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220 ohm 1/8 watt resistor
This resistor is the current limiter for the LED. If you do not wish to use the output LED (for example to conserve power), you can choose to not install this part along with LED1. This might make debugging the circuit more difficult, however. It has no polarity. This resistor can be identified by the first three color bands (left to right) as red, red, brown.