When you hit your car’s turn signal stalk and that little arrow flashes at you on the dash, you always hear a rhythmic clicking sound in sync with that flashing arrow. Here’s what that is.

That sound you hear originates in the late 1930s, though turn signals go back farther back to the early 20th Century when people were coming up with odd mechanical signals. (Actress Florence Lawrence devised one, for instance, which, “when placed on the back of the fender, can be raised or lowered by electrical push buttons,” according to the New York Times.)

There was also a light-based system in the 1920s, but the real clicky goodness started in the 1930s, after Joseph Bell came up with the idea of a flasher, and then, in the late 1930s, Buick made flashing turn signals standard on some of its vehicles.

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Shortly thereafter, other automakers followed suit, and in the ’50s, turn signals became mandatory. Ever since, new cars all have made that familiar clicky sound when signaling a turn.

Thermal Style Flashers

Early vehicles used an old-school thermal style flasher to send current to the lights. The way this worked is, a bi-metallic spring would heat up as soon as you activated the turn signal. Since the two metals comprising the spring had different thermal expansion coefficients, the metal strip would want to bend and contort from its original shape.

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As it did this, the bimetallic spring eventually made contact with another terminal, completing a circuit, and sending current to the turn signal lights.

You can see the large curved bimetallic spring mated to the small strip of high resistance spring steel. As that little strip of spring steel receives current, it heats up, forcing the large curved spring steel to straighten out and make contact with the terminal directly above it (pointed out in red above).

At that point, the lower contact (pointed out by the orange arrow) is no longer touching, so the smaller, high-resistance spring steel cools down, and so does the bimetallic strip, so it returns to its original location, at which point, the lower contact touches, heats up again, bends until it activates the light, and the cycle continues.

That bi-metallic spring bending as it heats up and cools down to touch the two different contacts is what creates the clicky noise you hear here:

Electronic Relay Style System

The blinker noise that you probably grew up hearing didn’t actually come from a thermal flasher like the one I mentioned above. More likely it came from an electronic style flasher, which uses a little chip to send a pulse to a relay.

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That relay is just an electromagnet, consisting of a set of wound coils that produce a magnetic field when current is sent through them. Working in a similar way as a typical solenoid switch, this magnetic field pulls up a hinged metal armature, and disconnects a set of contacts and cuts current to the lights.

Once the small chip stops sending current to the coil, the magnetic field collapses, and no longer pulls the metal armature, and a little coil spring will pull the contacts closed again, sending juice to the lights.

The video up at the top of this article shows how it works. And if you’d like an even closer look, here’s a video of the turn signal in action under the dashboard of my other Jeep Cherokee:

So there you have it: that classic turn signal noise comes from either a bimetallic spring bending back and forth as it heats and cools, or from a standard old relay activated by a little chip.

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Modern cars do things a bit differently, with on-board computers sending the signal to turn on the lights to—in some cases—silent solid state relays. The sound is sometimes simulated through speakers, and sometimes there’s a relay under the dash.

New cars don’t really need to make that clicky noise, but it’s a remnant from the olden days when bimetallic springs and little electronic under-dash chips and relays did all the hard work. It’s comforting and familiar, and let’s hope it stays around.