L78 series are three-terminal positive regulators to limit the voltage to a constant voltage, for example, L7805 limit the voltage to 5v, L7809 to 9v, L7812 to 12... etc..
This kind of regulators is low-efficiency components which lose 30% of the efficiency in heat, also it requires an Input 2 volts or higher than the limited output, that means if you want a 5v output you need 7v at least in the input, more difference between input and output voltage, more inefficient will be.
Circuit
To connect the L78 component it recommends to use two capacitors to maintain a constant output voltage.
L78 series are three-terminal positive regulators to limit the voltage to a constant voltage, for example, L7805 limit the voltage to 5v, L7809 to 9v, L7812 to 12... etc..
This kind of regulators is low-efficiency components which lose 30% of the efficiency in heat, also it requires an Input 2 volts or higher than the limited output, that means if you want a 5v output you need 7v at least in the input, more difference between input and output voltage, more inefficient will be.
Circuit
To connect the L78 component it recommends to use two capacitors to maintain a constant output voltage.
Reference
L78 datasheet L78 series wikipedia Make your own arduino board first part is about add L7805