please need some help. how can i start the countdown at 20 seconds instead of 99 seconds.

 

hi Mamz,
Welcome to AAC.
The 74LS192 is BCD Up/Down counter, it also has a parallel Load feature.
E

 

At the moment there is nothing to define what count the counter powers up at. MR is connected to ground so will not reset the counters to zero. NOT PL is floating and as it is 74LS logic it will probably float to a one state so a parallel load will not occure. You need to preset it to a value of 20 at power on and also on the next clock pulse after a count of zero. If you can tolerate it displaying 99 for a few tens of nS then you could decode the 99 state and force a load of 20. You may need to add a shot delay with this method to ensure 8 flipflops change state. (This is because the first flipflop to change state from the 99 state will negate the decode of 99 and remove the signal to the parallel load input.)

Les.