My wife bought a kitchen timer few days ago. Like this:
But then she said to me: "It is ticking! It annoy me!"
Well, I decided help her.
In attached file you will find source and firmware.
"Why did you use tiny13?" can ask you. Because I have had it.
There isn't full schematics, because it is very simple. Two buttons pin - gnd, piezobuzzer connected immediately to MCU, buttery and capacitor. That all.
How it is works?
Every half second MCU reset by watchdog timer. Most time MCU is in a power down mode. After power on it do little initialisation. Every next time MCU prepares I/O pins and check buttons. One button adds time with step 1 minute, other - 5 minutes. When user press both - MCu reset time (to zero). If If time was ending, MCU starts playing for 30 seconds.
For better precision you can tune WDT reset time by correct next line:
#define T1MIN (50*2)