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Pinball machine - switch closure timing problem

Discussion in 'Electronic Design' started by frenchy, Mar 22, 2005.

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  1. frenchy

    frenchy Guest

    I'm posting this to a few electronics newsgroups poking around for a
    possible answer...

    For years the pinball companies used a small .047 uf capacitor across
    some switch terminals for switches that could be closed and reopened
    very quickly like a target getting slammed particularly hard. The
    capacitor would increase the 'closure time' that the CPU would read the
    switch as closed, long enough to help it pick up the switch closure.
    These switches are just simple two blades and contacts, with one wire
    to the cpu, the other wire thru a diode to isolate it within the matrix
    of switches in the game, and the capacitor may or may not be across the
    terminals also.


    I'm having a problem with a particular swtich that the cap fix is
    working, but not completely. Some hardhits will still not register.
    Tried putting bigger cap on it with improved results but I've read that
    too big of a cap can cause 'ghosting' where it might start causing
    false closure reading on other switches in the matrix. So my question
    is....


    Is there a simple formula where one could increase the size of the .047
    cap or .1 cap or whatever is being used, but put in in series with a
    resistor, to lengthen the time that it is doing it's thing of
    lengthening of the closure to the cpu? I.e if I wanted to use a cap
    10x bigger, what resistor could I put in series with it to be sort of
    like the smaller cap without the resistor, but length of discharge
    would be stretched out? Or is this possible? Maybe a diferent more
    complicated approach is necessary to get this one switch to be sensed
    as 'closed' longer. I am pretty good at fixing pinballs, but
    electronics theory is not my calling. thanks for any assistance
    anybody can give me. Thanks!
     
  2. Ken Taylor

    Ken Taylor Guest

    Take a look at such circuits as car dome-light extenders - basically you
    need to use the existing switch contact to trigger another relay (or MOSFET,
    whatever) via an appropriate timing circuit. It's "trivial" but the easiest
    thing to do would be to get a kit from a local hobby store (or on-line). I
    doubt that I'm in your area so maybe a suggestion from the gallery......?

    Cheers.

    Ken
     
  3. frenchy

    frenchy Guest

    Was hoping to do it by modifying the way it is done now with the cap.
    This switch closure only has to be extended in time by a fraction of a
    second to be able to be picked up reliably by the cpu. My thought was
    maybe since it can currently be extended with a small cap, maybe a
    larger cap could be used in conjuction with a certain value resistor so
    that it would still perform the same function, but 'bleed off' slower
    and extend the time. A last resort would be to hook up some kind of
    timed relay. Maybe a simple circuit that did the same thing with only
    a few parts like a simple transistor etc. would not be too bad, but
    have no idea on what it would need to perform the function of a 'timed
    switch closure'. thanks...
     
  4. Zak

    Zak Guest

    I'd think that the problem is the switch doesn't close very well, and
    doesn't discharge the capacitor. Maybe there is some resistance in the
    switch/cap path, maybe the old cap is broken, but most likely the switch
    doesn't make too good a contact.


    Thomas
     
  5. Mark

    Mark Guest

    i don't understand how there can be ghosting in this case....a bigger
    cap would be like the switch was held closed for a longer time.... if a
    bigger cap can cause ghosting, then holding the switch closed for too
    long would also cause ghosting it seems... i would try a bigger cap
    and see what happens

    Mark
     
  6. frenchy

    frenchy Guest

    gallery......?>>

    was hoping was possibly a simpler way of just extending using the cap
    method they already use (cap across the switch). Like bumping size of
    cap up and adding a resistor to 'bleed off' the cap slower (and
    longer.) Any other solution is welcome, something with solid state
    parts like maye a transistor and cap and resistor, preferably that
    would not need additional power supplly and could just be connected to
    the switch itself. thanks
     
