555 one-shot trigger switch

vinodquilon · Dec 21, 2012

What would be the output of 555 1 second monoshot if I press the triggering push switch for 5 second ?

cowboybob · Dec 21, 2012

A one second duration, negative going square wave pulse from the the level of Vcc.

audioguru · Dec 22, 2012

cowboybob said:
A one second duration, negative going square wave pulse from the the level of Vcc.

No.
The output voltage pulse is positive, not negative going.
The output pulse from an ordinary 555 is 1.5V LESS than Vcc if it has no load current and is 2.5V LESS than Vcc with a 100mA load current.

jpanhalt · Dec 22, 2012

vinodquilon said:
What would be the output of 555 1 second monoshot if I press the triggering push switch for 5 second ?

From the datasheet:

Should the trigger be held low, without AC coupling, for a longer duration than the timing cycle the output will remain in a high state for the duration of the low trigger signal, without regard to the threshold comparator state. This is due to the predominance of Q15 on the base of Q16, controlling the state of the bi-stable flip-flop. When the trigger signal then returns to a high level, the output will fall immediately. Thus, the output signal will follow the trigger signal in this case.

In other words, if the trigger is longer than the one-shot output pulse, the output will stay high as long as the trigger is low.

John

cowboybob · Dec 23, 2012

Audioguru is correct, it's a positive going pulse .

Dyslexia has an odd way of manifesting itself sometimes.

However,

vinodquilon said:
What would be the output of 555 1 second monoshot if I press the triggering push switch for 5 second ?

jpanhalt said:
In other words, if the trigger is longer than the one-shot output pulse, the output will stay high as long as the trigger is low.

the OP did not specify the circuit being used.

That is the circuit I assumed he meant:

Trigger pulse duration is rendered moot with appropriate RC selection(s).

jpanhalt · Dec 23, 2012

That is addressed in the datasheet and is what that document calls AC coupling.

I felt it important to add what happens without AC coupling as in the common embodiment shown in the datasheet, which is what I assumed the OP was asking.

John

Mr RB · Dec 24, 2012

It depends how you wire the circuit? If you use the button to short the timing cap, the monostable period is the period the cap is charging after the button is released.

So you could set it up to have output ON for the period the button is held down, then + the monostable timed period after the button is released.

555 one-shot trigger switch

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555 one-shot trigger switch

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