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8 switches

Discussion in 'Electronic Basics' started by [email protected], Aug 7, 2005.

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  1. Guest

    i'm creating a basic device,

    i have 8 basic open switches.

    if i close one switch, i want the main circuit to be complete, however
    if more than one are closed, i want the circuit to remain open.

    i'm new to electronics and i was considering 8 seperate relays but i'm
    assuming this would require 8 seperate switch circuits with 8 seperate
    batteries, however each relay would close the same main circuit.

    can someone advise on a better design ?
     
  2. Why are you circumspect about your intended purpose? What is the
    purpose? Is this a question from a book or test? Or is this for a
    game of some kind?

    Jon
     
  3. Guest

    well i can see how my post could be seen as annoying, im designin a
    device that according to my research has not been created yet and i'd
    like to design and test it before revealing its purpose.

    it's not a question from a book or test. however the design could be
    seen as an educational tool. i'll be creating a webpage if the design
    is successful and beneficial to students, i can see it being very
    beneficial and naturally forcing students to use good form and
    technique.

    it won't take long to construct but the materials used must be very
    light to keep the instruction realistic and non-fatigueing, and this
    will be expensive as custom made machined parts will be necessary. can
    you guess what it is ?

    i'll post and graph student's progress and results including my own
    when this project is complete. hopefully the prototype will immediately
    eliminate bad habits. any help on the electronics side would be
    appreciated. im thinking relays or some kind of boolean logic chip :)
     
  4. It's difficult for me to suggest an approach. I've no idea if relays
    are required, at all, from the description. Maybe, or maybe not. What
    is being driven? How must the resulting switch function be attached?
    What are the voltage levels that the switch function must handle? How
    precisely must be all of the timing of observing the input switches
    and developing the control output? Must the output be held for some
    duration even if a second switch is then operated? I don't know what
    is acceptable and what isn't to you. Just way, way too vague.

    Sometimes, this is helped a great deal by knowing the application. If
    it is something within our reasoning and experience, we can fill in a
    lot of the gaps by common sense and don't have to beg for information.
    But perhaps, if you won't talk about it directly, we can play a little
    game and find out indirectly what you'd consider and not consider by
    just shooting in the dark, blindly tossing darts so to speak, until
    you clue us all in a tiny bit at a time. A kind of 20 questions?

    I'm good for one go at it.

    For me, the simple approach would be to use a 16-pin PIC, scan the
    input sources and debounce them in software, test them as per your
    spec, and drive _something_ from one of the pins on the basis of that
    result. But then, I program such things a lot.

    There's my tossed dart. You tell me where it hit.

    Jon
     
  5. Guest

    i feel bad not writing a more descriptive requirement .. thanks for
    your input john, ill get back to this post in a few days when i have
    time and can provide more details
     
  6. Tom Biasi

    Tom Biasi Guest

    Start out by making a truth table of what you want.
     
  7. Si Ballenger

    Si Ballenger Guest

    Well, you probably could use 9 relays, one being the master
    normally closed relay for the master circuit, and the other eight
    normally open operated by the switches. Connect the eight NO
    relay conatacts to a power supply and a resistor, and parallel
    the outputs to coil of the master relay. Size the resistors such
    that current thorugh only one isn't enough to operate the master
    relay, but current through two or more in parallel is sufficient
    to operate the master coil, opening the master circuit.
     
  8. How about an analog approach. Tie each switch to a positive supply,
    and put an equal resistor in series with each. Parallel all resistor
    switch pairs and connect to zero volts through a 9th resistor equal to
    about 70% of one of the other 8. Something like 8 of 10k resistors
    and 1 of 6.8k. Then use a window comparator to sense the voltage
    across the 9th (lower value) resistor is higher than what you get with
    one switch closed, but less than what you get with 2 or more switches
    closed. The reference voltages for this comparator could be a divider
    made of 3 resistors across the same supply that powers the switches
    with series resistors. Something like supply to 10k to 3k to 2k to ground.

    A good dual comparator for this job might be an LM393.
    http://www.fairchildsemi.com/ds/LM/LM393.pdf

    Then you can energize a relay or whatever with the window comparator
    output.

    Total parts count 12 resistors and a dual comparator, and what ever
    you need to drive the relay.
     
