A CLR (current-limiting resistor) is placed in series with a device that has a low or non-linear internal resistance, such as a light-emitting diode. It drops the excess voltage and sets a safe operating current. Without it, the component would draw too much power and burn out almost instantly.
Don't forget to put a 330 ohm CLR on that indicator LED or it will pop.
To calculate the CLR value, subtract the LED forward voltage from the supply voltage, then divide by your desired current.
Related terms
Resistor
A passive two-terminal component used to limit current flow, divide voltages, or terminate transmission lines.
Diode
A two-terminal semiconductor component that restricts electrical current to flow in only one direction.
In series
A circuit configuration where components are connected end-to-end so the same current flows through all of them.