7400 Logic Combination Lock
Reading Hackaday the other day I came across this article about a combination lock built from logic gates only. This reminded me of a similar circuit I designed and built at school for my electronics AS-Level and spurred me enough to dig it out and share it here
- A bank of momentary switches represents a keypad with ten digits (0-9), a ‘Reset’ and an ‘Enter’. This is similar to many mechanical door locks available.
- A bank of simple SP-ST switches act as a memory block and are used to store the combination. For the combination ‘1234’ simply close switches 1, 2, 3 and 4.
Inputting the correct combination and hitting ‘Enter’ will result in the output going high for around 5 seconds. In the circuit above the output is represented by an LED although in a real application this would most likely go to a transistor or relay to drive a solenoid for the lock mechanism.
How it works:
- A latch comprising of an Or and an AND on each input digit which holds high once the digit is pushed until the reset is triggered.
- An XNOR for each digit compares the value of the latch with the value of the corresponding ‘memory’ switch.
- A tree of AND gates evaluates the outputs of all digit blocks (the output of each XNOR) and outputs high if the combination was correctly entered.
- When the ‘Enter’ button is pressed:
- If the combination was incorrect an XOR gate is used to trigger the reset.
- If the combination was correct the a 555 monostable is triggered which holds the output high long enough for the user to enter the door.
- The moment the output of the monostable goes high, the reset is triggered.
The only real flaw that I see with this design is that you can enter the code in any order. However since many commercial mechanical locks have the same limitation I think I can live with it.
Does it work in real life?
I did actually build this thing many years ago,
but I cannot for the life of me find any photos of it. I can confirm however that it did work and have found the photo evidence!:
Additionally for those interested, you can download the circuit simulation from here and run it yourself in the quite fantastic Yenka simulation suite. Yenka is a free download and is free for home use.