  9. John Fields

    John Fields Guest

    Sure! :



    Vcc Vcc
    | |
    | [4K7]
    [0.499K] |
    | +----[1M]-+
    | | | 74HC86
    +---+---|+\ | +-----+
    | | >--+-------+--|A1 |
    | +-|-/ | | Y1|--+
    [2.49K] | Vcc +--|--|B1 | |
    | | | | | | | |
    | | [4K7] | +--|A2 | |
    | | | | | | Y2|--+
    Vcc | +-|---[1M]-+ +--|--|B2 | |
    | | | | | | | | | |
    [1000] +---+-|-|+\ | | +--|A3 | |
    | | | | >---+---+ | | Y3|--+
    +-----//----+------|-----+-|-/ +--|--|B3 | |
    | | | LM393 | | | | |
    [1000] [1000] | | +--|A4 | |
    | | [2.00K] | | Y4|--+
    D Q1 D Q8 | +---- |B4 | |
    IN1>---G 2N7000 +--G 2N7000| +-----+ |
    S | S | |
    | | | | +-----------------------------+
    | | | | |
    GND>----+-------|---+------+ |
    | | VCC VCC
    IN2>---TO Q2-G | | | |
    | | [1N4001] COIL- -|
    IN3>---TO Q3-G | [100R] | | O->| <--O-NO
    | | +--------+ | |
    IN4>---TO Q4-G | | | | O K1
    | | C | |
    IN5>---TO Q5-G | +-----B | +-----COM
    | E |
    IN6>---TO Q6-G | | +--------NC
    | GND
    IN7>---TO Q7-G |
    |
    IN8>------------+


    Typical for all inputs, a SPST NO switch closing to Vcc:

    Vcc--O--> |
    |
    O
    |
    +--->to Qn gate
    |
    [10K]
    |
    gnd



    With no switch or with more than one swith closed, the circuit
    between the common and the normally closed contacts of K1 will be
    open. With any single switch closed, the relay will make the
    contact between COM and NO.

    Need a circuit description?
     
  10. Guest

    these are all awesome ideas ! thanks everyone ..

    which one these designs would be

    1) most compact
    2) the lightest
    3) the cheapest to manufacture in small (50) quantities ?
     
  11. BobG

    BobG Guest

    can someone advise on a better design ?
    ====================================
    separate has 'a rat' in it
     
  12. Mine.
     
  13. Randy Day

    Randy Day Guest

    John's is an excellent suggestion. You
    can't get much smaller than 1
    easy-to-find IC and 12 resistors.

    That doesn't include the output transistor
    and the relay/diode, of course.

    Matt: The other neat thing about John's
    suggestion is that, if you decide you need
    more than 8 switches, all you have to add
    is one resistor for each new switch.
     
  14. John Fields

    John Fields Guest

    ---
    Looking over John Popelish's response(s), I see that the MOSFET
    switches I was using are unnecessary and the circuit can look like
    this:


    +5V
    |
    [1000]
    |
    +----+---+-------+-------+-------+-------+-------+-------+-->Eo
    | | | | | | | | |
    [1000] | [1000] [1000] [1000] [1000] [1000] [1000] [1000]
    | | | | | | | | |
    O| | O| O| O| O| O| O| O|
    O| | O| O| O| O| O| O| O|
    |S1 | |S2 |S3 |S4 |S5 |S6 |S7 |S8
    +----|---+-------+-------+-------+-------+-------+-------+
    | |
    GND |
    |
    |
    +--+
    |
    Vcc | Vcc
    | | |
    | | [4K7]
    [0.499K] | |
    | +-|--[1M]-+
    | | | | 74HC86
    +---+-|-|+\ | +-----+
    | | | >--+-------+--|A1 |
    | +-|-/ | | Y1|--+
    [2.49K] | Vcc +--|--|B1 | |
    | | | | | | | |
    | | [4K7] | +--|A2 | |
    | | | | | | Y2|--+
    | +-|---[1M]-+ +--|--|B2 | |
    | | | | | | | | |
    +---+-|-|+\ | | +--|A3 | |
    | | | >---+---+ | | Y3|--+
    | +-|-/ +--|--|B3 | |
    | LM393 | | | | |
    | | +--|A4 | |
    [2.00K] | | Y4|--+
    | +---- |B4 | |
    GND +-----+ |
    |
    +---------------------+
    |
    |
    | +V +V
    | [1N4001] [COIL]- -|
    [100R] | | O-> |<--O-NO
    | +--------+ | |
    | | | O K1
    | C | |
    +------B | +-----COM
    E |
    | +---------NC
    GND


    With a Vcc of 5V, the values chosen for the reference divider will
    allow one millampere to flow thrugh it and its output voltages will
    be:


    5V
    |
    [499R]
    |
    +-----4.5V
    |
    [2.49K]
    |
    +-----2.0V
    |
    [2.00K]
    |
    GND



    With none of the switches pressed, Eo will be at 5V, and since the
    inputs to the non-inverting inputs of the comparators (from the
    reference divider) will be at 4.5V, the outputs of both comparators
    will be low.


