Electric doorbells
Last updated: January 6, 2010.
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 some 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?
When I was about eight or nine, I built a very crude doorbell for
my bedroom. It was a simple electric circuit containing a
battery, 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.
Clapper doorbells
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 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. That makes
the clapper fall back again, 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
run out!
Photo: How a doorbell clapper works as part of a self-interrupting circuit. For the sake of simplicity, this picture doesn't include the electromagnet or the battery.
Chime doorbells
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.
