Hello , i was asking is there a way to make an audio changeover , between 2 sources , if 1 primary source line in is out , then after x second timer 555 it switch to second line in source 4066 .i tried to make that before but failed , and this was with the help of a freind ,forget to say that if main source detected back , it should be back , all that to complete the hobbie .It exist a high price microcontroller units but they are far to be obtained,or if a felow have a to write a software with a pic that it good . the pic control the inputs and switch to an analogue ic

There is a small PC software that does this. It is called "silence detector" it was on the pira.cz website.
I have never tried it but what you describe matches what that pira software does.

There are many simple switches designed to activate a recorder or bug by sound using a relay, and they switch off when silence returns - this could be adapted to do what you want using a double throw double pole relay.
http://www.circuitdiagram.org/audio-act ... witch.html
https://easyeda.com/guest/Audio_Detecto ... -yYuM4mfxP

@krakatoa thankl you already know that software but i need something hardware .

@yellowbeard thank you .

yellowbeard wrote: ↑Tue Nov 16, 2021 6:50 pm http://www.circuitdiagram.org/audio-act ... witch.html

That one won't work, and would tend to shunt the audio!

You may have a further problem if you're running stereo or have RDS - the modulation path is never "silent" because of the RDS signal or the stereo pilot tone.

My hardware Modulation Monitor gives an alarm output after a set number of seconds of silence. It can also handle stereo multiplex and RDS baseband signals - there are notch filters at the critical frequencies.

The approach I use is to filter the modulation - including 19 kHz and 57 kHz notches - and then feeds the audio to a "diode pump". This configuration detects the presence of audio, by converting it to a varying DC level. This DC signal is then used to feed a comparator, with the threshold set for very quiet (but present) audio. The output of the comparator then is used to reset a timer - the timer is held at zero by the presence of audio. If there is silence for a predetermined period, a bistable is "set" and an alarm raised. The output of the bistable can also be used to switch to an alternative source of audio.....

Albert H wrote: ↑Mon Nov 22, 2021 3:19 pm The approach I use is to filter the modulation - including 19 kHz and 57 kHz notches - and then feeds the audio to a "diode pump".

I guess it depends whether you are trying to detect 'silence' or a lack of valid programme. I've done both in the past, i.e. detected the 19 kHz tone to check that the incoming feed is intact, and then alternatively/also detected a lack of audio to see whether there was anything leaving the studio. I've even used these to change the transmitted RDS so that it's clear on the receiver where/what the problem is. Mind you as the RDS and pilot tone are now often generated in the same place, this is harder to do.

Way back, I used the 19kHz pilot to drive an NE567 to pull in a relay, and turn on the PA! Obviously, that assumed that you were using stereo.

I also used sub-audio tones over the link to switch the rig on and off.

A recent system I did has a silence detector at the transmitter end, and a USB-stick in a USB audio player as back-up programme for a few hours if the link is lost.