LED

Light-emitting diodes conduct in one direction only and light up once the forward voltage is reached, so they need a series resistor to keep the current within their rating. In community circuits LEDs serve as status indicators, polarity testers, the visible output of transistor and 555 flashers, and markers that show which half-cycle of an AC signal is passing. For example, "A Blinking LED Circuit" makes an LED flash using a resistor and the simulator's power source.