i'm using jk flip flop to latch on a state given by 2 switches to either stay ON or stay OFF, knowing that one of them could stuck on ON which is why i use one of them as the clock input for the 4027B which works fine on simulation but not in the real pcb

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i've read that the transition time on this flipflop could cause a problem but i'm new to this so i was asking for help to why this won't work (all the circuit is revised and voltage is correct on all terminals of the 4027 but the output which sometimes output around 2 volts and other time doesn't)

link to a similar problem caused by the transition time Slow clock edge causing issues with D flip flop behavior

also i've achieved the same design using the 4013B which is a D flipflop, would the new one work? is there an easier alternative other than using a microcontroller ?


1 Answer 1


I don't understand what you're trying to accomplish (try using shorter sentences), but one glaring problem in your design is the fact that you have the J and clock inputs of your FF tied together. This means that the J input will be changing at the same time as the clock edge, which almost certainly violates the setup and/or hold time requirements of the FF. Why don't you just tie the J input high?

  • \$\begingroup\$ the problem was with the j being tied with the clock input of the FF, i still duno why it violates the "setup and/or hold time requirements of the FF", thought if it works on the simulator it should work on the pcb, anyway thanks for helping me out :) \$\endgroup\$
    – Andrew
    Jan 4, 2016 at 1:54
  • \$\begingroup\$ "thought if it works on the simulator it should work on the pcb" Most problems in life are due to the fact that simulators are ideal... and the pcb adds a lot of parasitic elements to your design... \$\endgroup\$
    – MadHatter
    Jan 4, 2016 at 2:38
  • \$\begingroup\$ @AndrewxXx, the simulator will always scan the inputs in the same sequence and always give the same result. The real one will have variations in temperature and turn-on points, the voltages will rise at a certain rate and the one that exceeds its threshold will win. Please also make proper use of capital letters in your posts. They look sloppy. \$\endgroup\$
    – Transistor
    Jan 4, 2016 at 7:25
  • \$\begingroup\$ +1 You've got a good eye. Setup/Hold Time ruins most beginner circuits. \$\endgroup\$
    – ammar.cma
    Jan 7, 2016 at 12:46

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