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NE555 as Monostable – Time ON

We can now reason some aspects of timing issues.

First of all consider the fact that we have assumed implicitly a fact: the trigger impulse duration is far quicker than the tON duration.
Maybe the previous diagram has helped to consider this, but it can be an insidious trap.
Nobody can know in advance the trigger mechanism, in particular its timing feature: in other words, R and C must be chosen in order to guarantee the trigger impulse is less than output duration.

If you noticed that the output doesn't return to low level (even with the circuit correctly connected), it could mean that the trigger input is much longer than tON, with the result that S=1 remains so even after R passes form 1 to 0, forcing high the output; in this way, by getting S=0 again, we simply confirm the previous state (according with the truth table), so keeping the high output.
Based on the situation you could have to change the trigger mechanism (too slow) or the RC couple (too fast).

But what about if RC is enough so that we can put more than one trigger impulse during the tON output?
Simply the circuit won't be affected by this, keeping the output high for all the tON time, the end of which signs the return of output to the low state.

Commonly you can find expressions like "the flip-flop does not latch the trigger impulse" or "latch is insensitive to any new trigger input during charging time".
While practically right, they subtly are incorrect.


These expressions seem to say that the latch cannot accept the new inputs: false.
Simply by looking at table of truth we have that while cap is charging, then R=0 until it reaches 2/3 Vcc.
So aside from the first good trigger pulse, the successive ones continuously alternate S=1 (closing switch) and S=0 (opening switch), which respectively put output high and keep the previous state (high).

Externally nothing changes, but the latch continuously accept inputs because it cannot do anything else, otherwise it could be able to refuse even the first good trigger impulse.
The result for now is that once the output is high, it remains so until the charging phase hasn't been completed!
It doesn't start a new one-shot cycle (Non-retriggerable)!

So... if we wanted to make the tON restart at every trigger impulse?

The answer is in the discharging of capacitor, as we'll see next time.