by Chris Woodford. Last updated: November 19, 2020.
Ding dong! Sometimes we love that sound, sometimes we hate it. But
if there's one thing I love it's the science behind it. When
someone's finger pushes on my doorbell, what I can hear is the
sound of impressively simple 19th-century physics—the science
of electromagnetism, to be exact. Just what happens when the
doorbell goes "ding"? Let's take a closer look!
Photo: A simple "push-to-make" switch activates the doorbell's electric circuit when you push it in. When you release it, a spring makes it pop out and it breaks the circuit again.
How does an electric doorbell work?
Photo: A piezoelectric buzzer makes a chirping noise—the "ring tone" in modern landline telephones. Although cheaper doorbells do sometimes contain buzzers like these, most have more sophisticated ringers.
When I was about eight or nine, I built a very crude doorbell for
my bedroom. It was a simple electric circuit containing a
switch, and an electric motor standing on a large cardboard box. When
a caller pressed the switch, the battery fed power to the motor and
made it spin around with a buzzing noise (a bit like the vibrating
alert on a cellphone or pager). Standing on the box, the motor made a
reasonably audible but rather dull humming noise. Real electric
doorbells aren't that different. Instead of using an electric motor
and a cardboard box, they use an electromagnet
(a temporary magnet whose magnetism can be turned on and off instantly by electricity) to make a more
attractive sound, either with an electric bell, a buzzer, or chime
bars struck by a magnetic hammer.
Animation: How a doorbell clapper works as part of a self-interrupting circuit. For the sake of simplicity, this picture doesn't include the electromagnet and the battery (which are wired into the circuit) or the spring that pulls the
clapper back to its original position; those are shown in the next picture.
Artwork: Inside a clapper doorbell. Press the button (red) and the electromagnets (green) attract the
clapper (yellow). As the clapper moves in, it breaks the circuit and its mounting spring lets it spring back out again. The
process repeats until you let go off the button. Artwork from US Patent 592,269: Electric vibrating bell by Henry F. Albright, courtesy of US Patent and Trademark Office.
Most doorbells have what's called a "push-to-make" switch
outside your door, like the one in our top photo. When you prod the button, your finger pushes two
electric contacts together to complete ("make") the circuit; when you release
the pressure, a spring moves the button back out again so the circuit
is interrupted. Like my own primitive doorbell, the circuit itself
contains only two basic elements: a battery and something that makes
a noise. The "something" is often an electric bell: a little
metal bell (like one on a bicycle) and a clapper (hammer) powered by an
electromagnet. When someone presses the button, the electromagnet is
activated and pulls on the clapper, which strikes the bell. But
here's the clever bit: the clapper is actually also part of the
circuit. When it flips out to strike the bell, it breaks the circuit
at the same time, cutting the power to the electromagnet. A spring
attached to the clapper pulls it back to its original position, whereupon it completes the circuit,
energizes the electromagnet, and rings the bell once more. This goes
on for as long as you keep the button pressed. Or until the batteries
Artwork: This slightly more sophisticated electric doorbell, dating from the early 20th century, uses two interlinked circuits. The first one, shown in red, connects a pressure switch (green, 12), battery (10), and lamp (yellow, 21). The switch is placed under a doormat so it closes, operates the circuit, and lights the lamp whenever someone approaches the house. The lamp is meant to be placed right next to the push-button doorbell switch (44)—perhaps even to shine right through it—so it indicates what the caller should press when he or she arrives in the dark. When the switch (44) is pressed, it breaks the red circuit and operates the blue one instead. Now power from the batteries energizes the electromagnet (pink, 15), bringing the bell clapper (green, 14) repeatedly in contact with the bell itself (orange, 16). From US Patent: 769,203: Electric Signal by William Wheeler, patented September 6, 1904, courtesy of US Patent and Trademark Office.
Ding-dong chime doorbells are slightly more sophisticated. They
have two metal bars at either side that make the two different
musical notes when something hits them. In between are mounted
batteries and an electromagnet with a freely moving "hammer"
inside it—essentially just a cylinder of magnetic metal that slides
back and forth inside a slightly bigger cylindrical plastic tube.
When you press the doorbell, the electromagnet magnetizes the metal
hammer and pulls it way to the right, compressing it against a spring
and striking the metal chime bar on the right: ding! At this point, the circuit is
interrupted and the electromagnet switches off, so the spring makes the hammer shoot back the other
way, striking the metal chime bar on the left: dong! The chime bars
are very lightly mounted on plastic fasteners so they vibrate for a
few seconds before the sound dissipates.
Photo: Inside a chime doorbell. The chime bars are on the right and left. The electromagnet and the moving hammer are in the black plastic box in the center.
There's one big problem with conventional doorbells: if you have a large house
(or you're often outdoors in a garden or yard), you may not hear them ringing.
One solution to that is to have a wireless doorbell, which has a conventional
push switch mounted on the outside of your door and a battery-powered ringer
unit you can carry from room to room (or outdoors, if you wish). The
switch contains a miniature, short-range radio transmitter
that can send a signal (433 MHz is typical) up to about 100 meters (300 ft or so) to the radio
receiver in the ringer unit. Most wireless doorbells allow you to select different
radio channels, so you can use several different ones in one building without
them interfering—and ensure your doorbell doesn't ring when someone arrives at a neighbor's house!
Doorbells for apartment blocks
Photo: How a rotary doorbell switch works. This design is by David Brown, an inventor from Chicago, and dates from 1935. In the lower figure, I've colored (in red) the circuit for apartment 12, showing how the doorbell pointer completes the circuit for that apartment when it points straight upward. Read more about this invention in US Patent: 2039581: Doorbell Switch. Drawing courtesy of US Patent and Trademark Office.
Head to the front door of an apartment block and you'll likely find a whole set of doorbells, one for
each household. It's not the most elegant bit of electrical engineering, but it works. Go to an older
building and, if you're very lucky, you just might come across an elegant, antique, rotary doorbell like the
one shown here. As with the doorbells described up above, this one is designed to activate a simple electrical circuit that
rings a bell inside someone's home; you simply turn the dial to point to the apartment number
you want then press the button in the center. What's clever about this bell is the rotary switch
on the dial, which allows the doorbell to activate one of 12 separate circuits, each running to
the bell inside a different apartment. How does it work? As you can see in the lower part of the diagram, the dial pointer
is a key part of the circuit. When it turns to one of the apartment numbers, it completes the circuit for that apartment only.
How do electric door entry systems work?
If you live on one of the upper floors of a modern apartment block, you probably have an electric door entry intercom or videophone, allowing you to let people into your building without having to trudge all the way down the stairs and back again. Systems like this don't have much in common with traditional electric doorbells. Typically, they use an intercom that links a combined microphone and loudspeaker grille on the front door of the building to a separate telephone-style handset in each apartment. Each handset has an extra button on it that links back to the latch on the front door. When you press the button, it triggers a relay, which energizes a solenoid (a type of electromagnet) that pulls back the latch on the door, letting people in.