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Timer circuit help needed please.

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btb45

New Member
I'm making a small project that will power a latching relay. I've got working using the normal circuit for a 555 timer;

10129-5367pg.gif


which I'm sure you all know works like this;


10130-dpg45l.gif


I now want to modify it so that it needs 3 trigger pulses within 5 seconds before it changes state. I don't know if I can do this with the 555 as it is, or if I need to add some counter to trigger the 555. Can anyone help me and suggest the simplest way to achieve this?

Thank you.
 

crutschow

Well-Known Member
Most Helpful Member
You could use a 2-stage (or larger) counter and a second 555 one-shot with a 5 second period.
This one-shot is connected to enable the counter when high and reset it at the end of the period, when this 555 output is low.
The trigger is connected to this one-shot trigger input and the counter clock.
The first trigger pulse triggers this one-shot and and also the counter clock. Additional trigger pulses will cause the counter to count but will not affect the one-shot period.
You use a NAND gate to detect count 3 of the counter and connect the output of the NAND gate to your original 555.
If the counter counts to three before the end of the 5 second period, then the NAND gate output will go low and trigger the original 555. Otherwise it is not triggered.
 

btb45

New Member
Thank you, that sounds like what I need to do. I've searched here for '2 stage counter' and not found any result that helps me get anywhere, do you know where I could find some example circuits? I think I would be able to work out how to use a NAND gate to to take the output of the counter and the first one shot to trigger my original circuit, but I feel out of my depth with the counter and detecting the third pulse. It such an interesting hobby to get into, though frustrating as I don't have any electronic background.
 

btb45

New Member
Well I've thought about it a bit more and wonder if this idea would work. What if I have 3 of 555 timers as in my first circuit, the first timer will change state on the first trigger, so I take its output and via an AND gate use it to trigger timer 2 the other input of the AND gate will be the same trigger source. This will change state on the second trigger pulse. I then take this output and again AND it with the same trigger source to a third 555, so this will change state on the 3rd pulse.

I think the only thing I have to be careful of is the there is a slight delay in each 555 triggering, so that they changes state after the trigger pulse has gone, or all the timers will just respond to the first trigger. Does what I'm proposing make sense, is it a way I can achieve a change of state on pulse 3? Should I be aiming to NOR them instead so they trigger on the rising pulse?

I'm sure this is all simple well know design, but I feel stuck with the logic states and the timing of the one shot compared to the trigger.
 

btb45

New Member
This does seem to work as a simulation, but it feels clumsy and I'm sure over complicated.

I've used a generator to simulate the pulses, and the top output changes state with pulse 3

10138-w7zvja.jpg


Can anyone help me simplify it?
 

ericgibbs

Well-Known Member
Most Helpful Member
hi btb,
I have used this type of circuit OK.
Set the 'vc' charge up times to suit your pulse ON times

EDIT:
improved plot
 

Attachments

  • 2N6027_555a.asc
    3.4 KB · Views: 104
  • AAesp05.gif
    AAesp05.gif
    32.3 KB · Views: 219
Last edited:

btb45

New Member
Thank you Eric, that looks very promising, much simpler than mine. I loaded your asc file in Ltspice but it can't find the component PUJT 2N6027. Do you have the component 2N6027 that I could add the Ltspice library please? Thank you.
 

btb45

New Member
hi btb,
I have used this type of circuit OK.
Set the 'vc' charge up times to suit your pulse ON times

EDIT:
improved plot

Eric, I like your circuit design, could you let me have the model for the 2N6027, or otherwise how to get it into the component library? Does anyone know how to add this PUT to LtSpice? Thank you.
 

crutschow

Well-Known Member
Most Helpful Member
Thank you, that sounds like what I need to do. I've searched here for '2 stage counter' and not found any result that helps me get anywhere, do you know where I could find some example circuits?
True, they don't make 2-state counter IC's. You can either use a dual flip-flop connected as a counter or a 4-stage (4-bit) counter of which there are many IC's available such as the CMOS CD40160B, CD40161B CD40162B, and CD40163B.
 

ericgibbs

Well-Known Member
Most Helpful Member
Eric, I like your circuit design, could you let me have the model for the 2N6027, or otherwise how to get it into the component library? Does anyone know how to add this PUT to LtSpice? Thank you.

hi,
Try this.
 

Attachments

  • 2n602.zip
    1.3 KB · Views: 97
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