Something that always drives me crazy when I think about it is how the turn signals of different cars are never blink at exactly the same speed. For a long time I figured maybe different models had different blinking speeds. Until I saw two cars, same make, same model, same color even with turn lights that blinked at different rates. So I appeal to the Straight Dope, does anybody know a reason this might be so??
Yes, I know how weird this question is 
Different brands of flashers, perhaps?
Also, you have to factor in different lengths of wirings and different resistances of the different cars (in their wiring harnesses).
A lot of the blinkers are actuated by temperature. A coil of resistance wire is wound around a bimetal strip. When current from the stop light is switched through the blinker the wire heats up which interrupts the stop light current. The strip then cools off which closes the circuit starting the current and turning the stop light back on and so on. The tolerances of the blinker unit are such that no two of them will blink at the same rate, which is immaterial for their purpose.
Yeah, like DS said.
Car blinkers use thermal relays, which rely on the heating and cooling of a piece of metal to control the blinking. (Electronic relays, on the other hand, can be designed to actuate at a concise frequency.)
How Stuff Works has a nice little article all about them.
Not only do no two blinkers blink at the same rate, but the same TWO blinkers will vary frequency slightly over a short period. For example, two flashers start off flashing in synch. But, in seconds, they suddenly are flashing out of synch until one is on while the other is off (180 degrees out of phase). Observing a little longer, you notice the frequency of the lights has varied enough to bring them back into phase again until the cycle repeats…
This all bugs me, too, while sitting out a traffic light…but it’s more fun than watching the grass to grow… - Jinx
PS - You can get a lot more info on turn signals here.
Actually, I was just a little off. They don’t use a bimetal strip but the thing uses the heat from the stop light current to make and break the circuit as described in the site.
You can also buy all solid state units that flash a constant rate get rid of the annoyance of having the signal flash faster if you hook of trailer lights. But then the rate doesn’t change when a signal bulb burns out so you have to go around and check them every so often. Thus imperfect are all man’s works.
Thanks, DS. I figured there might be something that changed the speed of blinking, but didn’t know exactly what it would be.
I remember years and years ago, it may (probably) have been back when the SD was on AOL, somebody (forgot who) started a MPSIMS type thread about how amazed they were that they saw two vehicles blinking at the SAME rate. I believe they were Chevy or Ford Vans in a parking lot. I also believe that the poster said they they sat in the parking lot for a couple of minutes to make sure, and sure enough they were exactly in sync with each other. Does anyone else remember that post? Or is it just a case of my horrible short term memory but my ability to remember pointless and useless information from random times in my life for no apparent reason?
I didn’t realize they used thermal flashers. How do they make it flash faster when a bulb has blown?
By the way, what Jinx described is the result of two flashers whose frequencies are constant and slightly different from each other, not from a varying frequency. It’s the same principle as the “beat” you get if you have two sounds with a slightly different pitch. (An extremely useful effect when trying to tune a musical instrument!) For example, look at the bottom diagram on this page - the red and blue sine waves are both constant frequencies. But since the frequencies are not exactly the same, they slowly come in and out of phase.
It’s automatic. Thermal flashers usually go slower when a bulb is blown and if you are used to your car, you can tell when a bulb is out. The rate changes because with one bulb out the heating current through the flasher is smaller so it takes longer to make, or break, the circuit. Conversely, if you add a trailer there is more current and the rate increases.
So the heating element is in parallel with the switch? (The diagram on that link was not clear on this point.) That makes sense - very elegant solution.
I’m still surprised that the first flash is not longer than the subsequent flashes. Christmas tree lights work on a similar principle, and they usually take some time to start flashing. Something to do with minimizing the thermal mass, I suppose.
On a semi-related but important note, how many kids growing up in the 70’s messing around in the family car figured out that if one pushed the brake pedal and flipped the turn signal just so, the radio would work without the key?