The point of the circuit is to protect your expensive headphones from the Thump or pop noise you sometimes get when powering up an amplifier with headphones connected.
The circuit disconnects the attached headphones for a period of time, say 5 seconds then the circuit engages the output.
The benefits are no damaged headphones.
On power up the led will illuminate until the 555 timer runs out then it will extinguish at which point the relay will turn on connecting the right and left channels in to the PCB headphone jack .The time can be adjusted using the Variable Pot.
The board layout is designed for a Neutrik jack which are cheap but of good quality there are around £2 delivered on eBay
The 555 was rescued from a Atari 2100 Console just for nostalgia the great thing is with this circuit is it will take anything from 5-18v its only dependents on what relay voltage you use. Remember to use caps of 25v+ if anyone decides to hit the 18v end after which you will have to regulate the voltage .I would say really anything over 15VDC should be regulated as your at the end of the 555's voltage scale.
I designed this with regards to the NP-100v12 headamp project ,but i can see it being recycled many times in other projects that need a simple timed event .With a larger relay the project could easily be adapted for delayed speaker switching or even line level in a pre-amp.
Eagle PCB Design files INC PDFs are available in the files section