Need a 555 guru...

Discussion in 'General Electronics Chat' started by Mike33, Sep 2, 2013.

  1. Mike33

    Thread Starter AAC Fanatic!

    Feb 4, 2005
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    Hi!

    Here's one for the 555 guys...I need to set up a bouncefree switch for a model train application. Need to fire a snap switch off, and mechanical switching leaves too much a risk of burning out the coil. The device only requires a voltage pulse for a brief time to work. So, I can use a 555 as a one-shot that goes H for 1/8 second or so, and uses a power transistor or relay as the switching device. No problem there.

    The question: How can I 'block' the 555 from resetting for a couple of seconds after each timing cycle? This is to prevent repeated button-pushing, which would also damage the snap switch. I've thought of something in the reset line, maybe RC, to delay the reset, but don't quite know how it should be set up. If I can 'lock out' another button press for a few seconds, this project will be done. Thanks!
     
  2. djsfantasi

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    For the non- model railroad guys, a snap switch is a dual coil solenoid used to change the directions of the track in a track switch or turnout. One coil for one direction; a second coil for the other.
     
    Last edited: Sep 2, 2013
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  3. tubeguy

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    Yes, a second 2-3 second one-shot on the reset would work. Trigger the second one-shot through a capacitor. Use
    an NPN to invert the signal to the Reset Line on the 1st 555.

    Do you need a large number of these circuits? You can make one-shots from CMOS inverters, for example.
     
    Last edited: Sep 2, 2013
  4. tubeguy

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    Simpler idea:

    The 555 one-shot would provide the long 2-3 second delay pulse. Couple its output to your switch machine circuit through a CR (high-pass) network to provided the short pulse. Connect a diode from the 555 output to its trigger input to hold it high and prevent re-triggering during the one-shot period.

    The trigger switch will need to be connected through a resitor to make this work.
     
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  5. Mike33

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    Thanks, Tubeguy! Your idea seems right on track.
    What do you mean the trigger switch will need to be connected thru a resistor? After the HP network, to whatever device I'm going to use to switch? (it will either be a relay or power transistor).

    I need to make several of these, like 10...ugh...
     
  6. tubeguy

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    So, it sounds like you need the whole circuit, correct?

    What are the specs for the switch machines?
    These are dual-coil, correct?
    Do you already have a power supply that you would like to use?
     
    Last edited: Sep 2, 2013
  7. #12

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    Nov 30, 2010
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    A 555 configured as a one-shot will not accept a re-trigger until it has timed out. Just make the one-shot last 2-3 seconds.

    (I hope I read the question correctly.)
     
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  8. tubeguy

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    Egg on my face -- you're right. :)
     
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  9. Mike33

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    Feb 4, 2005
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    Hey, great replies, guys! Thank you.

    12 volt system. I believe the snap IS a dual coil deal. My father in law has one in the mail to me...it's his train, I help him out trying to improve it. He's burning them out by holding his momentary-on toggles too long, and retriggering them too soon. The coils are pretty sensitive. As soon as I mentioned a modified bouncefree switch, he was sold on the idea, ha ha.

    I am liking the idea of one 555 as a one-shot, 5s output thru HPF and diode to generate a fast pulse to trigger a relay or something (this works in LT Spice). Next issue is dealing with the two coil issue. It is either that, or you reverse the current...either way, seems sort of like a flip-flop kind of situation. So, more thought needed on the mechanics at that end!
     
  10. Mike33

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    Some research shows that the snap switches do have a common terminal, and a 'hot' terminal on either side, showing that they have 2 coils.

    So - will the HPF idea work if I were to connect the 555 to a flip-flop? In order to generate a very brief switching pulse. FF stays high, but HPF peaks and dies off...

    The most simple of ideas thus becomes complex, argh...it's all a learning process, though.
     
  11. tubeguy

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    Simple would be a one-shot circuit for each "coil".
    Are there 2 momentary toggles for each turnout machine, or 1 center-off type ?

