Red & Yellow LEDs

The red and yellow LEDs (D1 & D5) are light emitting diodes, 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 for red and 2V for yellow); the brightness increases. A high current will burn out an LED, so the current must be limited by other components in the circuit. LED's block electricity in the "reverse" direction.