7809 is a voltage regulator integrated circuit(IC) which is widely used in electronic circuits. Voltage regulator circuit can be manually built using parts available in the market but it will take a lot of time to assemble those parts on a PCB. Secondly, the cost of those parts is almost equal to the price of 7809 itself so professionals usually prefer to use 7809 IC instead of making a voltage regulator circuit from scratch. Before you start using 7809, you will need to know about the pin structure of IC 7809. Apparently, it looks like a transistor. It has three pins. For a better understanding, I have given an image of 7809 bellow. Please take a look.
You can easily see the V in and V out pins as well as the ground pin. It is really easy to use 7809 for voltage regulation purposes. I have also included a circuit diagram of 7809 so that you may learn how to use it in a circuit diagram.
It is wise to use two .1uF capacitors on both input and output sides to filter any ripple or distortion in voltage but it is not necessary. In the image, you can see that 12V are being supplied on the input side of 7809 but the out put side of 7809 is outputting Regulated 9V. As long as the input voltage remains above 9V, output voltage of 7809 will remain smooth and regulated.
Please note that input voltage of 7809 can be up to 23V but under my experience, it is wise to avoid input over 15V. 7809 is claimed to output 9V and almost 1.5A Current but again, I have experienced that we should not put a load over 9V and 1A on it. Since we are using it in power supply, the transfer of power will result in heat output. We will need to use a heat sink with 7809 otherwise this heat can damage it. It is advised to use a 1A fuse on the output side of 7809 and a 1.5A fuse on the input side of 7809 to avoid damage in case of short circuit.