Hardware stores sell dozens of different
doorbell models you can install in your
are one of those everyday devices we take for granted. On the
surface, they couldn't be simpler: You press a button, which
completes a circuit, which turns on some sort of noisemaker.
In a world full of computers, cars
that doesn't seem so impressive.
But when you take off the cover and see what's going on,
conventional doorbells are one of the coolest machines around,
precisely because of their simplicity. Doorbells put the basic
principle of electromagnetism to work in remarkable,
innovative ways. In this edition of HowStuffWorks,
we'll look inside some standard doorbells to see how these
devices translate electric current into buzzes, rings and
Magnetic Motor The core of a conventional
doorbell is an electromagnet.
If you've read How
Electromagnets Work, you know that an electromagnet is a
coil of wire, often wrapped around a piece of magnetic metal.
Electromagnets work on a very simple principle. Running
electrical current through wire creates a very small magnetic
field around the wire. Coiling the wire amplifies this
magnetic field, so it has a substantial effect on any magnetic
objects around it.
Just like the magnetic field around a permanent magnet, the
magnetic field of an electromagnet has a polar orientation --
a "north" end and and a "south" end -- and it is attracted to
When you hold down a doorbell button, it closes an
electrical circuit so that household current flows
through the electromagnet (or electromagnets) by way of a transformer.
The transformer is a simple device that takes the 120-volt
household current and steps it down to a 10-volt current (see
a Power-Cube Transformer to find out how). This current is
then passed through the electromagnet wire.
The magnetic field of the electromagnet is put to work to
drive some sort of a noisemaking apparatus. There are a number
of different ways to configure the doorbell components in
order to produce different sorts of noises. In the next couple
of sections, we'll look at three common systems: the
buzzer, the bell and the chime.
Buzzer and Bell As we saw in the last
section, an electromagnet consists of a single length of wire
wrapped in a coil. In a buzzer, the simplest sort of
doorbell, an electromagnet is used to operate a
self-interrupting circuit. You can see how this system
works in the diagram below.
Click and hold the doorbell button to see how the
buzzer works. When the circuit is closed, the electromagnet is
magnetized so it pulls up on the contact arm. This breaks the
doorbell circuit, which shuts off the electromagnet. The arm
falls down, closing the circuit again and the process
One end of the electromagnet wire is connected directly to
one end of the electrical circuit. The other end of the wire
connects to a metal contact, which is adjacent to a
moving contact arm.
The contact arm is a thin piece of light, conductive metal,
with a thin iron bar soldered onto it. The anchored end of the
contact arm is wired to the electrical circuit. When the
electromagnet is turned off, the free end of the arm rests
against the contact point. This forms a connection between
that end of the wire and the electrical circuit. In other
words, electricity can flow through the electromagnet when the
circuit is closed.
Closing the doorbell circuit (by pressing the button) puts
this mechanism in motion. Initially, the electromagnetic field
attracts the iron bar, which pulls the contact arm off the
stationary metal contact. This breaks the connection between
the circuit and electromagnet, so the electromagnet shuts off.
Without a magnetic field pulling it back, the contact arm
snaps back into position against the stationary contact. This
reestablishes the connection between the electromagnet and the
circuit, and the current can flow through it again. The
magnetic field draws the contact arm up, and the process
repeats itself as long as you hold down the buzzer button. In
this way, the electromagnet keeps shutting itself on and off.
The buzzing noise you hear is the sound of the
rapidly moving arm hitting the magnet and the stationary
contact dozens of times a second.
A bell system works on exactly the same idea, except the
moving arm is attached to a long clapper, which rests
alongside a circular bell. As the arm moves back and forth,
the clapper hits the bell repeatedly. This is the same system
used in old-fashioned fire alarms and school bells.
A standard bell ringer design works just like
a buzzer except the contact arm is attached to a long
clapper that hits a metal
These days, most people don't have buzzers or bells in
their homes. The standard doorbell design today makes a softer
chiming sound. In the next section, we'll see how this
Ding Dong! A chime doorbell uses a
specialized sort of electromagnet called a solenoid.
A solenoid is just an electromagnet where the coiled wire
surrounds a metal piston. The piston contains
magnetically conductive metal, so it can be moved backward or
forward by the electromagnetic field. Press the button in the
diagram below to see how this works.
The common chime design uses a solenoid to hit two
tone bars in sequence. Press the button to see how this system
In this design, the solenoid piston consists of an iron
core mounted to a non-magnetic metal bar. When there is no
power to the electromagnet, a spring pushes the piston to the
left, and the iron core extends outside of the wire coil. When
you turn the electromagnet on (by pressing the doorbell
button), the iron core is drawn to the magnetic field, so it
slides into the center of the coiled wire.
As the iron core slides to the right, the end of the piston
strikes the right-hand tone bar. The tone bar vibrates,
producing a particular note. This is the "ding" sound.
As long as you hold the doorbell button, current will flow
through the electromagnet and the piston will remain in this
position. But when you release the button, the current will
stop flowing through the electromagnet and the magnetic field
will collapse. The spring snaps the piston back to the left,
where it hits the tone bar on the other side. The second tone
bar produces the "dong" sound.
You can make more elaborate tone sequences by adding more
solenoids and tone bars. Some variations on this design
produce a different tone pattern when someone rings the back
doorbell than when someone rings the front doorbell. Each
doorbell button closes a separate circuit, connected to
separate solenoids in the doorbell mechanism. In the simplest
design, one of the solenoids hits a bumper instead of the
second tone bar. In this way, one doorbell button will trigger
a "ding-dong" tone and the other button will trigger only a
This doorbell has two solenoids, each wired
to a different ringer button. One solenoid hits two tone
bars, while the other hits only
In recent years, more and more people have been installing
electronic doorbells in their homes. An electronic
doorbell doesn't have any electromagnets or tone bars.
Instead, it has an integrated circuit (IC) that registers when
the button has been pressed. The IC triggers a digitally
recorded song or message. A lot of electronic doorbells have a
wireless control system so the doorbell mechanism doesn't have
to be specially wired.
Electronic doorbells are growing in popularity, but most
homes still use conventional electromagnet doorbells. This
classic design may not play elaborate songs or messages, but
it is so simple and sturdy that it should be around for a long
time to come.
For more information on doorbells, including tips for
repairing a broken doorbell mechanism, check out the links on
the next page.