The notches turn when you turn the steering wheel.

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.

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 hub.

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 centered position.

Next, let's take a look at a recent innovation in turn-signal technology.