Great PIC project, source and schematic included —
When the unit is turned on the last used starting count, minutes 0-99, seconds 0-59, is showing on the display. The start count is held in data EEPROM of the PIC16F84. Countdown starts when the start button is pressed. An alarm is sounded when the count reaches zero. The alarm continues until start is pressed again. This press also returns the starting count to the display. Pressing start before reaching zero also returns to starting
The start count can be changed if the set button is pressed before countdown. Each digit is lit in turn, incrementing from zero until the set button is pressed again. The new start count is saved in EEPROM after the final press of the set button.
There are 15 settable start counts. You cycle through them using the select pushbutton. The set button changes only the starting count presently displayed.
Gennette Bruce’s countdown timer – Link.