Timer Trigger Question

Discussion in 'The Projects Forum' started by Old_Dog, Oct 13, 2013.

  1. Old_Dog

    Thread Starter New Member

    Oct 13, 2013
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    I'm trying to sort out a pump run-on timer.

    I've looked for 555 circuits - all the ones I find need an instantaneous trigger, either positive or negative, but my trigger will be persistent, so I'm struggling. I don't want the timer to stay latched if the trigger stays at 0V.

    For most of the time, the pump will be powered and the timer will see 12V. When the power drops, the timer will see 0V, at which point I want it to begin timing and run on for a few minutes. But it may stay at 0V for quite some time, and once the we have timed out I want the pump to stop.

    If power is restored, the timer needs to reset and only recommence timing if the 12V drops to zero again.

    I can deal with how to control the pump (by relay or solid state), but not with how to trigger / reset / trigger the timer based on just a 12V /0V signal.

    Can anyone help?

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  2. JohnInTX

    Moderator

    Jun 26, 2012
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    Check out his 555 pages here, particularly this one.

    There are also lots of threads here at AAC covering 555 driving relays etc. available for the searching.

    Have fun and welcome to AAC!
     
  3. Old_Dog

    Thread Starter New Member

    Oct 13, 2013
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    John,

    Thanks for the very prompt reply. Super forum by the way!

    I'd looked through just about everything on the forum before I posted the question and tried to figure it all out, but couldn't find any circuit that achieves what I want. Outside the forum, I must have puzzled over a hundred or more 555 circuits on the internet, but they all present the same problem.

    If I understand the 555 correctly, then Pin 2 (Trigger) .... "must not remain lower than 1/3 V+ for a period of time longer than the timing cycle....... if the pin is held low longer than that then the output will remain high until the trigger is driven high again"

    So, since my trigger might stay at 0V for several minutes, hours even, the pump would just continue to run and run. I can't find a circuit anywhere on the forum, including all the references and experiments - that doesn't need an short pulse trigger.

    I've clearly reached the limit of my talent. Can you help me with conditioning the trigger signal so that a long duration 0V will still give me a short duration timing cycle please?





    p.s. Just so I understand it correctly......... If I connect my trigger to input A on Bill's monostable circuit, will it work?
    i.e. long duration 0V = timer cycle then stop, long duration 12V = reset and wait for next 0V trigger?

    And finally, in this configuration, what would be the back current along the trigger when it is at 0V? I don't want to flatten the battery if the trigger stays low for a long period. I'm thinking it is 1.2mA (12V/10KΩ R3)

    Sorry to be so thick, you might have given me the solution but I can't see the wood for the trees.



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    Last edited: Oct 14, 2013
  4. elec_mech

    Senior Member

    Nov 12, 2008
    1,513
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    Welcome to AAC.

    You are correct - if the trigger input of a 555 is held low beyond the timing cycle, the output of the 555 will continue to be high (ON) for as long as the trigger is held low.

    What you want is referred to as an edge-trigger. This will only act on a change to the trigger.

    Take a look at this then let us know what questions you may have.
     
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  5. Old_Dog

    Thread Starter New Member

    Oct 13, 2013
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    Yes! That's it. Almost.

    The fog is clearing at last. I can use this as an edge trigger to kick the timer off. However, the trigger voltage will also swing back up to 12V when the pump is under normal power. Which prompts 2 questions:

    a) Will this be okay, having 12V (via a cap) on the trigger Pin 2? I'm assuming it isn't a problem, I can't see one myself but as we've established, I'm out of my depth.

    b) How do I use this 12V to reset the timer? Bearing in mind that I can't just connect it to the reset pin, because sometimes it will be sitting at 0V.
    I hope I've explained this okay.........

    • Trigger at 12V for most of the time - no action.
    • Trigger drops to 0V, sets off the timer as an edge pulse for a one-shot period then the timer shuts off.
    • Trigger stays at 0V, nothing happens, no big leakage backward, battery doesn't go flat, quiescent current is low.
    • Trigger returns to 12V - timer resets ready for the next timing cycle.
    I've decided I'm going to control the pump through a relay, so if it helps I can use a spare pair of contacts on the relay for the reset.




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  6. JohnInTX

    Moderator

    Jun 26, 2012
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    It should. The capacitor couples the edges to the input, removing the DC component. When the input goes high to low, a single low-going spike will appear at pin 2 then pin 2 will return to a high level regardless of the input DC level. A high-going spike will also appear when the input returns to a high level and for this reason I would consider a Schottky diode from pin 2 to Vcc to clip the spike (cathode to Vcc).

