Here is the 7805 5V voltage regulator datasheet the 7805 voltage regulator IC. They are designed to provide a constant 5V.
More about 5 volts DC power supplies
See 5V 3A power supply circuit using LM323K or LM350 or 7805 by helping of the power transistor. For digital load, microcontroller, even raspberry-pi.
This is 5v 12v dual power supply circuit using 2N3055 and Zener diode for 12V then to LM309 or IC-7805 dc regulator IC for 5V output which easy circuit [...]
If you are looking for 5-volts DC regulator without transformer. This circuit may help you. It is suitable for low current digital. Use the MOSFET is based.