Tutorials/Combination locks

Combination locks are a type of redstone circuit. They generally have a number of components which must be set in the right combination in order to activate something such as a door. Combination locks can be very useful in creating adventure maps. Note that if you are playing in survival multiplayer, other players will still be able to break into the mechanism and cause it to activate without knowing the password.

Lever lock
A lever lock is one of the easiest redstone locks to create. Each lever doubles the number of combinations the lock has. The possibilities can be found using 2 ^ n, where n is the number of levers used.

When constructing, you will probably first want to place the levers and put them into the desired positions. Next, you should place the redstone torches and repeaters. Torches go behind levers that are down and repeaters go behind levers that are up. When you are done, all of the torches and repeaters should be turned off. If not, double check any components that are turned on.

Item frame lock
Item frame combination locks are a step up in complexity from lever locks. Items in item frames can be rotated into 8 different positions. This rotation can be detected by a comparator which will output a signal strength from 1 to 8 depending on the position. Since each item frame has 8 possible states, the number of possible combinations can be found with 8 ^ n, where n is the number of item frames.

The redstone paths on the floor decrease the signal emitted by the comparator down to a fixed strength of 1, and then feed it into an "A OR NOT B" gate that is true if the strength is either too small (B is not powered, i.e. false) or too large (A is powered, i.e. true). These gates are all ORed together so that the overall result is true if any item frame in the lock is misplaced; finally a torch (optional) inverts the result.

If you want to make it extra tricky, it is also possible to detect an empty item frame by when a comparator doesn't output anything. Simply leave a repeater connected to the comparator and feeding into the vertical redstone wire on the right side; a filled item frame will give a signal strength of 15 out of the repeater and keep the door locked.

RS NOR Combo Lock
Connect a series of buttons to the S-input of RS Latches. Feed the Q or Q (choose which one for each latch to set the combination) outputs of the RS Latches into a series of AND gates, and connect the final output to an iron door. Finally, connect a single button to all the R-inputs of the RS Latches. The combination is configured by using either Q or Q for each button (Q means that the button would need to be pressed, Q don't press).

Example:



With the automated reset it causes the correct combo to cause a pulse instead of a "always on" until reset.

AND Combo Lock
The AND gate based combo lock uses switches and NOT gate inverters instead of the RS NOR latches in the previous design. This makes for a simpler design but becomes less dynamic in complicated systems and it also lacks an automated reset. The AND design is configured by adding inverters to the switches. Example:



OR Combo Lock
The OR combo lock is actually an AND combo lock without unnecessary repeaters, override lever and last inverter. Output is off when the code is correct.

Due to its compact size and fast response time, this combination lock is also ideal for use as an address decoder in the construction of addressable memory (RAM)

Design A. Code is set by torches on inputs (1001):



It is possible to remove the spacing between the levers by replacing redstone wire behind the levers with delay 1 repeaters.

You can expand on this by creating a new level on top of the first and using the same principle as the first level, keep creating them.

Design B. Code is set by inverters in the blue area (001001):



N = number of inputs. K = number of 1's in code.

Order-sensitive changing code XNOR Combo Lock
Opens when a certain order of switches are pressed. You can change the order. (Note: A moderate understanding of logic gates is needed for this device.) Have 4 blocks near each other with a switch and a sign saying 4,3,2 or 1 respectively. 10 blocks to the right and 2 blocks down place a block then place 2 more with a 5 block space. 6 right and 3 up place the block. Label them 4-3-2-1 respectively. Have a 11 block wire or 13 for left to a repeater on the 9th block or 11th for left and on the right side. Place a repeater 2 blocks over with the same wire from it. Connect the left repeaters. to the code changing module. (You may use bridges of cobblestone for getting over other wire and repeaters for boosting the signal. Construct a XNOR Gate where). 2 of the wires meet. Connect to adjacent outputs with AND gates. These Outputs are connected to final AND gate. Final AND gate if connected to iron door.

Order-sensitive RS NOR Combo Lock
A door that opens when buttons are pressed in certain order.

(Note: A moderate understanding of logic gates is needed for this device.)

Make a series of buttons, and connect only one to an RS NOR latch. Then connect both the RS NOR latch and a second button to an AND gate, which feeds to another RS NOR latch. Do this continually until you have either filled all of the buttons or are satisfied with the lock. Connect the final RS NOR latch to a separate AND gate with a signal from an enter button. Feed that to the output RS NOR latch. Then connect any of the left-over buttons to the enter button and send reset signals to all of the RS NOR latches. A pressure plate next to a door can reset the door. This type of lock has severe limitations its security. For example, not all the buttons could be used in the pin or there would be nothing to reset the system.

For a lock that can have a combination of any size, using all the buttons, and still have a wrong entry reset the system, you need a different way for it to reset. To construct this, hook up a panel of buttons (any number, but four or more is preferred) to a parallel series of adjacent repeaters. Invert as necessary so that all the repeaters are powered and are unpowered by the press of the corresponding button. These repeaters power a row of blocks. On top of the blocks, place a torch corresponding to the incorrect buttons for the fist number in the PIN. For the correct button/number, place dust under the powered block which leads to a RS NOR Latch. Place a row of blocks above the torches for the incorrect buttons, with redstone dust on top. Then connect this dust to the reset of the first RS NOR Latch. Only the correct button will set the RS NOR Latch and all others will reset it. Connect the output of the RS NOR LATCH to half of an AND gate. After the first row of blocks with the reset torches, place another row of repeaters and another row of blocks. Again place torches for the incorrect buttons and dust under the correct button's line. Power will be fed from the buttons through the rows of repeaters and blocks for as many rows as there are digits in the PIN number. Connect the dust from the correct button to the other half of the AND gate coming from the first RS NOR Latch. Only if the two conditions are met, that the first button was pushed correctly, setting the first RS NOR Latch, and the second button is pushed correctly will the AND gate send a signal to set the second RS NOR Latch. Again, connect a reset line from the incorrect button's torches to the reset of the second RS NOR Latch. Delay the reset signals by one full repeater to give time for the next RS NOR Latch to be set before the reset happens. Continue building the array in the same manner until you reach the desired number of digits. In operation, when a button is hit, each RS NOR Latch checks (through the AND gate) to see if the previous RS NOR Latch is set, and the correct button for this RS NOR Latch has been pushed. Only when the correct buttons are pressed in order, will the signal progress through the conditional RS NOR Latches to the end. Connect the output of the last RS NOR Latch to a door and attach a line to a pressure plate inside the door to reset the last RS NOR Latch.

Tutorial Video
Combination Lock RS NOR

There is also another way to make order-sensitive combination locks. It is based on several RS NOR latches that is placed on a row. The RS NOR latches are connected together, and each latch is connected to one button. The combination lock opens when all the latches are activated. To activate all of them, the latches have to be activated in the right order. If wrong button is pressed, the lock automatically sends a reset signal to the first latch, and resets the entire lock. The circuit also has a T flip-flop that controls the output. The T flip-flop turns on and stays on when the right combination is pressed. When the lock is open, all the buttons works like a reset button. This makes it easy to close the door from the outside. Just press a random button. It is also possible to connect buttons that overrides the lock and makes the output signal toggle like on a normal T flip-flop.

Combination lock with order-sensitive reset
This is an order-sensitive combination lock with order sensitive reset function. It works like an ordinary order-sensitive combination lock, but in addition it has a function that resets everything when a button is pressed too early. The function consists of AND-gates that sends a reset signal if the previous button hasn't been pressed yet. The lock does not need a reset button because it resets automatically when the code is wrong.

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