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Simplest latch imaginable

Discussion in 'Electronic Design' started by MTB, Apr 24, 2004.

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  1. MTB

    MTB Guest

    Hi,

    I want to ignite the second stage of a model rocket by using a mercury
    switch to detect the deceleration of the first stage motor stopping.
    My elementary knowledge of electronics seems to indicate some sort of
    "latch" since the switching action of the mercury switch may only be
    "on" for a few milliseconds. This latch would have to turn on as soon
    as the mercury switch did and then remain on. The "load" in the
    circuit would be the second stage igniter (essentially a fuse) which,
    when it blew, would turn the circuit off again.
    Whole thing would be powered by a small 9V battery.

    Since the last electronics I did was when discrete transistors were
    the norm, my abilities are small. I seem to remember though that a
    monostable might serve this purpose?

    Question is, what is the simplest (and therefore the lightest) circuit
    to achieve this?

    Any experts out there care to help?

    Rgds,

    /\/\
     
  2. Fred Bloggs

    Fred Bloggs Guest

    Please view in a fixed-width font such as Courier.


    7V
    R1= ---------------
    ignitor ignitor current

    +----o~~~~o------+------/\/\--------+----+
    | | R1 | |
    | | | /
    | | | 10K
    | | | /
    | / e \
    | 100K \| |
    | / 2n2907 |--+
    --- \ /| |
    - 9V | c |
    | | | c
    | | / 470 | |/
    | +----o o---/\/\---+--| 2n2222
    | | sw | |\
    | | / e
    | === 10K |
    | |0.47u / |
    | |metal film \ |
    | | | |
    +----------------+------------------+----+
     
  3. Active8

    Active8 Guest

    Is the deceleration going to be enough to cause that heavy mercury
    to move all the way to the end of the bulb? I'm thinking of the
    switches found in home heater themostats.

    Maybe using the switch to trigger an SCR would be best. The igniter
    in the anode circuit, once blown, would reset the SCR. This is
    really unneccessary since the open circuit won't be drawing any
    current anyway.
     
  4. Fred Bloggs

    Fred Bloggs Guest

    I suppose it would help to make allowance for arming the thing- this
    uses a two pole shunt- move to ARM position after battery voltage is
    thrown ON:
    Please view in a fixed-width font such as Courier.

    7V
    SHUNT R1= ---------------
    ignitor current
    +--o ARM o SAFE o---+
    | | |
    +-+--/\/\---+---o~~~~o--+---------/\/\--------+----+
    | 100K ignitor | R1 | |
    | | | /
    | | | 10K
    | | | /
    | | e \
    | | \| |
    | | 2n2907 |--+
    --- | /| |
    - 9V | c |
    | | | c
    | | / 470 | |/
    | +------o o----/\/\---+--| 2n2222
    | | sw | |\
    | | / e
    | === 10K |
    | |0.47u / |
    | |metal film \ |
    | | | |
    +-----------------------+---------------------+----+
     
  5. Fred Bloggs

    Fred Bloggs Guest

    Please view in a fixed-width font such as Courier.



    Make that:



    7V - ignitor voltage
    R1= ---------------------
    ignitor current


    SHUNT

    +--o ARM o SAFE o---+
    | | |
    +-+--/\/\---+---o~~~~o--+---------/\/\--------+----+
    | 100K | ignitor R1 | |
    | | | /
    | | | 10K
    | | | /
    | | e \
    | | \| |
    | | 2n2907 |--+
    --- | /| |
    - 9V | c |
    | | | c
    | | / 470 | |/
    | +------------------o o----/\/\---+--| 2n2222
    | | sw | |\
    | | / e
    | === 10K |
    | |0.47u / |
    | |metal film \ |
    | | | |
    +-----------+---------------------------------+----+
     
  6. First of all: don't use real mercury switches in rockets - they're bound
    to break and spill their toxic content in the environment. Electrolytical
    types are OK though.

