Me complace ver que mi pregunta inició un diálogo entre personas que conocen el tema y aún hay cierta controversia. Todo lo dicho hasta ahora escapa a mi conocimiento.
Me tomará tiempo analizar y estudiar las alternativas propuestas y así entender un poco más.
Al menos en mi imaginación pensé en dos temporizadores idénticos, el primero inicia la cuenta regresiva y cuando termina dispara el segundo que también limita la duración del sonido del beeper y todo queda a la espera de un nuevo inicio.
Gracias a todos por vuestro interés.

 

At least in my imagination I thought of two identical timers, the first one starts the countdown and when it finishes it triggers the second one which also limits the duration of the beeper sound and everything is left waiting for a new start.
Thank you all for your interest.

 

Hello. Yes, that's exactly what you told me before with the addition of the electrolytic capacitor for timing. Thank you very much. I'm making the shopping list. I'll tell you the result later.
Regards.

 

Well... Finally! Everything works perfectly. Thanks for your interest and help.
I got carried away with the tests in the simulator. I was intrigued by the reason why it didn't work. I downloaded several from different sources and one of them did work. The Livewire is old, the last version I found is from 2004. It will be time to try another one.
Now it will be a matter of tackling the PCB design.
Thanks to all!

 

Hi, instead using a pair of 555, could be using a 556?