    Yes but, if that is an issue, so will the power supply current for the LM555 of ~10ma. Consider an LMC555 (CMOS version, ~150uA). The input currents are much lower as well so the pullup resistors can be made much larger. The CMOS part has a lower output current though...

    Most 'edge triggered' 555 circuits have some sort of capacitive coupling as shown in the various references here. I'm not generally a fan of such things due to noise sensitivity and other things but lots of designs use it with success.

    Good luck!
     
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  7. elec_mech

    Senior Member

    Nov 12, 2008
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    I've thrown together a circuit. I don't know how long you want the timer output on for, so you may need to change R1, VR1, and/or C5. As long as you plan to use 12VDC to power the circuit and your relay coil uses 12VDC, you should be good.

    John knows far more than I do on this subject, so I suggest deferring to him. If someone can suggest a specific common Schottky diode, I'll happily add it to the schematic.

    As John mentioned, depending on the 555 you choose (yes, there are a few flavors with different power ratings among other specs), you may be limited in the power going to the coil. If you could let us know the make and model of the coil you plan to use, we may suggest adding a transistor. Depending on the power requirements of your load, you may be able to get away with a transistor in place of the relay.

    Look at the timing diagram shown below to verify it works as you expect. Note the timer ON time is always the same.
     
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  8. Old_Dog

    Thread Starter New Member

    Oct 13, 2013
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    Yes, that's pretty close and the variable resistor means I can trim the time. I'm guessing it will need around five minutes, maybe a smidge longer and as you say, I can play with the timing components. I know that 300 seconds is a bit of a stretch for a 555 but it doesn't have to be absolutely perfect.

    I'm very grateful that you took the time to sketch something for me.

    Sadly however, when I look at the timing chart, it doesn't quite achieve what I'm looking for. As soon as the trigger goes back up to 12V, I need either:

    a) To stop timing and drop the relay, then reset the timer ready for the next 0V trigger pulse

    or

    b) The 12V trigger just keeps the relay continuously latched (but at the same time, it resets the timer ready for the next 0V trigger /5 minute run-on).

    Is there anyway I can use a positive voltage on the trigger to reset the timer?

    The PC I'm using right now doesn't have any useful software for me to draw up the sort of timing diagram I'm looking to acheive, but I can draw one tomorrow when I'm back at my desktop if this helps.


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  9. JohnInTX

    Moderator

    Jun 26, 2012
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    You could probably do all of that by using a CMOS inverter (CD40106 etc) or transistor inverter with its input connected to the DC side of your control signal. When the 12V control signal is high, RESET/ of the 555 is driven low, resetting the timer. When it goes low, RESET/ is released and the 555 is triggered by the cap-coupled input on pin 2. This setup has the added bonus of being somewhat transient resistant as the control signal has to be present throughout the timing cycle.

    Not sure what you are saying here.
     
  10. Old_Dog

    Thread Starter New Member

    Oct 13, 2013
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    I'm not sure I'm explaining it well either. Thanks so far, I'll post an idealised timing diagram tomorrow, then maybe you can give me a final steer.
     
  11. elec_mech

    Senior Member

    Nov 12, 2008
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    I think the schematic below achieves this, but it is untested.

    To do what John describes, simply remove D4 and R5 completely and remove C5 but short the connection where C5 was. My only concern with this is a race condition - we're relying on reset to trigger just before the trigger input.
    I haven't used 555's like this before, so I'm not sure.

    I was thinking along the same lines, but added an edge-trigger to the reset input. In this way, the reset will only trigger when the input signal goes high and the timer will only trigger when the input signal goes low. That's the theory anyway. It's late and I'm exhausted, so I may be way off. As always, I invite feedback/improvements. I've also added the Schottky diodes for the edge triggers that John suggested.
     
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  12. ronv

    AAC Fanatic!

    Nov 12, 2008
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    I think it will work, but C5 needs to be removed.
     
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  13. Old_Dog

    Thread Starter New Member

    Oct 13, 2013
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  14. elec_mech

    Senior Member

    Nov 12, 2008
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    Thank you for the advice. I was hoping it would act as a power-on reset, but I was worried it might have negative effects with everything else on the line. I've removed it.

    Yes, in theory, the circuit below will do just that. Good luck and let us know if you run into problems.
     
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