    Second: I'm not sure whether a liquid tilt switch in general is the best
    solution for the problem. Consider what happens:
    1. First stage fires -> acceleration, G-forces -> all liquid stays in
    the bottom of the switch (where it was in the first place).
    2. First stage stops -> free fall = zero-G condition -> liquid doesn't
    move in any particular direction, maybe doesn't even close the contacts
    for several seconds.
    I'd say this is rather unreliable - when you're out of luck, the second
    stage fires only when the nose already points downward (or has reached
    the ground, in extreme cases). Then again, when the rocket makes a sudden
    movement sideways for whatever reason, the liquid might slosh up and close
    the contact prematurely.

    So what you actually need, is some sort of crude acceleration measuring
    device, which fires the second stage when zero-G occurs. I'd choose one of
    those levered microswitches, with the lever weighed down sufficiently to
    keep the lever pushed well down at 1G (in rest).
    Only during zero G will the lever come up and activate the switch. And the
    best thing is: this situation will persist until the rocket accelerates
    again, so you don't need any extra electronics to keep the current flowing.


    Richard Rasker
     
  7. Fred Bloggs

    Fred Bloggs Guest

    This is getting ridiculous, scratch this:


    Use this:
    Please view in a fixed-width font such as Courier.



    Make that:



    7V - ignitor voltage
    R1= ---------------------
    ignitor current


    SHUNT

    +--o ARM o SAFE o---+
    | | |
    +-+--/\/\---+---o~~~~o--+---------/\/\--------+----+
    | 100K ignitor | R1 | |
    | | | /
    | | | 10K
    | | | /
    | | e \
    | | \| |
    | | 2n2907 |--+
    --- | /| |
    - 9V | c |
    | | | c
    | | / 470 | |/
    | +------o o----/\/\---+--| 2n2222
    | | sw | |\
    | | / e
    | === 10K |
    | |0.47u / |
    | |metal film \ |
    | | | |
    +-----------------------+---------------------+----+
     
  8. Dan Charette

    Dan Charette Guest

    You know... since you're doing a multistage rocket, I'm assuming that
    you are wanting to go higher and faster with the second stage. So,
    why detect when the rocket slows and then fire a stage stage at that
    point? And, carrying an extra ciruit to detect this would just
    increase the payload. Why not take a simpler approach by taking your
    first stage motors and time the burn cycle in a test on the ground to
    see how long they go. Then, instead of carrying a relatively heavy 9V
    battery, why not use an old fashioned fuse that has a specific burn
    rate that you cut to length based on your measurement of the burn
    cycle on the first stage. I know this isn't all that hip and circuit
    driven, but if you are going for distance and speed, don't let the
    rocket decelerate and then have to use another motor to regain your
    momentum.
    If a little payload weight isn't of concern, then instead of a fuse,
    design a smaller timer circuit that is started when the first stage
    ignitor is triggered. The timer then counts until the first stage
    comes into about 95% of it's burn. Then, the timer would trigger a
    switch to hit the ignitor for the second stage so it can take over and
    pick up with the first stage burns out. You may even just use a large
    RC circuit tied to a comparator to do the timing and the triggering of
    the second stage turn on. Just find a combination of high R and a
    good sized electrolytic that will give you a few seconds of time to
    get that first stage burned almost all the way through.

    Have fun!
    Dan Charette {}
    Remove the "FUZZ"
    from my e-mail address
    to contact me.

    "I may not always be right, but I'm never wrong."
     
  9. mike

    mike Guest

    This is a disaster waiting to happen.
    Add a fail-safe to make absotively, posilutely CERTAIN that the
    second stage cannot ignite before the first stage.
    Pressure switch, conductor that the first stage burns thru...
    anyhting to keep the second stage from firing when you
    bump it.

    SCR does the same thing, saves the weight of a transistor.
    Don't know how much energy the ignitor takes, but charging
    that cap might be dangerous.

    I prefer switches that fail safe when a wire breaks loose.

    This one requires that the ignitor shunt work.

