Engineering & Technology



Buzzers produce a loud noise whenever a DC voltage is connected to its leads. Unlike piezoelectric transducers they don't need any circuitry to make them work.

It is important to remember that they are polarised, the red lead is the positive and the black lead is the negative.

Buzzer symbol

Bells aren't so common now since more advanced sound producing technology is available.

The are electromechanical devices, where connecting it up to a power supply uses electromagnets to move a hammer against a bell. The electromagnets are wired up in such a way that the hammer moves backwards and forwards against the bell continously.

Twp types of tilt switch
DC Motors

DC (Direct Current) motors are simple devices that cause a metal shaft to rotate when a voltage is applied to its terminals.

The direction of the motor can be changed by reversing the power supply direction. These motors spin very fast and usually need a gearbox of some kind to make them practical. 

DC Motor symbol
DC Motor
Geared DC motor

A solenoid consists of an iron rod enclosed within a coil of copper wire. When no power is applied, the rod is pushed outwards by a spring.

When power is applied the coil of wire becomes and electromagnet and pulls the iron down.

The animation shows the power being turned on and off continously.

Solenoids are used in automatic water valves, cars and systems when linear movement is needed.

Solenoid circuit symbol
Solenoid in action

Speakers are devices that convert electrical currents into sound. Magnetic speakers consists of a coil of wire held withing the field of a permanent magnet. When current flows through the coil it produces a magnetic field which interacts with the magnetic field of the magnet, causing the diagragm (the black bit at the top) to move. This movement creates sound.

Speaker symbol
Piezoelectric Transducers

The most basic piezoelectric transducers consist of a thin layer of quartz crystal sandwiched between two metal surfaces.

Piezoelectric materials are able to expand and compress when an electric field is present. Piezoelectric speakers exploit this property, applying a varying voltage to the crystal causes it to vibrate in sympathy, thus creating sound. Smoke alarms use these.

Piezoelectric transducers can also generate small electric currents when they are stretched or compressed. Some gas lighters use this to greate a spark to ignite the gas. They can also be used to detect motion and vibrations in security systems.

Piezoelectric element
A piezoelectric sounder