  7. John Fields

    John Fields Guest

     
  8. James Beck

    James Beck Guest

    There are two separate switch systems used in most pinball machines, the
    switches that are attached to the switch matrix and what is usually
    referred to as "continuous" switches. A switch matrix allows a
    relatively small number of wires to read a large number of switches by
    stepping or strobing through a certain number of rows of switches. What
    they do is in effect move the ground from one bank of switches to
    another. In fact, since they all used 8 bit port on their systems the
    standard switch matrix was an 8 by 8 matrix, which allows the reading of
    64 switches with 16 control lines. These switches are by default
    debounced to what ever strobe rate the matrix works at. Adding a
    capacitor here can help, but only to a very small extent. Putting a
    large capacitor might actually cause the switch to get worse. It all
    depends on how good the strobe driver is. It is just a bipolar
    transistor and is no where near a great ground for the cap. If the
    switch you are having problems with is on the matrix then I wouldn't
    play with adding caps, I would look at things like contact resistance,
    contact gap, mechanical problems with actuation, or even the 1N914
    blocking diode that is on the switch.
    On the other hand the "continuous" switchess were just directly read
    input ports that were used for switches that were attached to devices
    that operated way too fast for the strobe rate of the matrix. Things
    like sling shots and, primarily, the thumper bumpers. The addition of
    capacitors there could make a difference, but the value would need to be
    selected to make sure that the response time to successive events isn't
    dampened to the point it ruins game play.

    Jim
     
  9. Rich Grise

    Rich Grise Guest

    You should crosspost - that way, when one person answers, everybody
    benefits.
    I used to work on pinball machines. Have you burnished the contacts?

    If that doesn't fix it outright, then just try stuff until it works. :)

    Good Luck!
    Rich
     
  10. frenchy

    frenchy Guest

    no this is a brand new machine and good switch, it is a common problem
    on this model (this particular switch is the only target on the game
    that can really get slammed quickly and commonly has trouble scoring).
    This manufacturer doesn't even put caps on their switches anymore. I
    am experimenting with caps to try to eliminate the problem (for me and
    other owners)...Frenchy
     
  11. frenchy

    frenchy Guest

    << A switch matrix allows a relatively small number of wires to read a
    large number of switches by
    stepping or strobing through a certain number of rows of switchesThese
    switches are by default
    debounced to what ever strobe rate the matrix works at. Adding a
    capacitor here can help,
    but only to a very small extent.>>>

    This is the type I am dealing with here. And in the old days (80s)
    they would put caps on these, .05 uf, if the switch was particularly
    prone to hard fast hits. This is where I am hoping to stretch the time
    out with maybe bumping the cap and adding a resistor.

    <<On the other hand the "continuous" switchess were just directly read
    input ports that were used for switches that were attached to devices
    that operated way too fast for the strobe rate of the matrix. >>

    Yeah this is like one company used in the 80's also for 'special'
    solenoids where they
    went with a 22uf cap and a 100 ohm resistor to stretch out the switch
    closure time read by the
    solenoid driver so it would pulse a kicker or bumper longer for more
    power. So I am wondering if
    I can have something in this set up and the one above....maybe a
    smaller capacitor than this, +
    a resistor of some value.
     
  12. frenchy

    frenchy Guest

    <<If the switch you are having problems with is on the matrix then I
    wouldn't
    play with adding caps, I would look at things like contact resistance,
    contact gap, mechanical problems with actuation, or even the 1N914
    blocking diode that is on the switch. >>

    This switch is on a brand new machine, of which other owners invariably
    have the same problem with this switch.
    So this is not a problem of faulty parts from reading rec.games.pinball
    and other owners reports. It's just a switch
    in the game that is really whacked, the only stand-up target in the
    game that is really whacked like this. All other switches in the game
    are microswitches, or softly hit targets, or infrared led sensor types,
    and don't have this problem. Targets are all gold-plated points.
    Again it is just a similar case of what pin makers dealt with in the
    80s of having to put caps on certain switches from the outset knowing
    they get fast hits, and this company does not use caps at
    all....Frenchy
     
  13. Ken Taylor

    Ken Taylor Guest

    I note the comments of others more familiar with pinballs in particular, and
    your comment that it appears to be a design fault in the machine.

    Given all that, it would seem to me that it's relatively easy to make up a
    small circuit which will run a MOSFET or relay using the existing switch as
    the trigger, and vary the amount of time you keep the output closed. This
    way you can do some controlled experiments in whether the technique of
    stretching the closure time is even valid in the first place. Once you know
    it works you can then decide how to do it on a more permanent basis.

    Cheers.

    Ken
     
  14. Ross Herbert

    Ross Herbert Guest


    In my experience the real problem has to do with the switch itself, no
    matter whether it is new or not. The contact tension should be
    increased for a start but since I don't know the switch construction
    this may not be possible. Contact bounce is a common problem with
    relays using long contact spring arrangement as used in older
    telephone exchanges and the spring tension is adjusted by 'stroking'.
    The suppression capacitor simply 'irons out' any remaining bounce
    artefacts and putting a larger cap in will not improve the situation.

