Output Devices

For example, in a smoke alarm, detecting smoke is the input function. The processing stage activates a buzzer, whose sound warns people. The buzzer is the output—not the part that detects the smoke.

Output devices convert electrical energy into another useful form. Buzzers and loudspeakers produce sound, LEDs produce light, and motors produce movement. Choosing between them means considering what the user needs the product to do, not simply whether the component can be switched on.

Buzzers provide simple audible warnings

A buzzer produces a simple sound from electrical energy. It is suited to attracting attention rather than communicating detailed information. Typical applications include smoke alarms, timer alerts and electronic toys.

Sound is produced by vibration. In a piezoelectric buzzer, an electrical signal makes a piezoelectric element vibrate. In an electromagnetic buzzer, an electromagnet causes a diaphragm to vibrate. Some buzzers have a built-in circuit that generates the repeated signal needed to make the sound; others require that signal from the surrounding circuit.

Buzzers are generally small, inexpensive and straightforward to use, making them useful where a product only needs a warning tone. An audible warning can attract someone who is not looking at the product. However, a buzzer cannot reproduce spoken instructions or music like a loudspeaker. Its simple sound may tell the user that something needs attention without explaining exactly what is wrong.

LEDs give a visible indication

A light-emitting diode, or LED, is a semiconductor component that emits light when current flows through it in the correct direction. LEDs can provide power indicators, status lights and displays. For example, a lit LED can show that an electronic product is switched on, while different colours can distinguish different operating states.

LEDs are polarity-sensitive: the anode connects towards the positive side of the supply and the cathode towards the negative side. In a simple indicator circuit, a resistor in series limits the current and protects the LED from excessive current. Connecting an LED directly to an unsuitable supply can damage it.

LEDs use relatively little electrical energy, have a long working life and switch on and off quickly. These properties make them useful in battery-powered products and indicators that operate repeatedly. Their main limitation as warning devices is that they provide a visual signal: someone looking away may miss it. A product may therefore combine an LED with a buzzer so that its warning can be both seen and heard.

Loudspeakers reproduce detailed sound

A loudspeaker converts a changing electrical audio signal into sound waves. Unlike a buzzer, it can reproduce a wide range of sounds, including speech, music and sound effects. Headphones use small loudspeakers to reproduce audio; a car satellite-navigation system uses a loudspeaker for spoken directions.

In a typical loudspeaker, a coil of wire sits in the magnetic field of a permanent magnet. The changing current in the coil produces a changing magnetic force, moving the coil backwards and forwards. The coil is attached to a cone or diaphragm. As this vibrates, it makes the surrounding air vibrate, producing sound waves.

The ability to reproduce complex audio is its main advantage. Spoken directions can communicate much more information than a simple beep. However, the loudspeaker needs a suitable audio signal, and larger speakers require an amplifier to provide enough power. This makes the overall system more complex than one designed only to operate a simple buzzer. For a basic timer alert, that extra capability may be unnecessary.

Motors make products move

A motor converts electrical energy into kinetic energy: movement. A common small direct-current, or DC, motor turns a shaft that can drive a fan blade, toy wheel or other mechanism.

A rotating output can also be connected to a mechanism that changes the movement. Gearing can reduce the speed while increasing the turning force, called torque. This helps a designer match the motor’s output to the job rather than assuming that the fastest rotation is always best.

Motors are appropriate when a system must physically act on something. For example, a fan needs a motor to turn its blades; an LED could show that the fan is on, but could not create the airflow itself. A motor’s advantage is that it provides useful powered movement. Its limitations include the current needed to run it and the need to connect its shaft to a suitable mechanical load. A microcontroller may therefore control the motor through a driver circuit rather than power it directly.

Select the output to match the task

The best choice depends on the information or action required. A buzzer suits a simple audible alert; a loudspeaker suits spoken instructions or music; an LED suits a visible indication; and a motor suits powered movement.

A justified choice links a property to a particular need. For a navigation device, a loudspeaker is useful because it can give spoken directions without requiring the driver to read a display. For a small power indicator, an LED is useful because it gives a clear visual signal while using little energy. Products can use several outputs together when one device cannot meet every need.