The flashing direction indicator (or blinking trafficator) operates on the electrothermal principal and uses bimetallic strips to open and close electrical contacts.

As its name implies, a bimetallic strip consists of two strips of different metals, with different coefficients of thermal expansion, bonded together. When a bimetallic strip is heated, it will curve towards the side where the metal with the lower coefficient of expansion is affixed.

Thus, if one end of the strip is gripped, the other end will thus move a certain distance away from its home position when cold, the magnitude of this distance depending on the temperature to which the strip is exposed.

The flashing direction indicator comprises two such bimetallic strips, both of which move in the same direction on being heated.

Each strip is provided with a silver contact at its end. In the neutral position these contacts touch each other.

 When the ignition of the motor vehicle is switched on, current from the battery flows through these contacts and through the heating coil of one of the bimetallic strips. The heat produced by this coil warms the strip, causing it to bend and thus open the contact. Now the current, instead of flowing through this contact, is passed through the heating coil of the other bimetallic strip, with the result that this strip, too, is deflected, in the same direction as the other strip.

The latter now receives less current, consequently cools a little, and moves back towards its initial position; the two contacts touch each other again; and the cycle is repeated. This continues as long as the ignition is switched on, even while the direction indicator switch is not actuated by the driver of the vehicle. When he wishes to turn a corner and actuates the switch, then current will flow directly to the direction indicator when the contacts are closed.

The indicator lamp then lights up. However, the process continues: the contacts separate, and the lamp goes out. The pilot light inside the vehicle always lights up just when the indicator light is out because the pilot light gets its current only when the contacts of the bimetallic strips are open.

When these contacts close again, the indicator lamp lights up, while the pilot light remains dark, as it is then short-circuited by the contacts. The frequency of blinking is independent of the outside temperature. Besides, when the engine is running, the flashing indicator is already switched on and therefore comes into immediate action when the driver actuates the indicator switch.

Another widely used type of flashing indicator is based on the hot-wire principle.

The thermal elongation of a thin resistance wire under the influence of a current passing through it is used for controlling the blinker contacts.