Timer Circuit for Darkroom

The project demonstrates the use of timer 0 using the interrupt routines which involves programming of the PIC16F84.

During the operation, the last starting count of 0-99 minutes and 0-59 seconds are showing on the display when the unit is turned ON. The data EEPROM of the PIC16F84 holds the start count and when the start button is pressed, the countdown starts. When the count reaches zero, an alarm is sounded which will continue until the start is pressed again. The starting count returns to the display by this press and pressing start before reaching zero also returns to starting conditions.

If the set button is pressed before countdown, the start count can be changed since there are 15 settable start counts. Each digit is lit in turn, incrementing from zero until the set button is pressed again. After the final press of the set button, the EEPROM saves the new start count. The select button is used to cycle the start counts while the set button changes only the starting count that is presently displayed.

TMR0 for the interrupt routine and instruction length timing for delays in switch are the two timing methods.

Rest of the project