# combination lock

Discussion in 'General Electronics Discussion' started by chagz, Apr 7, 2010.

1. ### chagz

7
0
Jan 4, 2010
I am currently working on an electronic combination lock circuit that operates as follows:
A user enters a four digit code that goes through a decimal-to-binary encoder. Because there are four input digits, it means that there are four identical comp[arator circuits. whenever an input is applied a pulse is sent to a counter circuit that switches the input from one comparator circuit to the next. This should be clear from the attached circuit. the problem is just that when the input of the fourth digit is applied to the fourth comparator - the counter returns to the first comparator and so implying that the fourth number is placed at the first input. The idea is to have a circuit that is always ready to accept the input from the user at any time, so I am having trouble trying to stop the fourth input from interfering with the first input. Can anyone help me on this one?
NB: The circuit has a facility for changing the preset code, using the SR flipflop near the bottom, which is working perfectly-it turns off soon after the fourth number is stored.

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2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,419
2,790
Jan 21, 2010
You've certainly picked a complex approach.

Is there any reason why you chose this way of doing it?

What is the reason you're doing this?

3. ### 55pilot

434
3
Feb 23, 2010
Unless this is a school project that requires you to do it this way, there is just no reason to go about doing it this way. Maybe 25 years ago this may have been one of the better options, but not today.

---55p

4. ### chagz

7
0
Jan 4, 2010
I understand that this is a very crude way of achieving the goal. I could have used a microprocessor with even better results. As it stands, I am a second year student at NUST, Zimbabwe. so this is a school project. The idea is to first make a working circuit using ICs, then translating the circuitry so that it is made up of discrete logic NAND gates using combinational logic. Changing the circuit to NAND gates only(except flip-flops only) is not a problem at all since, from my circuit, some parts are repeated four times. So basically, the aim of the project is to show a deep understanding of combinational logic-which I can do. It is now out of interest that I feel that I need to perfect the operation of this circuit. I think the experience would greatly benefit me.