Anti-terrorist Lock Project
Jennifer Lee
The pictures above are my beautiful exposed lock. This lock contains five IC chips, a JK flip flop (7476), nonretriggerable monostable multivibrator (74221), demultiplexer (74151), AND chip (7408), and an inverter chip (7404). There are two AND gates used and two monostable multivibrators. The monostable multivibrators were used as one shot triggers. One shot 1, as indicated on the lock schematic, contained a resistor of 56kΩ and capacitor value of 22µF for a activated time of 1.4 seconds. The second one shot has a resistor and capacitor value of 43kΩ and 47µF, resulting in a activated time of 2.22 seconds. The demultiplexer ensures that only a certain combination of the switches will produce an output from the multiplexer that will affect the rest of the lock. A red LED indicates that the lock is indeed locked, and a green LED is lit once the right combination is input. The lock is controlled by three single pull double throw switches. The last page of the report contains the lock schematic and timeline of the operation of the lock.
How it works:
The switches on the lock schematic are shown as they appear on the actual switch pad. There are three combinations of the switches that work to activate the lock. The three combinations are, in order and in abc format, (101, 001,010). Before analyzing the actual combinations, the first thing to consider when looking at my lock is that the demux sets the different outputs to high when not selected and low when selected. The inverter chip was used to remedy this problem so that the outputs of the demux would be high when selected and low when not selected.
The first combination to be analyzed is 1-0-1. When A and C are high, but not B, the selected output of the demux is set to low. This output triggers the first one shot, which controls several functions within the lock. The first function is the overall timing of the lock. When the first one shot is triggered, the user has 1.4 seconds to activate the rest of the combinations, or else the one shot will reset. The second function that the first one shot has is controlling the JK flip flop. The output of the one shot is set to the J input and the RST input of the JK. The purpose of the one shot being connected to both of these outputs is so that the combination 1-0-1 must be input before the JK can be clocked. When the one shot is not triggered, the reset on the JK is triggered constantly so that regardless of whatever clock comes into the JK, no high output will occur. Connecting the one shot to the J input ensures that when the one shot is triggered, the output of the JK when clocked will be high. The last function of the one shot is that it is set into an AND gate in order to ensure that all of the right combinations are triggered to unlock the gate.
The second combination that is needed for the lock is 0-0-1. This combination acts to trigger the one shot. As stated in the previous paragraph, the first triggered one shot enables the JK so that when triggered it will produce a high output. Once the one shot is triggered, using 0-0-1 combination the output is set to high for the rest of the time remaining on the one shot.
The last combination needed to unlock the lock is 0-1-0. This combination goes into the AND gate that includes the one shot signal. The combination of 0-1-0 has to be the last combination triggered for it is not controlled by any time regulating device such as the one shot or JK. Because the output of the one shot and the JK should be high, in order to produce a high output out of the second AND gate, and ultimately triggering the output that will unlock the lock.
Once the output of the second AND gate is high, the second one shot is triggered. Once the one shot is activated, the green LED and the buzzer will activate for a time of 2.22 seconds, indicating the lock has been unlocked.
Comments on my design:
I designed this lock the way I did by trial and error. Initially I had a good amount of AND gates ultimately triggering a multiplexer to an output. I then replaced the AND gates with a demultiplexer, which reduced the number of chips as well as made the wiring easier. The one thing I would change about my lock design would to be to connect all of the unused outputs of my demultiplexer to the clear on the first one shot so that at the instance a wrong code is input into the lock, the entire system would reset. In addition, I would probably decrease the time of the one shots. The first one shot I would decrease the time to make unlocking more difficult, and the second one shot I would decrease the time so that the buzzer doesn’t sound for too long. I found after testing my lock numerous times that the sound of the buzzer is not as pleasant as I initially thought.
The EWB simulation of my lock did not work due to the faulty functioning of the monostable multivibrator on EWB. Because my lock depends on the one shot to unlock it, I could not do an EWB simulation. Luckily my lock ended up working.
I was motivated to work hard on this project so that I could prove to the Board of Trustees that out of state students are superior to in state students and that our tuition should not be raised any higher than theirs. (I’m just kidding, I love my fellow in-state classmates). I tried to spend as much time in Phillips Hall working on my lock as John Edwards has working against poverty. I think I may have almost reached this goal.
My lock is awesome because it doesn’t need a cover to hide its secrets, it bares all for everyone to see, daring anyone to unlock it. That’s pretty much all I have to say about that.