The pulse detector is categorized as a mono-stable multi-vibrator because it has only one stable state. By term stable, we mean to say a state of output where the device is able to latch or hold to forever, without any external prodding. A latch or a flip-flop, being a bi-stable device, can hold in one state either "set" or "reset" state for an in-definite period of time. Once it is set or reset, it will continue to latch in that state unless impelled to change by way of an external input. A monos table device, on the other end, has the only capacity to hold in one particular state indefinitely. Its other state can only be held momentarily when triggered by an external input.
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A mechanical analogy of a mono-stable device would be a momentary contact pushbutton switch, that spring returns to its ordinary (stable) state when pressure is removed from its button actuator. Likewise, a standard wall (toggle) switch is a bi-stable device. It can latch in one of the two modes: on or off. All mono-stable multi-vibrators are timed devices. That is, their unstable output state will hold only for certain lowest amount of time before returning to its stable state. With the semiconductor mono-stable circuits, this timing function is naturally achieved through the use of capacitors & resistors, making the use of the exponential charging rates of RC circuits. A comparator is sometimes used to perform comparison of the voltage across the charging or dis-charging capacitor with a steady reference voltage, and the on (off) state of output of the comparator used for a logic signal.
