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Launch ICBM

The "two guards" launch protector. As you are probably alone, the two launch buttons are made a toggle buttons and placed close so you can press them in a rapid succession. The schema passes the launch signal if the time gap between clicks is less than the delay time in the delay element
The two guards system is the system that allows some highly critical operation only if initiated by the two operators, each turning his own key in its matching keyhole, at the same time. This ensures that a single person (probably intruder) cannot initiate the operation even when possessing the both keys. You have probably seen this in Terminator 2, where turning two keys simultaneously opens the high security door. There are some unverified talks that the intercontinental missile launch sequence may include a similar authorization.

While probably we do not plan to build missiles, high security doors are probably acceptable, and it may be interesting to discuss the design. The two guards (or operators) must turn they keys at the same time. Surely, as the guards are not machines, they just cannot turn the keys exactly at the same time, some very short disagreement time should be allowed. The XOR gate suits well as a disagreement detector, producing signal when its inputs disagree (01 or 10). But we also need some flip-flop to keep the activation suppressed also after the inputs finally agree. This flip-flop can be automatically reset after both keys are turned to the "off" state.

The activation signal is trivially obtained using the AND gate but it must be delayed to allow the previously mentioned XOR gate and flip-flop to detect the disagreement and put veto. I the example shown, this delay is the double of the disagreement time.

This probably would be all, but... when testing such a system, I have observed that the activation LED may transiently go on when we are returning from the "disagreement lock" state to the initial state. Indeed, after we switch both keys to off, there is no longer disagreement but the activation delay line (the two delay elements in our case) may still hold the "logical one" in the input of the final AND gate. To prevent this, we have made this final gate a three input gate to make sure that the activation signal exists (direct input), activation signal persists (delayed input) and finally there is no veto from the disagreement detector.

To make sure that a single operator can anyway test our schematics, we have made both buttons toggle buttons and placed them close to each other. In order to get the activations signal (LED in our case), you need to press them both, with disagreement duration less that the delay time in the single delay line. The more realistic design (that also could be tested on the ordinary PC) would require the two computer mouses connected, so the both buttons could be really pressed near at the same time.