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using current to measure a time

Discussion in 'Electronic Basics' started by Panther, Dec 1, 2005.

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  1. Panther

    Panther Guest


    I am making something to measure how much time two objects are in contact
    when one of them is accelerated towards the other. To do this, I'm going to
    put a wire on one object and another on the other, and make the objects
    complete the circuit when they touch obviously. What happens with this is is
    these wires link to a stopclock/watch (i can give the model name if you
    want) and this stopwatch starts timing when it receives a current and stops
    when it receives ANOTHER current. I mean, you give it a current. Starts
    time. You "break the circuit." It still times. You give it another current,
    it stops. This isn't useful because I need it to start timing when it
    receives a current and stop when that current is taken away (as the objects
    are no longer in contact; no more current).

    What can I do to make this work? Is there anyway I would generate/make a
    current come when the circuit is broken? How does a potential divider fit
    into this? I am grateful for help. Thank you.
  2. Dan Hollands

    Dan Hollands Guest

    Conceptually it is simple

    Sense the current that starts the watch

    When the current goes away, fire a one shot that will generate a current
    pulse to stop the watch


    generate a current that is normally connected to the Stop input.

    When current is detected switch that current to the Start input until the
    current goes away

    Other Thoughts

    If I had this problem I would simple use an oscilloscope to monitor the time
    that the objects touch. If the objects are metalic do you think they will
    touch long enough to run a clock?


    Dan Hollands
    1120 S Creek Dr
    Webster NY 14580
  3. Andy Baxter

    Andy Baxter Guest

    Panther said:
    I don't know if this would work, but you could try doing it like this:

    Build a current generator circuit with a fast response rate that didn't
    mind trying to drive an open loop.

    Connect the output of this to a capacitor through the circuit made by the
    two objects.

    When they collided, a pulse of current would flow between, and the voltage
    on the capacitor at the end would measure the impact time.
  4. Nick.

    Nick. Guest

    Blimey... I remember doing those experiments at school on equipment using
    Nixies tubes! (Kicking a tinfoil covered football down the corridoor IIRC)

    Forget the toy stopwatch. Use (or get) a frequency counter with a "count"
    facility. Buy a 1MHz crystal oscillator (example link below). Connect one
    object to the osc, the other to the counter input and you have a timer
    with 1uS resolution...
  5. Panther

    Panther Guest

    Thanks, but I'm afraid this too complex for me :(

  6. Panther

    Panther Guest

    Thank you for the assistance, but I am not allowed to use that (even if I
    was, I wouldn't know how as I'm quite new to this stuff). But thanks anyway.
  7. Panther

    Panther Guest

    What does a one shot mean? Sorry I'm new.

    This looks like it would lead to inaccuracies because of slowness. Either
    way, I don't think there is a Stop input or anything of that sort.
    Unfortunately I can't do that anymore :(

    Thanks anyway.
  8. Nick.

    Nick. Guest

    You can't use a counter or a 'scope. An analogue (capacitor) solutiom is
    too complex. What answer was you hoping for?
  9. Panther

    Panther Guest

    How I could use a potential divider to set a pulse when the circuit breaks?
    Or a more complicated circuit design?
  10. John Fields

    John Fields Guest

    I'm assuming you've got two balls hanging from the ends of wires and
    you're going to pull on eway from the other and the let it go so
    that it'll hit the other one, and what you want to measure is how
    long the collision lasts.

    Try this: (view in Courier)

    GND +5V HFCK
    | | |
    | [100R] | +-------------+
    | | +--|> |
    + +-----A |_ COUNT _| A---+
    / | OR Y---+-O|E Q0 R|O--Y NOR |
    / | +--B | +---------+---+ B---+
    / | | | | |
    / | | A--+ +--A |
    / | +---Y AND NOR Y---+ |
    / | B---------------+--B | | +5v
    / \ / \ | | | |
    \_/ \_/ | A---+ | O |
    +--Y NOR | |<-ARM
    B--------+--O |
  11. Panther

    Panther Guest

    I'm sorry but this is way too complex for my level. Thanks anyway.
  12. Rich Grise

    Rich Grise Guest

    You should also learn to bottom-post.

    Anyway, I think I have an idea what you want - a current source
    that when the balls are touching, lets current flow for time T.

    OK, take a power supply, a resistor, a capacitor, and a voltmeter.

    PS+ ---- R ----- (B1) (B2) ----+---- Voltmeter +
    === capacitor
    PS- -----------------------------+---- Voltmeter -

    Use a high-impedance voltmeter, like a DVM.

    Record the voltage while the balls are apart. It will very probably be
    very close to zero. You can assure this by momentarily shorting the
    capacitor terminals.

    Drop the ball. While the balls are in contact, current will flow and
    start to charge the cap through the resistor, with a time constant
    of T = RC.

    The voltage across the capacitor at the exact moment that the balls
    separate will tell you T by using that exponential equation that I
    can't remember now, but since you're in school you should look it up

    Good Luck!
  13. John Fields

    John Fields Guest

    It seems that no matter how simple we make it, you can't quite get

    How about specifying what you want/need in a little more detail, if
    you can.

    Otherwise, well... we've offed trolls here before.

    BTW, here in Rome we bottom post so, when in Rome...
  14. Yes, please.
  15. Nick.

    Nick. Guest

    You cannnot. A potential divider divides potential.
    Some simple designs (e.g. a resister and a capacitor) have already been
    suggested but are too complex (?)

    If you really must use the stopwatch you mentioned what you really need is
    an edge-triggered monostable. You may get away with a simple inverter,
    depending on the input logic of the stopwatch. Either way, you will
    probably run into problems with "switchbounce".
  16. Panther

    Panther Guest

    Thanks but I think the poster above you's suggestion is simple enough to do.
  17. Panther

    Panther Guest

    Thank you!!!!!!!!!!!!!! This looks very simple so I will give it a go. But
    could you please explain what the time constant T = RC means? I've no idea.
    So you're saying, for example, if I were to do the practical now and record
    the voltages, I would be able to work out the time section using some
    formula? IE I don't need the equipment? As that would very handy.

  18. Rich Grise

    Rich Grise Guest

    T = RC is the "time constant". It's the amount of time it takes for the
    charge to reach 63% (or something) of its final value, R is in ohms, and
    C is in farads. With T on the horizontal axis, the cap voltage rises at
    some exponential rate...
    Maybe one of these sites will be more helpful:"time+constant"

    Good Luck!
  19. John Fields

    John Fields Guest

    It seems to me that by your not cross-posting, by your off-handed
    rejection of several solutions which have been presented to you,
    and by not using in-line references, you're not really looking for a
    solution to your "problem", but instead are more interested in
    playing games.

    Am I wrong?
  20. John Fields

    John Fields Guest

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