A buzzer or chime 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 doorbell is so penetrating that it is noticeable even in a very noisy environment. Buzzers/beeps are generally used as audible alarms. In many applications they are used to provide an audible indication in response to some action or event. A doorbell can be used to produce the sound of a click, beep, or ring.
Types of Doorbells
Doorbells come in a variety of constructions, sizes and specifications. Different types and sizes of doorbells are used for different applications. Based on construction, there are the following types of doorbells:
- Piezoelectric Buzzers
- Magnetic Doorbells
- Electromagnetic doorbells
- Mechanical doorbells
- Electromechanical doorbells
Piezoelectric and magnetic buzzers are most commonly used in electronic applications. Buzzers are designed to be used as a transducer or indicator in any circuit.
Doorbell as Transducer vs. indicator
Buzzers/Beepers are designed to operate as a transducer or indicator in electronic circuits. Doorbells designed as transducers do not have any built-in trigger circuit. When these doorbells are interconnected in a circuit, they require a square wave input for their operation. An external drive circuit is required that feeds the square wave input to the doorbell. The advantage of transducer bells is that they can be activated to produce different frequencies according to the needs of the application. However, this adds more complexity to the design and explicitly requires designing an external drive circuit for the doorbell.
Buzzers designed as indicators have an integrated driver circuit to produce a fixed frequency or tone. Such bells only require a current source for activation. The indicator buzzers can be activated by simply applying a DC voltage to them. Often these buzzers can be driven via digital I/O from a controller/computer with or without supporting circuitry. These types of doorbells are used most frequently. As they generally do not require any complex external circuitry, they are easy and practical to use.
The indicator and transducer buzzers can be used to produce continuous tones and slow/fast pulse sounds. To produce a constant tone, a transducer buzzer needs to provide a continuous square wave of fixed frequency, while an indicator buzzer needs to provide continuous DC voltage. To produce a slow/fast pulse sound, the transducer buzzer needs to deliver square wave pulses of fixed frequency. In contrast, indicator buzzers need to be switched on and off alternatively, such as by applying a PWM signal.
High/low tones, siren or buzzer sounds can be produced just by using a buzzer transducer. High and low tones can be generated by quickly switching the square wave signal to a transducer buzzer between two frequencies. The sound of a siren can be produced with a transducer buzzer by periodically increasing the square wave frequencies for a transducer buzzer from low to high. The sound of a bell (like a doorbell) can be produced by a single slow cycle of high and low square wave frequency for a transducer bell.
Piezoelectric Buzzers
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 the metal plate are held together by an adhesive. The piezoceramic disc has electrodes attached to it. The piezoelectric disk expands and contracts diametrically when an alternating current is applied to it. This produces vibrations in the piezoelectric element and generates sound of a certain frequency or range of frequencies.
Piezoelectric Buzzer Working Principle
The piezoelectric element is supplied with alternating current from an oscillator circuit. In indicator-type piezo buzzers, the oscillator circuit is integrated to produce a fixed frequency or range of frequencies. In transducer type piezo buzzers, an external oscillator circuit is required. This oscillator circuit is generally a square wave generator.
Many piezoelectric buzzers have a feedback line. In these buzzers, the piezoelectric element is divided into two electrically isolated parts. When the main piezo element is actuated, it compresses 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.
Piezoelectric buzzers have a wide operating voltage ranging from 3V to 250V. Most piezoelectric buzzers used in electronic circuits have an operating voltage between 3V and 12V. These doorbells have high sound pressure levels. They have very low current consumption. Higher is the frequency/tone of the piezoelectric buzzer; the lower your current consumption is. Buzzers used in electronic applications have current consumption as low as 30 mA. Piezoelectric buzzers occupy a large area and are preferred for use in cost-sensitive electronic applications.
Piezoelectric doorbell construction
Magnetic doorbells
In a magnetic doorbell, there is a ferromagnetic disc attached to a post. There are magnets around the pole that keep the disc in a resting 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 disc's vibrations. When an oscillating signal is applied to the coil, the electromagnetic field generated from it also fluctuates, causing vibrations in the ferromagnetic disk. In this way, the sound of frequency equal to the frequency of the applied oscillating signal is produced by the magnetic bell. Magnetic buzzers come in transducer and indicator configurations.
Magnetic Doorbell Construction
Magnetic buzzers have a narrow operating voltage ranging from 1V to 16V. These buzzers produce lower nominal frequencies and have low sound pressure levels. These have a slightly higher current consumption compared to piezoelectric buzzers. Its current consumption can reach 100 mA. These doorbells take up little space and are generally used in high-end consumer applications.
Doorbell Specifications
A doorbell has the following important specifications:
Type – A buzzer can be a transducer or indicator type. Both types of doorbells have different capabilities and applications.
Operating voltage – It is important to observe the operating voltage before using it in an application. Piezoelectric buzzers have a wide operating voltage range (3~250V), while magnetic buzzers have a narrow operating voltage (1~16V). Large doorbells generally 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 Pascal decibels. The SPL rating is most useful for comparing two audio output devices. Indicates how loud the sound the device produces. The SPL is generally proportional to the input voltage (buzzer operating voltage) and generally decreases by 6 dB when doubling the distance from the buzzer.
Decibel is not a unit, but a proportion. A value expressed in decibels increases exponentially rather than linearly during counting. For example, 20 dB is ten times the power of 10 dB. The use of decibels allows expressing a considerable range of values in a relatively small space. The sound pressure levels to which human ears are sensitive can range from 20 uPa to 20×10 12 uPa. Therefore, SPL levels can be from 0 to 120 Db Pa. The most common doorbells have SPLs of 80 dB, 85 dB, 90 dB and 95 dB. Some doorbells have sound pressure levels of up to 105 dB.
Resonant Frequency – 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 while consuming less input power when they vibrate at their resonant frequency. Indicator buzzers are generally designed to only operate at/near their resonant frequency.
Impedance – Impedance is the relationship between applied voltage and current. The impedance of a buzzer fluctuates with frequency.
Frequency Response – For different frequencies, a buzzer has different SPL values. Therefore, a doorbell produces different sound frequencies with different intensities. A graphical representation of a doorbell's SPL for various frequencies is called SPL – frequency response, or simply frequency response. The frequency response is plotted over the entire audible range of the human ear (20 Hz to 20,000 Hz). Frequency response is an essential factor when transducer buzzers are used in an application. The graph is extremely useful in designing the circuit to drive transducer bells.
Doorbell applications
Doorbells can produce clicking, beeping, or ringing sounds. They are also used to create siren or doorbell sounds. Buzzers are used for alarms/warnings or as audible indicators. They are used in alarm devices (such as alarm clocks, fire alarms, intruder alarms, etc.), timers, input devices (such as mice and keyboards), electronic metronomes, annunciator panels, and many consumer electronic devices. Buzzers are also used at sporting events and game shows. Different sizes and types of doorbells suit different applications. How a doorbell is electronically actuated depends on its type, configuration, operating voltage and other specifications.