Red LED

The LED (D1) is a light emitting diode, and may be thought of as a special one-way light bulb. In the "forward" direction (indicated by the "arrow" in the symbol) electricity flows if the voltage exceeds a turn on threshold (about 1.5V); brightness increases. A high current will burn out the LED, so the current must be limited by other components in the circuit. LED's block electricity in the "reverse" direction.