get it on and off continuously. The circuit is using a well known NE555 timer IC which is a very famous IC due to its uses in the wide variety of electronic circuits. In this circuit this IC is connected as a astable multivibator and the 12 volt relay is derived through transistor 2N4401 which is working as a switch. The ON and OFF rate per second can be controlled by adjusting the 100K pot.
Here is a very useful project of an automatic on off relay circuit. The circuit can be used for many purposes for example as a car headlight or brake light flasher or flashing an AC lamp etc. The circuit will continuously ON and OFF the 12 volt relay therefore you can connect any appliance with the relay to