In the American southeast, which up until recently was
comprised mainly of small towns, it's said that drivers don't
use their turn signals because everyone knows where you're
going anyway. Turn signals may be the most underutilized
device on a car. Yet signaling is one of the most important
actions you can take as a driver, warning other drivers of
your impending moves to minimize hazardous surprises. You
might not see a car in your blind spot, but that car's driver
might see your turn signal start to blink before you move into
his lane. Failing to signal may be the cause of quite a few
accidents, and is probably a big source of "road rage" as
We've used it thousands of times (well, most
of us have), but what makes this lever
To most drivers, turn signals seem pretty simple: You push
a lever up or down, causing your turn signals to flash. But
there is actually some cool technology at work there. In this
edition of HowStuffWorks,
we'll take a look at the unusual device called a thermal
flasher that makes your signals flash, and we'll learn how
turn signals cancel themselves after you've made your turn.
But first, let's see how turn signals are wired.
The Wiring Let's take a look at how the
turn-signal circuit is hooked up.
The turn-signal circuit gets power when the ignition
key is on. The power goes through a fuse panel into the
thermal flasher. From there it goes to the stalk on the
Depending on the position of the turn-signal stalk, the
power either stops in the switch or gets sent to the left or
right turn-signal lights (including the indicator lights on
the dashboard). Power flows through the filament of the lights
and then is grounded.
This is all fairly straightforward, but you might be a
little surprised at how the thermal flasher works.
The Thermal Flasher This small, cylindrical
device is sometimes located in the fuse panel under the
dashboard of the car. It costs about $3 in the auto parts
store and works reliably for years.
In this vehicle, the thermal flasher is
located in the fuse
Inside the thermal flasher there are a few simple
An electrical contact that conducts electricity
into the wire
A piece of gently curved spring steel to which
the electrical contact attaches
A resistive wire wrapped around a smaller
piece of spring steel
When you push the turn-signal stalk down, the thermal
flasher connects to the turn-signal bulbs by way of the
turn-signal switch. This completes the circuit, allowing
current to flow. Initially, the spring steel does not touch
the contact, so the only thing that draws power is the
resistor. Current flows through the resistive wire,
heating up the smaller piece of spring steel and then
continuing on to the turn-signal lights. At this point, the
current is so small that the lights won't even glow dimly.
After less than a second, the small piece of spring steel
heats up enough that it expands and straightens out the
larger, curved piece of spring steel. This forces the curved
spring steel into the contact so that current flows to the
signal lights unimpeded by the resistor. With almost no
current passing through the resistor, the spring steel quickly
cools, bending back away from the contact and breaking the
circuit. The cycle then starts over. This happens at a rate of
one to two times per second.
Let's take a look at the mechanism that cancels the turn
signal when you finish turning.
Self-Canceling Signals Most cars have a
mechanism that shuts off the turn signal when you are finished
making a turn.
You are driving straight down a road and put your right
turn signal on. You slow down and turn the steering wheel to
the right. The turn signal is still blinking away. As soon as
you make the turn and turn the steering wheel back to the
left, the turn signal goes off and the lever pops back to its
original position. How does it do that?
On the steering shaft (the part that spins when you turn
the steering wheel), there is a notched hub.
The notches turn when you turn the steering
There are four notches equally spaced around the hub. When
the turn signal is on, a plastic lever on the
turn-signal switch is pushed into the path of these notches.
Inside the turn signal switch: When the turn
signals are on, the black plastic tip on the white
plastic lever engages the notches on the
When you lift the turn-signal stalk to signal a right turn,
a spring-loaded roller falls into a notch in the switch
housing, holding the stalk in place. At the same time, a
plastic lever thrusts out into the path of the hub.
As the hub continues to rotate clockwise, the notches hit
the plastic lever, which rocks to let each notch pass. When
the wheel turns back to the left, the hub turns
counterclockwise, pushing the plastic lever in the other
direction. This forces the spring-loaded roller out of its
notch in the switch housing, so the stalk springs back to its
Next, let's take a look at a recent innovation in
Turn Signals in Mirrors Recently, many
carmakers have started offering cars that have turn signals in
the side mirrors. This is a better spot for the turn signals
because if a car is in your blind spot, its driver might not
be able to see the back of your car.
These mirrors contain high-intensity light-emitting diodes
(LEDs), usually arranged to form an arrow that can
point either left or right. The LEDs are positioned behind the
mirror glass so that from inside the car, the driver sees only
a dimly lit arrow, but outside the car, other drivers see a
very bright arrow.
Photo courtesy CustomVan.com From inside the car, the driver sees a dim
Photo courtesy CustomVan.com The signal is bright when viewed from an
angle outside the car.
Since the lights in these mirrors are LEDs, the system has
an additional benefit: LEDs light up about a fifth of a second
quicker than incandescent
light bulbs. That may not sound like much, but at 65 miles
per hour (105 kph), your car covers 19 feet (5.8 m) in a fifth
of a second. LEDs could give someone the extra time and space
needed to avoid hitting you.
For more information on turn signals and related topics,
check out the links on the next page.