Much like car turn signals, the directional indicators on motorcycles are electrical and work on a standard electrical circuit. Depending on the make and model of the motorcycle, the turn signals are powered either by a small alternator or, on simpler models, directly by the battery itself. As with all electrical components, the turn signals on a motorcycle are fed by a positive lead. The wire, often red in color, that connects to the power source and a negative lead which is a black wire that connects either directly to the negative battery terminal or to a grounding point on the motorcycle frame.
On a standard electrical circuit designed to turn on light bulbs, all lamps on the circuit should illuminate when power is supplied. This situation would not be effective for directional indicators, though, so motorcycle manufacturers design turn signal circuits with diodes that alter the function of the circuit. When the turn signal switch is pressed to turn on the right indicator, for example, a diode blocks the electrical current from flowing to the left lamp. A separate diode performs a similar function to prevent the right lamp from coming on when the left indicator is activated.
In many jurisdictions, a simple lamp is not compliant with driving regulations. Instead, the lamp needs to flash in order to effectively serve the purpose of a turn signal. To comply with this requirement, motorcycle manufacturers insert a special relay known as a flashing relay that uses capacitors to control the flow of electricity to the activated turn signal. The effect of the relay is the widely recognized flashing turn signal.
