Hi everyone,
I'm working on an LTspice simulation with an NE555 timer in monostable mode. I want the timer to start counting when the trigger goes to 0V, keeping the output at 5V for 2 seconds. If the trigger remains at 0V after this time, the output should switch to 0V. However, if the trigger returns to 5V before the 2 seconds are up, the output should remain at 5V without switching. The issue is that even if the trigger goes back to 5V, the timer still completes the cycle and switches to 0V. How can I properly configure the circuit so that the timer only completes when the trigger remains at 0V for the full duration? Thanks!

 

Hi Paula,
Welcome to AAC.
Please post your LTSpice asc file.
E

 

Thank you for your reply.
This is my current circuit, but it is not working as expected. I want the timer's output to go to 5V when the trigger is at 0V and remain there for a duration t. If during this time the trigger returns to 5V, the timer should ignore this change and stay at 5V, without resetting or affecting the counter.
My goal is to send the timer's output to a counter. The counter should only register a transition from 5V to 0V if the trigger has remained at 0V for t seconds. However, if the trigger goes back to 5V before t is completed, the timer should not generate an output change, and the countdown should reset.

 

Hi Paula,
Downloaded your circuit, I will check it out and post back.
E

BTW: Is this a homework or college assignment?

 

Thank you so much.
It's for a college course.

 

Hi Paula,
As this college work, I cannot post a working circuit.

As I understand the problem, the push Trig switch operator will have to press and hold down the Trig switch for say 2 Secs [ the same time period as the Mono] in order to generate a Clock pulse for the Counter input, is that correct?

Is pressing the Trig switch longer than the Mono time acceptable, or must it be exactly the same time period as the Mono???

If yes, you will require additional logic adding to your circuit.
E

 

Thank you for your reply! I’m currently studying this topic in a course, but it’s not for any specific assignment.
What I want to achieve is that when the trigger input goes to 0V, a 2-second timer starts. After those 2 seconds, the input needs to be checked again: if the input is still 0V, the output should switch to 0V; if the input has returned to 5V, the output should remain at 5V. The key requirement is that the output is only updated after those 2 seconds, based on the state of the trigger input at that moment. Do you have any suggestions on how to implement this efficiently?
Thanks in advance!

 

Hi Paula,
The circuit operation you are stating sounds like a standard Mono configuration??
Is this example in line with your requirement?
E

 

The key requirement is that the output is only updated after those 2 seconds, based on the state of the trigger input at that moment. Do you have any suggestions on how to implement this efficiently?

Your requirements are not complete.
What happens when the input is high at 2 seconds so the output stays high, and then goes low after that (which normally would trigger the 2 second pulse)?

 

Hi, thank you very much for your response.
However, in this model, the timer starts counting when the trigger transitions from 0V to 5V. I would like it to work the other way around—starting when the trigger goes from 5V to 0V. How can I modify the circuit to achieve this?

 

Hi Paula,
The circuit operation you are stating sounds like a standard Mono configuration??
Is this example in line with your requirement?
E

View attachment 343870

Hi, thank you very much for your response.
However, in this model, the timer starts counting when the trigger transitions from 0V to 5V. I would like it to work the other way around—starting when the trigger goes from 5V to 0V. How can I modify the circuit to achieve this?