# Why do blinkers of two cars never blink synchronously?

I dont know if I can ask this question here but have been trying to find an answer for this one for a long time.

Take three cars of the same make/model/year. With the precision of microseconds, turn on the blinkers (indicators) of one side. How is it possible that the frequency of blinking is never the same?

Or if you look at a few cars waiting to make a left turn with their blinkers on. There will be a moment of time when all blinkers will blink together and then fall out of their synchronous-ness. Why does this happen?

• It's probably to do with the state of the charge of the various batteries. A less fully-charged battery may take slightly longer to make the thermal relay open up. Sep 13, 2013 at 8:40

A blinker relay uses a thermal switch which is a piece of curved, spring steel and a thinner piece of spring steel near it that has a wire coiled around it with a resistive property.

When current is applied to the relay the coil of wire heats up and expands the small piece of spring steel until it pushes it to make contact with the larger piece of spring steel. All of the current then travels directly between the two pieces of spring steel. Since no current is flowing through the coil of wire any more, (electricity always takes the path of less resistance), the small piece of spring steel begins to cool down.

Once sufficiently cool, the small piece contracts and pulls away from the larger piece and breaks contact. Current again begins to flow through the coiled wire and begins the cycle again, causing the light to blink.

No matter how precise the relays are manufactured, there are far too many variables for any two relays to blink at the same rate. The resistance in the wire can be slightly different. The temperature of the relays could be different, perhaps because one car has the A/C on. Many different materials could be used by different manufacturers. There are many reasons for slight differences.

Any slight difference between the two relays will end up heating and cooling at different rates. Meaning they will always blink at a different frequency.

For a fairly simple explanation look on How Stuff Works.

• Good write up on thermal switches. More recently solid-state flasher relays have been entering the marketplace that do not operate on this principle and may theoretically be far more consistent in their blink rate. These are often used when someone has converted from incandescent to LED bulbs for turn signal lighting, as they fix the "fast blink" condition that would result with a thermal relay and low-current-draw LEDs.
– mac
Sep 13, 2013 at 14:34
• @mac Unfortunately, I was in a hurry when I posted this and didn't include the solid state relays. They suffer from the same types of variances in the manufacturing process, however. Particularly, the capacitor and the resistor in the circuit. They are both subject to an assumed 5% difference from their rated functions for the most part. Higher tolerances can be used but, they still have a rated percentage of variance that would make it practically impossible for two blinkers to share the same frequency. I didn't realize they were becoming more common. I'll have to update my answer. Thanks Sep 13, 2013 at 17:31