But that is literally the definition of a monostable multivibrator, no?
Yeah, I'm unsure now. I don't think these definitions are standardized and Wikipedia doesn't link to any sources for the monostable/bistable multivibrators. There seems to be some overlapping terminology between latches, flip-flops, multivibrators etc. Or sometimes you might have an application or use-case that doesn't fit exactly under one definition? Not really my area of interest to dive into these semantics. Maybe someone that studied electronics theory more in-depth would be able to clarify the terminology?
I did study electronics formally.
The monostable/bistable/astable terms certainly are well-defined, and their names hint at the distinguishing characteristic of each.
In this context , think of "stable" as meaning "happy to remain in this state".
An
astable multivibrator has
zero stable states. It is not "happy" in either state, so it will constantly switch between the two states. No trigger is needed. In other words, it's a type of oscillator.
A
monostable multivibrator has
one stable state. It's "happy" only in one state, and "dislikes" being in the other, and so it "wants" to go back to that state as soon as it can. When triggered, it temporarily goes into the "unhappy" state, but then eventually reverts to the stable state. In other words, a trigger event produces a pulse. (These come in retriggerable and non-retriggerable. Retriggerable ones allow the trigger to reset the timer at any time, producing a pulse that can be extended indefinitely. Non-retriggerable ones ignore subsequent triggers while in the "happy" state, meaning that they always produce a pulse of a fixed length.)
A
bistable multivibrator has
two stable states. It's happy in either state, so it will never switch on its own. When triggered, it switches state, but can stay in that state indefinitely, until triggered to switch again. In other words, it is a flip-flop or toggle. (Note that there can be one single toggle input, or there can be separate inputs to say "go to a specific state".)
As you can see, the first two are essentially timing circuits, while the third is more like a memory. They share a common topology, which is why they're all called "multivibrators", but the functions are quite distinct.
In the case of a latching continuity tester, you can easily see which one of these it is, by comparing its operation with all three types. The "trigger" is continuity.
If it were an astable multivibrator, it would be switching the beeper off and on constantly, with no need for a trigger. So it's not that.
If it were a bistable multivibrator, it would stay on indefinitely once triggered. So it's not that, either.
That leaves us with the monostable multivibrator, whose behavior matches what we see: a trigger, even a really short one, produces a pulse of a defined length. In the case of a continuity tester, it does extend the pulse indefinitely as long as the trigger continues, so it is a retriggerable monostable multivibrator.
Of course, in actual multimeters with latching continuity, they don't use a monostable multivibrator circuit, but just do it in software. But the principle remains the same.
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