    Turnout = lingo as mentioned earlier, to differentiate Model RR 'switch' from well, a switch.:)

    EDIT: Even small coils will need much more current than a typical flip flop
    can supply.

    What gauge is the model train ?
     
    Last edited: Sep 2, 2013
  12. Mike33

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    Hey, alright, Tubeguy, looks like you know what I'm dealing with!! :D
    My FIL is a great guy, but doesn't know how to express himself in electrical terms, hence my surprise at finding out I have TWO discrete circuits to deal with, not one, ha ha. I thought they latched, but nah, 2 coils, I'm pretty sure.

    Taking a stab, I'd guess he has them set up as 1 toggle, one coil. "Hit this one, track goes right...hit THIS one, it goes left". He told me he has like 20 toggles already on his panel, so doesn't want to go to pushbutton as I suggested. :confused: We'll be talking tonight.

    Yes, I think I'll need to board up a 556 and take 2 toggles to each board, sending separate pulses out to trip each snap switch. He already spent $60 on the darn switches...

    Quick Q: Do you have ANY clue what the max draw of those things is?? That will determine my choice of inexpensive transistor to switch, or pricey relay. I bet the 'operate' on 1, 2A but the inrush may be huge...
     
  13. tubeguy

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    The model gauge, and type of turnout snap switch will make a difference in the current requirements. One additional trick is to use a large capacitor on the power supply as a cushion or reservoir to handle the momentary current spike. When you receive the turnout snap switch, you can measure the ohms.
     
    Last edited: Sep 2, 2013
  14. ian field

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    Oct 27, 2012
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    IWHT a single 555 is sufficient to generate a single one-shot pulse - there are plenty of application examples (even whole e-books) online.

    If you have 2 solenoids to pulse - a 556 dual timer will save you a couple of pins.
     
  15. #12

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    Seriously. This is getting quantitative now. Must have measurements to get down to specifying actual part numbers.
     
  16. tubeguy

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    Yes, agreed ! See post #4. :):)
     
  17. Mike33

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    Well, not really...I can't just "pulse" the snap switch...the current requirements for it will exceed the 555's ability to source (200mA), plus I need to also lock out repeated attempts to activate the mechanical switch. If I had a 1/8 second output, for example, and the toggle was held for longer, it would trigger again (possibly, given 'bounce'). The output period is so short, I need to delay the 555 reset, if you will. I've used 555s a lot in the past, just not in this kind of 'demanding' application. I COULD use 2 of them, but am trying to find the most economical way to do this.

    The best idea I've heard is a long output pulse from the 555 which is also used to 'spike' something via HPF (power MOSFET maybe) very briefly, 'firing' the snap switch. Just remotely actuating a relay. And yes, I'd have to use a 556 to create TWO of the same circuit so you can trig each solenoid. I just have to decide on what kind of device to put at the end of it all, based on....the ohms (current draw) of the thing when it gets here.
     
  18. tubeguy

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    In the meantime, I just measured some basic N-scale Atlas snap switch machines at 18.6 to 18.8 ohms per coil. So about 2/3 amp at 12 volts.
    If yours are similar, maybe look for simple solenoid driver IC's with built-in protection diodes. May be available in 4 or more per chip.
     
    Last edited: Sep 2, 2013
  19. Mike33

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    That sounds like it fits the bill nicely, Tubeguy - thanks!
     
  20. ian field

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    You can avoid retriggering by using the capacitor discharge method: charge a decent size electrolytic via a resistor too high to activate the solenoid - when activated the solenoid activates and simultaneously spends the charge, it can't retrigger until the capacitor charges again.

    A small (TO92 or TO126) SCR would do the job, once the charge depletes and can't maintain the SCR holding current, it commutates and is ready for the capacitor to start charging again - you do have to make sure the charging resistor isn't low enough to sustain holding current.
     
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