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hope you are all well and healthy during those times. I have a question which I cannot solve. Question is like this:

Design a counter which counts from 0 to 5 and then stops and turns on a led. The process is initiated by pressing start push button. (Please use J-K Flip Flop with preset and reset asynchronous inputs).

I reached at a certain point in the question and I want to share these with you:

Initial state of the circuit outputs below indicate 0

Count is 1:

Count is 2:

Count is 3:

Count is 4:

Count is 5:

Count is 6 and LED is turned on (sorry for black-white photo):

enter image description here

When the counter is at 110 (6) the LED turns on. When it gets to 7 LED turns off.

After 111 (7) the counter resets which is natural since I am using 3 Flip-Flops. How can I stop this counter (does my teacher mean resetting by stop?) and turn on the led.

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  • \$\begingroup\$ Currently, the LED never turns on long enough to see ( it only turns on for a fraction of a microsecond)....just long enough to reset the count to 000. After five counts, the LED should stay on. \$\endgroup\$
    – glen_geek
    May 5, 2020 at 19:44

2 Answers 2

Reset to default
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You probably want to put a resistor in series with your LED. It may be dragging the output of U3:A down so far that U4:B isn't seeing a high on its input.

Of course, if you reset all three the LED will go off immediately. However, you could reset just U1:A (if you really want it to stay at 6 instead of 5) or more generically you could tie J and K of U1:A to the output of U4:B so they both go low once you achieve whatever final count you set it to.

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  • \$\begingroup\$ Yes I tried what you said and it worked. Thank you so much for your help. \$\endgroup\$
    – kucar
    May 5, 2020 at 22:03
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Without giving too much away. there are things you can do with the J and K inputs that will stop the counter without resetting it. Stopping just the first stage may be enough.

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  • \$\begingroup\$ Yes, I attached U4:B to firs J-K inputs and it worked. Thank you so much \$\endgroup\$
    – kucar
    May 5, 2020 at 22:04

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