Needing help with a 555 timer switch

[patsroom]

What I am hoping to do is get a one shot non-retrigger-able timer switch of one or two seconds. One that no matter how many time you try to press the on switch after the first time it will not pay attention to the other pressing of the button.Thank You for reading..................Pat

 

[(*steve*)]

A 555 is retriggerable.

one option is to have one 555 set up as a bistable which is triggered by your input and its output triggers the second monostable 555.

when the second minostable returns to its rest state it resets the first 555, allowing I to be triggered again.

 

[patsroom]

Thank you for your reply steve. I was think that if I did not connect pin 4 to the circuit it would not reset so that a new signal would not affect what it is doing until it's power was completely off to the 555 to re set the 555. And to control the time would be to use different resistor and or caps............Pat

 

[Herschel Peeler]

Both Reset and Trigger are active low inputs. They never go low in the circuit, If Reset and Trigger both go low at the same time will it ever trigger?

 

[Harald Kapp]

What I am hoping to do is get a one shot non-retrigger-able timer switch of one or two seconds. One that no matter how many time you try to press the on switch after the first time it will not pay attention to the other pressing of the button.

How is the timer intended to be armed for another trigger? Will there be a reset button, will there be a timeout...?

Steve's post #2 would arm the 1st 555 after the timeout of the 2nd 555.

If you want a reset button, I suggest you use an RS flip flop which would be set by the trigger button, reset by a reset button. the output of the rs flip flop would trigger the 555 to genrate a single pulse when the flip flop is being set. Multiple pushes on the trigger button will not impress the rs flip flop unless it has been reset by the reset button.

 

[(*steve*)]

when referring to a monostable, "retriggerable" and "non-retriggerable" refer specifically to the behavior of the device if a trigger pulse arrives while the monostable is in its "unstable" state.

If the OP has his own definition of "non-retriggerable" which differs from the conventional meaning, it would be best if it is defined for us.

 

[Harald Kapp]

From post #3 I take it that the op's intention is a real single-shot until reset by power off-on cycling. Not the conventional meaning of 'non-retriggerable'.

 

[Herschel Peeler]

A 555 is retriggerable.

one option is to have one 555 set up as a bistable which is triggered by your input and its output triggers the second monostable 555.

when the second minostable returns to its rest state it resets the first 555, allowing I to be triggered again.

Is the 555 retriggerable? I don't think so.

 

[(*steve*)]

Is the 555 retriggerable? I don't think so

Yes. If you have a 555 set up as a monostable with a time period t and you send trigger pulses with < t between their falling edges, the 555 will remain triggered for ever.

if it was non-retriggerable, the output would change state for time t then trigger again on receipt of the next falling edge.

Actually its worse than that, the 555 is level sensitive, so while it is triggered on the falling edge, the time period does not start until after the rising edge. Typically you capacitively couple the trigger so this is not a huge issue.

 

[(*steve*)]

For the OP's case, the two 555 solution would work, removing the ability to reset the bistable would do the trick.

note that it is important that the bistable starts in the right state. An appropriate power on reset will ensure this.

 

[Alec_t]

Here's how you could do it with a CD4013 or other dual D-flip-flop :-