This project is an automatic line-level audio switch.
It has 2 line-level inputs and 1 output.
When there is no signal on input 1, input 2 will be on the output.
When a signal is detected on input 1, the relais will switch the input 1 to the output.
Then when there is no signal on input 1 for about 15 seconds, the relais will switch again to input 2.