I am trying to figure out how to create a combination lock using logic gates, so that a password must be input (let's say pressing numbered buttons) in the correct order.
I understand logic gates completely, and have some of a grasp on flip-flops and latches, but I'm definitely getting stuck on how to use them in the right combination.
So as an example, if there are 3 buttons (call them 1, 2, & 3), I would like to be able to set a password such as 1 2 3 1 (hardcoded is fine), where they have to be input in that exact order, otherwise you have to start at the beginning again (1 2 3 3 3 3 1 will not work).
Any tips, links, hints on this?
Best Answer
I would probably use two bits to encode the three switches, and the general approach would be to have a string of pairs (one for each bit) of D or JK flip-flops to accept the sequence of input digits, essentially a shift register for pairs of bits. Then there would have to be de-bounce circuitry for the switches, and some way to generate a clock signal from the key presses (probably on release) to shift the digits through the register. Finally, XOR gates between the stages of the shift register and the digits of the key (hardwired to 1231 or use switches, etc) would produce signals to indicate when each digit is a correct match (logic Low on match), and then a multi-input NOR of the XOR outputs would give a final output signal that would indicate when the entire sequence in the shift register matches the key.