    Having said all that, aren't there commercial solutions for two-stage
    rockets? Dedicated newsgroups? Check there. Your MUCH better off
    with recommendations from people who are currently doing successfully
    the exact same thing. Solutions from a bunch of engineers with no clue
    as to the gotchas in model rocketry may not be optimum...or safe.
    Be careful that you quest for minimum weight doesn't lead you to
    maximum blindness when it goes up in your face. Safety first!!

    mike

    --
    Return address is VALID.
    Bunch of stuff For Sale and Wanted at the link below.
    Toshiba & Compaq LiIon Batteries, Test Equipment
    Honda CB-125S $800 in PDX
    Yaesu FTV901R Transverter, 30pS pulser
    Tektronix Concept Books, spot welding head...
    http://www.geocities.com/SiliconValley/Monitor/4710/
     
  10. Some people are *very* afraid of mercury. There are many reports of
    using evacuations and "Moon Suits" to clean up after broken
    thermometers.
     
  11. John Larkin

    John Larkin Guest


    batt+
    |
    |
    +----------+
    | |
    | |
    | |
    a | |
    ----- |
    \ / |
    SCR \ / |
    ===== switch
    k |\ |
    | \ g |
    | \-------
    |
    |
    squib
    |
    |
    batt-


    The SCR could be one of those umpteen-amp TO-220 Teccor things from
    Digikey. Don't use a "sensitive gate" SCR... they trigger too easily.

    More interlocks are strongly advised, but you know that.

    John
     
  12. Jim Thompson

    Jim Thompson Guest

    I have a friend who, in High School, decided that boiling mercury
    would make a dandy temperature reference.

    Landed him in the hospital and gave him *permanent* near death
    sensitivity to more mercury.

    Very nasty stuff for the nervous system.

    ...Jim Thompson
     
  13. Garrett Mace

    Garrett Mace Guest


    Don't do this, for the following reasons:

    1. Mercury switches will probably slosh up and trigger right when the rocket
    is initally fired
    2. Mechanical switches might do the same
    3. Neither switch will sense the deceleration when the rocket motor burns
    out: rockets coast for quite a while
    4. If the switch is sensitive enough, then it will trigger on the launch pad
    if you bump or tilt the rocket the wrong way: very unsafe!

    Multistage model rockets use a primary engine designed for that purpose,
    without an ejection charge in the top. The fuel burns to the top, and a jet
    of flame goes up and ignites the second rocket. This accomplishes the task
    very simply and well.

    This link shows how to ignite the second engine if it is far away from the
    first engine; up to about ten inches:
    http://www.apogeerockets.com/education/how_to_multi-stage.asp In addition to
    running a narrow tube from the first to the second rocket, you need to
    provide vents at the top so that the hot exhaust reaches the second engine.
     
  14. John Larkin

    John Larkin Guest

    Is the liquid stuff that bad? It's fairly non-reactive, and the vapor
    pressure is very low.

    John
     
  15. I read in sci.electronics.design that Aubrey McIntosh <
    ima.austin.tx.us> wrote (in <
    ..com>) about 'Simplest latch imaginable', on Sat, 24 Apr 2004:
    Yes, well, it runs after you to kill you, if you run away downhill. (;-)
     
  16. I read in sci.electronics.design that Jim Thompson
    .... and the vapour is SUCH a lovely green colour, too!
     
  17. I read in sci.electronics.design that John Larkin <[email protected]
    techTHISnologyPLEASE.com> wrote (in <[email protected]
    4ax.com>) about 'Simplest latch imaginable', on Sat, 24 Apr 2004:
    No, that why it was considered safe to use in dental amalgam until the
    scare-mongers started in.
    Quite. However, mercury compounds, especially organic mercury compounds
    ARE bad news. But again, at the parts per zoctillion level or whatever
    can be detected these days, ONLY if there is long and continuous
    exposure.
     
  18. Fred Bloggs

    Fred Bloggs Guest

    Please view in a fixed-width font such as Courier.

