Motor vehicle directional signals have been around almost as long as cars, and as early
as 1909, a device was patented that had hand-shaped lights that indicated to other drivers which
way a vehicle intended to travel.
Obviously, that one didn’t catch on.
Neither did the mechanical signaling device invented by silent-film star Florence Lawrence
in 1914, which, after a button was pushed by the driver, a sign would pop up from the
rear bumper indicating which way the car was turning.
The first modern directional signal was patented in 1925 by Edgar A. Walz, Jr., although no
one was interested in it either, at least until after his patent expired 14 years later.
That’s when, in 1939, the U.S. automaker Buick first introduced the “Flash–Way
Directional Signal,” which operated in the familiar way – a stick, mounted on the steering
column.
In its original incarnation, this turn signal only flashed the rear lights; by the next
year, front lights were flashing as well.
By the end of WWII, directional signals were standard in most American automobiles.
While there are several different designs out there for turn signal switches, they classically
used some form of a thermal switch (in this case called a thermal flasher).
The core of this switch includes a resistor that heats up when current flows through it.
This is attached to a bimetallic spring- essentially an arched strip made up of two sandwiched
metals with very different thermal properties, one that expands quite a bit as it warms and
the other that doesn’t.
The result is that when current is flowing (when you turn on the blinker), the resistor
heats up the arched bimetallic spring which in turn is connecting the two terminals, allowing
the lights to come on.
As it heats up, one of the metals begins to try to straighten itself out as it expands,
but initially can’t because the other metal is holding it in its arched form.
Eventually, the heat builds enough where the metal trying to straighten itself out wins
the battle and the bimetallic spring straightens, disconnecting the two terminals.
At this point, no electricity is flowing and the lights are off.
Because no power is flowing, the resistor and the bimetallic spring rapidly cool to
the point where the other metal pressing to reform itself into the arched shape wins,
and there is another snap as it once again bends back into shape and re-connects the
terminals, starting the process over again.
So in these thermal flashers, the clicking noise you hear is just this bimetallic spring
snapping back and forth from its arched form to a straightened form and back again.
One tell-tale sign that you have a thermal flasher in your car is that when a light goes
out, the flash rate of your blinkers will slow down- less electricity = more time needed
for sufficient heat to build to snap the spring.
In the extreme, where too little current is flowing (like if you try to swap out your
incandescent bulbs for LEDs), your blinker will simply not blink, staying on.
On the other end of the spectrum, if you hook up a trailer or something with more lights,
it will speed up the blinking rate and in the extreme perhaps even damage your thermal
flasher.
Another common device used to create the blink in your blinkers is an electronic flasher.
Instead of using heat and a bimetallic spring, this device simply uses some electronic circuit
to control a relay.
Inside a mechanical relay, applied electricity to an electromagnet creates a magnetic field
that switches a metallic armature, which in turn makes an audible clicking noise.
When the electricity is no longer applied to the electromagnet in the relay, a spring
moves the armature back the other way, either connecting the circuit or disconnecting depending
on the type of relay (whether it’s designed to be switched on or off by default).
This type of system has the advantage of not inherently changing the blink rate based on
the electrical draw of the lights.
So switching to LED lights or hooking up trailer lights won’t necessarily affect anything,
though this does vary somewhat depending on the design of the circuit, particularly in
how the system is designed to detect and respond to a burnt out light bulb.
But, in general, as long as you’re not drawing more power than the wires, fuse, etc. can
handle, the system should blink at a constant rate, with the relay clicking away as it switches
the lights on and off.
There is also another type of electronic flasher that is a solid state flasher, meaning it
has no moving parts (making it incredibly long lasting and reliable), perhaps using
a solid state relay.
This type of system does not inherently click unless the designer specifically chose to
add something to do so, possibly added as an extra indicator to the driver that the
blinker is on.
In most cases, though, there will be no click with a solid state flasher.