    The real solution is to incorporate 'bounce elimination' using simple
    logic so that a '1-and-1-only' switch closure is produced.

    This page describes the problem and how to overcome it (2nd cct
    applies to single make switch)
    http://www.play-hookey.com/digital/experiments/manual_pulser.html

    A paper on debouncing goes into more detail
    http://www.ganssle.com/debouncing.pdf

    And to do six switches at once use the MC14490
    http://thayer.dartmouth.edu/~engs031/databook/mc14490.pdf
     
  15. frenchy

    frenchy Guest

    In my experience the real problem has to do with the switch itself,
    no
    matter whether it is new or not. The contact tension should be
    increased for a start but since I don't know the switch construction
    this may not be possible. Contact bounce is a common problem with
    relays using long contact spring arrangement as used in older
    telephone exchanges and the spring tension is adjusted by 'stroking'.
    The suppression capacitor simply 'irons out' any remaining bounce
    artefacts and putting a larger cap in will not improve the situation.>>

    These are brand new gold-contact switches. No relays, this is solid
    state stuff on a new machine. Pretty sure they already have put
    debouncing circuitry or software in pinballs for a long time to prevent
    false excess scoring etc. Again the basic problem is just sometimes
    certain pinball switches don't quite close long enough for the matrix
    to pick it up, even if the switch itself is perfect, was done by the
    factories for years on certain switches they knew would have this
    problem due to a ball really zipping over a rollover switch extremely
    fast or a target getting hit hard. In fact the switches in question
    work better when softly hit since then the contacts are closed longer.
    Hard hits where the contacts are really making even better contact and
    with longer stroke due to more movement of the switch, but the total
    time of switch closure is shorter due to the speed and rebound off the
    switch. Particular swtich in question is same thing, works fine on
    glancing hits since then it is closed longer than hard slams...Frenchy
     
  16. James Beck

    James Beck Guest

    Once upon a time I had to deal with a prototype Williams Dracula that
    had a problem recognizing a matrixed switch even if it were engaged very
    positively, so I know it does happen. I ended up making a little
    circuit with an opto as the interface to the matrix and an Op Amp set up
    as a comparitor to stretch the length of time it appeared to have a
    closed switch. I know it worked for the duration of the trade show.
    Williams had +12VDC available under the playfield for use with their
    opto boards and so on, so it was a snap to steal a little power. I
    don't know what resources you have available on your particular pinball
    machine, but it is something to think about.

    Jim
     
  17. frenchy

    frenchy Guest

    Ok I will keep this in mind, there is nothing that will piss a pinball
    player off more than something that is supposed to score when you hit
    it, and it doesn't. Bugs in the software can be tolerated to a point,
    but when it comes down to a target acting like you never even hit it in
    the first place, that's another story. (Electromechanical games did
    this routinely since they could only do one thing at a time but that
    was expected by players...not so with electronic games.) Right now I
    have a 4.7 uf cap and a 150 ohm in series across the switch and seems
    to be working flawlessly (so far anyway). If this ends up not being
    foolproof I may get back to you on how complicated this opto thing
    would be. thanks!....Frenchy
     
  18. Zak

    Zak Guest

    Well,

    It sounds like it is a matrix. There is only one ball: thus only one
    switch is engaged.

    The capacitors extend the time that a single switch seems engaged. A
    rapid succession of two switches could make it appear that two other
    switches are also engaged...


    Thomas
     
  19. frenchy

    frenchy Guest

    Nope, multiple balls on modern games. I don't see a problem in simply
    extending the switch closure on one switch, the games routinely have
    mulltiple balls going at once, have some switches that are closed for
    long periods or all the time, etc. As for ghosting, not sure what is
    meant by that but was told this can happen if the cap is made too
    large. AS if it could cause false reads on other switches when the
    problem switch was not even closed, due to the cap doing something
    unintended. Maybe would make a difference if cap was polarized or non
    polarized? Maybe that could make a difference. But if the worst thing
    a cap can do is make the switch read as closed all the time, I don't
    see that as ghosting either - just a stuck switch.
     
  20. Mark

    Mark Guest

    Even if it is martixed, I think a cap directly across the switch in
    question may help. Note I'm saying the cap should be across the
    switch, not from the switch to ground.

    Did you try it?

    If the switch closure is short and the scanninbg is slow, I can see how
    it could miss, but the cap should help because when the switch closes
    even breifly, it will discharge the cap and the cap will "remember"
    that the switch was closed until the scan comes around to read it.

    Mark
     
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