    ..
    ..
    ..
    ..
    ..
    ..
    .. 7V - ignitor voltage
    .. R1= ---------------------
    .. ignitor current
    ..
    ..
    .. SHUNT
    ..
    .. +--o ARM o SAFE o---+
    .. | | |
    .. +-+--/\/\---+---o~~~~o--+---------/\/\--------+----+
    .. | 100K ignitor | R1 | |
    .. | | | /
    .. | | | 10K
    .. | | | /
    .. | | e \
    .. | | \| |
    .. | | 2n2907 |--+
    .. --- | /| |
    .. - 9V | c |
    .. | | | c
    .. | | / 470 | |/
    .. | +------o o----/\/\---+--| 2n2222
    .. | | sw | |\
    .. | | / e
    .. | === 10K |
    .. | |0.47u / |
    .. | |metal film \ |
    .. | | | |
    .. +-----------------------+---------------------+----+
    ..
    ..
    ..
    ..
    ..
    ..
    ..

    All rockets have ARM sequences relying on a similar circuit- the shunt
    comes with the ignitor- and no well in hell does 90uA peak current
    ignite any ignitor if the shunt fails- the shunt protects against EM and
    ESD ignition, and the capacitor severely limits dV/dt across the SCR or
    its transistor equivalent and also filters EM and ESD into the circuit
    from parts unknown. So you're going to have to go with a non-failsafe
    mode when a wire breaks loose no matter what.
    The circuit is generic and the mercury switch can be replaced by a
    surplus MILSPEC single axis totally shock proof acceleration switch that
    stays NO for >0.5G or less, some go for $1.50. The hobby products are
    overpriced junk by comparison- good reading for entertainment though-
    like your post.
    such as yourself...

    Okay- you are capable of saying at least one reasonable thing.
     
  19. Ben Bradley

    Ben Bradley Guest

    I agree with everyone else, don't do that. Put the mercury switch
    in a 'dangerous items from the past' museum.
    I suspect you wouldn't want to wait that long (if it makes any
    difference), you'd probably want to fire the second engine when the
    acceleration drops below one gee.
    You can do quite a lot with electronics nowadays that weighs a lot
    less than a standard 9 volt battery. IMHO, the 9V is too heavy, I'd
    make something that runs on a couple of AAA's or watch batteries (the
    minumum that could provide enough power to the fuse, even if three
    volts goes through a swithching supply to get to 9 volts). I presume
    you would prefer to minimize weight over a battery that lasts through
    hundreds of launches.
    That's good, surface mount components are small...
    The lightest is not neccesarily the simplest or best.
    If you knew what you can do with electronics nowadays, it might
    motivate you to learn more about it.

    For this app, I would use a microcontroller and a MEMS
    accelerometer chip (as used in automobiles to trigger air bags in an
    accident). The microcontroller would fire both (or all) stages. I'm
    sure many here would have reservations about using a microcontroller
    in such an application, but it can be made a lot more reliable than
    the negative-gee-activated switch idea, especially with the problem
    others addressed of accidentially firing the engine prematurely.
    A serial connection (contacts on the bottom of the rocket laying on
    contacts on the launchpad) to a handheld controller sending a
    CRC-checked command sequence would tell the processor to go, which
    would immediately fire the first engine. I presume these have some
    minimum and maximum running specs, say five to ten seconds. After
    (let's say) three seconds the processor would look for acceleration to
    drop to 1 gee, meaning the first stage is down to just keeping up with
    Earth's gravity and no longer applying an increase in speed to the
    rocket. It's at this point the processor should ignite the second
    engine.

    ISTR an article several years back, I think it was in "High Powered
    Rocketry" magazine or some such for 'serious' rocketry enthusiasts (I
    didn't buy it, I just read this on the newsstand - I really am
    interested in model rocktry, but I already have enough
    techno-hobbies). It described a microcontroller/accelerometer/EEPROM
    device which was mounted inside a rocket and measured acceleration
    throughout the flight.

    As for construction, you could make it with old-fashioned
    through-hole parts and sockets, and have it weigh no more and not be
    too much larger than a 9-volt battery. But without much trouble you
    can go surface mount and make a smaller board and use less solder
    (which being about 50 percent lead, you want to minimize to keep
    weight down).
     
  20. MTB

    MTB Guest

    Guys, When I were a boy, they used to hand the stuff out in chemistry
    classes and let you play with it. Apart from wearing glasses I have no
    visible side effects, but then who knows what tomorrow will bring. Too
    much scottish salmon can reputedly have a greater effect......
     
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