# Problem triggering a NE555 monostable clock with an input signal

Discussion in 'The Projects Forum' started by old_owl, May 11, 2009.

1. ### old_owl Thread Starter Member

May 3, 2009
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0
The basic idea is that I want a monostable clock to be triggered when a certain input signal is high. I know no other way of doing this than to hook up the signal to power, however the clock never triggers doing this.

Right now, I have power hooked up to pin 8, a resistor connecting pins 8 and 7, a capacitor connected from pin 7 to ground, a wire connecting pin 7 and 6, a capacitor hooked up to pin 5 which is hooked to ground with pins 1 and 4 being tied to ground and power respectively.

http://www.datasheetcatalog.org/datasheet/stmicroelectronics/2182.pdf

I figure that there must be a correct way of doing this, but I have no idea what it might be.

2. ### Ron H AAC Fanatic!

Apr 14, 2005
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Where does your input signal come from? How wide is the monostable output pulse?

3. ### old_owl Thread Starter Member

May 3, 2009
10
0
The input signal is produced from a pretty large circuit. However, I do know that the input signal is acting exactly as it should be and I'm hooking it up to the power pin right now.

As for your second question, I'm not quite sure what you mean. My problem is that the output of the clock is low no matter what the input signal is. I've tried using both small and large resistors but it is to no avail.

I'm not sure if I've made it clear enough in the first post but I will try to explain here: I want the 555 timer to pulse when a certain signal is high. I've tried hooking this signal up to power but that does not pulse at all.

4. ### Bernard Expert

Aug 7, 2008
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If you have pin 4 tied to ground ,output will be permanetly reset-ground.; tie to +.

Last edited: May 11, 2009
5. ### old_owl Thread Starter Member

May 3, 2009
10
0
Pin 4 is set to power.

6. ### CDRIVE AAC Fanatic!

Jul 1, 2008
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This is how you should be triggering the 555.

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7. ### Ron H AAC Fanatic!

Apr 14, 2005
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Do you mean you want it to oscillate when the input is high? You originally said it was a monostable. A monostable is also called a one shot. It produces one pulse of a fixed width (time) each time the input is pulsed.

8. ### old_owl Thread Starter Member

May 3, 2009
10
0
I mean pulse as in a single pulse (i.e. monostable).

9. ### Wendy Moderator

Mar 24, 2008
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IMO, CDRIVE post (#6) is the way to go. Normally the 555 triggers on the negitive going edge, so you have to invert it.

10. ### old_owl Thread Starter Member

May 3, 2009
10
0
1. Is the circuit in CDRIVE's post retriggerable?
2. If it is, would you mind drawing a more clear schematic (i.e. showing all pin connections)?
3. If it isn't retriggerable, what recourse do I have?

11. ### Wendy Moderator

Mar 24, 2008
21,477
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The base 555 monostable is not retriggerable. Is this what you need?

If so we could redraw a schematic, basically put a transistor across the capacitor on pin 7, so it would discharge when the base recieved a trigger signal.

The 555 is very open ended, you just need to define what you want precisely before you use it.

12. ### old_owl Thread Starter Member

May 3, 2009
10
0
I want a single pulse whenever the input signal goes from low to high.

13. ### Wendy Moderator

Mar 24, 2008
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OK, but one of the concequences of retriggerable is if the input is held at a high the pulse is started, but not timing. It's what I mean by defining what you want. A simple cap on the input will eliminate this, but you have to define it.

Definition of retriggerable, the timer will start timing from scratch every time a new pulse is recieved. Side note, if the input is held high the timer doesn't start timing until it goes low, this can be designed out though.

14. ### old_owl Thread Starter Member

May 3, 2009
10
0
I'm not sure what you mean in the bolded sections. Thanks, by the way, for remaining patient.

15. ### Wendy Moderator

Mar 24, 2008
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I believe this will do what you want...

Looking at it, I'm going to redraw this sucker... BRB.

• ###### retriggerable 555.GIF
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Last edited: May 12, 2009
16. ### Wendy Moderator

Mar 24, 2008
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This will do it (I'm drawing these on the fly)...

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17. ### CDRIVE AAC Fanatic!

Jul 1, 2008
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Yes, it's retriggerable.
There's no reason to redraw the whole circuit because Bill posted a link in post 9 that will give you more than you need.

Nice job Bill!

EDIT: It's retriggerable after the RC timing components have completed their pulse cycle.

Last edited: May 12, 2009
18. ### Wendy Moderator

Mar 24, 2008
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Part of the reason I went through definitions is retriggerable in the world of monostables has a different definition than the OP may have been thinking.

Forrest Mimms had a circuit that was a missing pulse detector. I need to look that sucker up sometime, it did something similar.

19. ### Ron H AAC Fanatic!

Apr 14, 2005
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The cap in the base circuit needs a discharge path. A resistor to GND from either end of the base resistor will do the trick.

20. ### Ron H AAC Fanatic!

Apr 14, 2005
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Old owl, have you looked at CD4538? It is a retriggerable monostable.