Hi folks,
I cascaded 2 74HC192's to build an 8 bit down counter with parallel loads etc that can be set to start countdown from the specified binary number on the dip switches. (attached) I also need to add a 15 second delay as soon as it reaches 0 seconds by mean of 555 monostable which I found some good standard values for resistor and capacitor.

In the question it specifies the use of an optoisolator which I used to isolate the clock input in the counters from the 555 timer output. I almost got it to work but the thing is that It does not stop at zero but turns to the parallel load before the 15 seconds delay starts. After the 15 sec delay it counts down from 20 and therefore the system is not perfect.

Is there any way I can make it stay at zero during the 15 sec period and then start again from 20 seconds? Any help would be really appreciated.

 

Kill the clock cycle when you reach 0(zero)

Running the clock cycle through a logic level mosfet, or disconnect the enable pin to kill the clock at 0.

An AND gate should be trigger enough.

 

Do you want the counter to freeze when it reaches 0 or do you want it to behave in a different way?

Do you want the 15s delay to stop the counter or just to be produced for another use?