GAL 1 is programmed in a manner that it counts from 0 to 9 and then resets by generating a carry at pin 21.
This carry goes to the clock pin of GAL 2 which counts from 0 to 5 on each pulse provided by GAL 1.
All GAL IC’s (GAL number 2 to 4) change their state whenever they pick a carry the previous IC’s EXCEPT GAL number 1.
When the direction input (dir) is held at ground, GAL cpunts from 0 to 9 normally. But when ‘dir’ is connected to ‘fc’ (force stop carry) it only counts from 0 to 5 and resets by generating a carry.
The last 2 IC’s are for hour purpose and they are dependent on each other.
So I connected the carry of last IC with ‘dir’ of GAL 5 and developed this logic.
A switch is provided from Vcc t0 carry pin 1 of each GAL, so carry can be provided from Vcc to adjust time yourself. High at pin changes 1 state of GAL IC.
For the clock, I used 555 timers with f = 1 Hz at GAL 1.
Schematic Diagram:
This Project is Made by Waqar Ahmed Wajahat Ali M.Shahruk Department of Applied Physics Karachi University