Monostable circuits are used to send out outputs of a specific length. This can be used in a wide variety of mechanisms, whether it’s simply opening a door for a specific amount of time or as part of a bigger, more complicated redstone circuit.
A pulse generator is exactly what it is, something that generates a pulse. The length of the pulse is determined by how fast the input is interupted, which is done by the active redstone torch in the picture below.
Pushing the button will deactivate the redstone torch, which will cause the off-state redstone torch to activate. However, the button will also activate the off-state torch, which means it stays deactivated. However, when the input is turned off, the off-state torch will turn on for a brief second, before it’s deactivated by the other torch again.
You can increase the length of the pulse by adding repeaters between the 2 torches.
A pulse sustainer is something that will take an input and extend it by a specific amount of time. It’s basically 2 different circuits, attached to the same input, which give off a signal at different times. The 2 signals will allign in such a way that the first signal ends when the second signal reaches the output.
As you can see in the image above, pushing the button will send a signal through 2 paths, one which has a repeater on 1 tick, and another which has 2 repeater on 4 ticks. The signal will reach the piston through the single repeater first, and as soon as that signal ends, the signal from the other 2 repeaters will have come through the repeaters and activates the piston as well, which means the piston will stay on longer than it would have if only the button was used.
A pulse limiter is also very straight forward. All it does is reduce the length of a signal, by cutting off the input. As you can see in the image below, pushing the button will deactivate the first torch, which will activate the second torch, which in turn activates the pison. However, the button will also send a signal to the repeater between the torches, which will send a signal to the second torch after a short delay, which will deactivate that torch and cut of the signal to the piston.
A monostable circuit is similar to a pulse limiter, as it will turn off a signal after a specific amount of time. However, unlike a pulse limiter, a monostable circuit will activate the output when the input is activated, but will only deactivate once the signal from the input has gone through a delay circuit.