Design a sequential digital circuit that detects a 4-BCD digits password. Assume that you are using # as an enter key where the password will be decided, by your circuit, to be right (output =1) or wrong (output = 0). Assign an unused BCD code for #. Assume that the password is 3542

I tried to solve it by converting each one of them to BCD , then I made the state diagram . But the the problem is I don't know how to do this state diagram correctly . Does anyone have any idea ?

By the way I don't need a code , I want to know theoretically what happens

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    \$\begingroup\$ Add the state diagram you currently have so we can see your progress so far. What BCD value did you assign to the # key? \$\endgroup\$ – David Dec 18 '13 at 15:53
  • \$\begingroup\$ If you don't know shift registers yet, which kind of logic parts do you know? \$\endgroup\$ – Geert Goeteyn Dec 18 '13 at 16:14
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    \$\begingroup\$ Come up with a state diagram...even if it's super wrong, it will be a starting point. \$\endgroup\$ – dext0rb Dec 18 '13 at 16:17

Each time you enter a code, shift it (the 4-bits) into an 16-bit shift register. Use a 4-bit compare to detect the "#" code, and when you have a match there, latch the output of a 16-bit comparator which compares the last 4 digits with the correct code, that's the 4 digits in the 16-bit register.

  • \$\begingroup\$ what is shift register? I think we can do it in another way , since we didn't take it yet \$\endgroup\$ – user3096919 Dec 18 '13 at 16:02
  • \$\begingroup\$ A shift register is a string of flipflops which store the status of a number of bits, here 16. Upon a clock pulse, all bits are shifted to the right, and the new digit is shifted in. That way it will always hold the latest 4 digits entered. If you have to it's possible without shift registers, but then it becomes much more complicated. \$\endgroup\$ – amadeus Dec 18 '13 at 16:06
  • \$\begingroup\$ Make sure to clear the shift register when you hit the '#' key. Otherwise your system is significantly easier to break into. I believe some car door keypads had this vulnerability. \$\endgroup\$ – Samuel Dec 18 '13 at 17:32
  • \$\begingroup\$ @amadeus: I'm not sure that a lack of shift registers would make things harder, since a shift register is nothing other than a set of latches which feed each other. \$\endgroup\$ – supercat Jul 9 '14 at 15:14
  • \$\begingroup\$ @Samuel: If one uses four 5-bit shift registers (or 5 cascaded 4-bit latches) and has the # key get shifted in like any other, I don't think it's necessary to clear the latches since a circuit set for 1234# would only open when the last five entries were one, two, three, four, and pound, in that order. \$\endgroup\$ – supercat Jul 9 '14 at 15:16

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