magnetic buzzers are most commonly used in electronic applications. The buzzers are designed to be used as a transducer or indicator in any circuit.
Buzzer as transducer vs. indicator
Buzzers/Beepers are designed to operate as either transducer or indicator in electronic circuits. Buzzers t designed as transducers do not have any built-in driving circuit. When such buzzers are interfaced in a circuit, they require a square wave input for their operation. There needs an external driving circuit that feeds square wave input to the buzzer. The advantage of transducer buzzers is that they can be driven to produce different frequencies as per the requirement of the application. However, this adds more complexity to the design-in and explicitly requires designing an external driving circuit for the buzzer.
The buzzers designed as indicators have a built-in driving circuit to produce a fixed frequency or tone. Such buzzers only need a source current for activation. The indicator buzzers can be driven by simply applying a DC voltage to them. Often, these buzzers can be actuated via digital I/O of a controller/computer with or without supporting circuitry. These types of buzzers are used most often. As they usually do not require any complex external circuitry, they are easy and handy to use.
Both indicator and transducer buzzers can be used to produce continuous tones and slow/fast pulse sounds. For producing a constant tone, a transducer buzzer needs to supply a continuous fixed frequency square wave, while an indicator buzzer needs to supply continuous DC voltage. For producing a slow/fast pulse sound, a transducer buzzer needs to supply square wave pulses of fixed frequency. In contrast, indicator buzzers need to be alternatively switched on and off like by applying a PWM signal.
The high/low tones, sounds of a siren, or chime can be produced only using a transducer buzzer. High and low tones can be generated by rapidly alternating square wave signal to a transducer buzzer between two frequencies. The sound of a siren can be produced with a transducer buzzer by periodically ramping square wave frequencies to a transducer buzzer from low to high. The sound of a chime (like a doorbell) can be produced by a single slow cycle of high and low square wave frequency to a transducer buzzer.
A piezoelectric buzzer operates on the principle of the piezoelectric effect. The main component of a piezoelectric buzzer is a piezoelectric element. The element is composed of a piezoelectric ceramic and a metal plate. Both the piezoelectric disc and metal plate are held together with an adhesive. The piezoceramic disc has electrodes attached to it. The piezoelectric disc expands and contracts diametrically when an alternating current is applied to it. This produces vibrations in the piezoelectric element and generating the sound of a particular frequency or range of frequencies.
The piezoelectric element is supplied alternating current from an oscillator circuit. In indicator-type piezo buzzers, the oscillator circuit is built-in to produce a fixed frequency or range of frequencies. In transducer type piezo buzzers, an external oscillator circuit is required. This oscillator circuit is usually a square wave generator.
Many piezo buzzers have a feedback line. In such buzzers, the piezoelectric element is divided into two electrically isolated parts. When the main piezo element is actuated, it squeezes the feedback component, producing a feedback voltage. The feedback signal is usually applied to a transistor/OP-AMP circuit, which blocks or amplifies the current supply to the piezoelectric element.
The piezo buzzers have a wide operating voltage that ranges from 3V to 250V. Most of the piezo buzzers used in electronic circuits have an operating voltage between 3V and 12V. These buzzers have high sound pressure levels. They have very low current consumption. Higher is the frequency/tone of the piezo buzzer; the lower is its current consumption. The buzzers used in electronic applications have current consumption as low as 30 mA. The piezo buzzers have a large footprint and are preferred to be used in cost-sensitive electronic applications.
In a magnetic buzzer, there is a ferromagnetic disc that is attached to a pole. There are magnets around the pole which maintains the disc in a rest position. There is a coil below the ferromagnetic disc that acts as an electromagnet. When current is supplied to the coil, the disc is attracted to the coil. When there is no current in the coil, the disc returns to its rest position. A weight above it controls the vibrations of the disc. When an oscillating signal is applied to the coil, the electromagnetic field generated from it also fluctuates, causing vibrations in the ferromagnetic disk. This way, the sound of the frequency same as the frequency of the applied oscillating signal is produced by the magnetic buzzer. Magnetic buzzers come in both transducer as well as indicator configurations.
The magnetic buzzers have a narrow operating voltage that ranges from 1V to 16V. These buzzers produce lower-rated frequencies have low sound pressure levels. These have a slightly greater current consumption in comparison to piezo buzzers. Their current consumption can be as high as 100 mA. These buzzers have a small footprint and are generally used in high-end consumer applications.
A buzzer has the following important specifications:
Type – A buzzer can be a transducer or indicator type. Both kinds of buzzers have different capabilities and applications.
Operating voltage – It is important to note the operating voltage before using it in an application. The piezo buzzers have a wide range of operating voltage (3~250V), while magnetic buzzers have a narrow operating voltage (1~16V). The large buzzers usually have higher operating voltages.
Sound Pressure Level (SPL) – Sound pressure level (SPL) is the deviation from atmospheric pressure caused by the sound wave. It is expressed in decibel Pascal. SPL rating is most useful for comparing two audio output devices. It indicates how loud the sound the device produces. SPL is generally proportional to the input voltage (operating voltage of the buzzer) and usually decays by 6 dB on doubling the distance from the buzzer.
Decibel is not a unit but a ratio. A value expressed in decibels increases exponentially rather than linearly when counting. For example, 20 dB is ten times the power of 10 dB. The use of decibel allows expressing a considerable range of values by a relatively small space. The sound pressure levels to which human ears are sensitive can range from 20 uPa to 20×1012 uPa. This way, SPL levels can be from 0 to 120 Db Pa. Most common buzzers have SPL of 80 dB, 85 dB, 90 dB, and 95 dB. Some buzzers have sound pressure levels as high as 105 dB.
Resonant Frequency – The resonant frequency is the frequency at which a device tends to vibrate. For buzzers, this is the frequency at which they have the highest SPL. Buzzers produce the loudest sounds consuming the least input power when they vibrate at their resonant frequency. The indicator buzzers are usually designed to operate at/around their resonant frequency only.
Impedance – The impedance is the ratio of applied voltage to the current. The impedance of a buzzer fluctuates with the frequency.
Frequency Response – For different frequencies, a buzzer has different SPL values. Therefore, a buzzer produces different frequencies of sound at different loudness. A graphical representation of a buzzer’s SPL for various frequencies is called its SPL – frequency response, or simply, frequency response. The frequency response is plotted in the entire audible range of the human ear (20 Hz to 20,000 Hz). The frequency response is an essential factor when transducer buzzers are used in an application. The graph is extremely useful in designing the driving circuit for transducer buzzers.
Applications of buzzers
The buzzers can produce clicking, beeping, or ringing sounds. They are also used to create siren or chime sounds. The buzzers are used for alarms/warnings or as sound indicators. They are used in alarm devices (like alarm clocks, fire alarms, intruder alarms, etc.), timers, input devices (like mouse and keyboards), electronic metronomes, annunciator panels, and many consumer electronic appliances. The buzzers are also used in sports events and game shows. Different sizes and types of buzzers suit various applications. How a buzzer is electronically actuated depends upon its type, configuration, operating voltage, and other specifications.