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Timer circuit help needed please.

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btb45

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

**broken link removed**

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


**broken link removed**

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.
 
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.
 
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.
 
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.
 
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

**broken link removed**

Can anyone help me simplify it?
 
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: 181
  • AAesp05.gif
    AAesp05.gif
    32.3 KB · Views: 278
Last edited:
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.
 
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.
 
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.
 
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.
 

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  • 2n602.zip
    1.3 KB · Views: 170
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