    With any single switch pressed, the switch divider will look like
    this:


    5V E1
    |
    [1000]R1
    |
    +---->Eo
    |
    [1000]R2
    |
    GND

    and Eo will be:


    E1 R2 5V * 1000R
    Eo = --------- = --------------- = 2.5V
    R1 + R2 1000R + 1000R


    Now, since that voltage will be on the inverting input of the
    comparator with 4.5V on its non-inverting input, (and will be less
    positive than 4.5V) that comparator's output will go high.

    However, since the other comparator's non-inverting input is at
    2.0V, its output will stay low


    Next, if we push two buttons, the switch divider will look like
    this:

    +5V
    |R1
    [1000]
    |
    +-------+---->Eo
    |R2 |R3
    [1000] [1000]
    | |
    +-------+
    |
    GND

    and because R2 and R3 are in parallel, their total resistance will
    be:


    R2R3 1000R * 1000R
    Rt = ------- = --------------- = 500 ohms
    R2+R3 1000R + 1000R


    So, the divider now looks like this:


    +5V E1
    |R1
    [1000]
    |
    +----->Eo
    |R2
    [500]
    |
    GND

    and Eo becomes:


    E1R2 5v * 500R
    Eo = ------- = -------------- ~ 1.67V
    R1+R2 1000R + 500R


    Now, since 1.67V is lower than the reference voltages on either of
    the non-inverting inputs of the comparators, both of their outputs
    will go high.

    If we make a truth table for all possible combinations of switch
    activations it will look like this, where 'n' is the number of
    switches pressed and 'OUT1' and 'OUT2' are the logical output states
    of the comparators:

    n OUT1 OUT2
    -----|------|------
    0 0 0
    1 1 0
    2 1 1
    3 1 1
    4 1 1
    5 1 1
    6 1 1
    7 1 1
    8 1 1

    Now, since the OP only wants an output when one buton is pressed,
    and the only time the outputs of the comparators are different is
    when one button is pressed, we can use an EXCLUSIVE OR to decode
    that state. Additionally, since a 74HC86 contains four EXORs, we
    can run them in parallel and use them to drive a moderate-current
    bipolar relay driver. (Or anything else, with proper modifications)
     
  15. John Fields

    John Fields Guest


    ---

    Changing some polarities and wire-ORing the outputs gets rid of the
    EXOR, and the relay contact designations were wrong, but are fixed
    below:

    +5V
    |
    [1000]
    |
    +----+---+-------+-------+-------+-------+-------+-------+-->Eo
    | | | | | | | | |
    [1000] | [1000] [1000] [1000] [1000] [1000] [1000] [1000]
    | | | | | | | | |
    O| | O| O| O| O| O| O| O|
    O| | O| O| O| O| O| O| O|
    |S1 | |S2 |S3 |S4 |S5 |S6 |S7 |S8
    +----|---+-------+-------+-------+-------+-------+-------+
    | |
    GND |
    |
    |
    +--+
    |
    |
    |
    VCC |
    | | VCC
    [510R] | |
    | | [1K]
    +-----|-|+\ | +V +V
    | | | >--+ |K |
    | +-|-/ | [1N4001] [COIL]- -|
    | | | | | O-> |<--O----->NC
    [2.4K] | | +--------+ | |
    | | | | | O K1
    | +-|+\ | D | |
    | | >--+-----G | +--------->COM
    +-------|-/ S |
    | LM393 | +------------->NO
    [2K] GND
    |
    GND
     
  16. Guest

    I thank you for converting my rambling description to a schematic. I
    didn't have time to do it, myself, last night.

    I think I like it with the switches pulling up, better, because it puts
    both sensing levels within the common mode range for lower power supply
    voltages (all the way down to 3). And if you use a PNP relay driver,
    it can be driven in series with the common pull up resistor on the
    outputs, so a normally open contact can be used, or the active high
    collector might be the actual output, itself. Can't say for sure,
    because we don't know what the intended load is.
     
  17. Guest

    Forget that part about the PNP driver eliminating the NC contact. The
    signal in in the "window" when neiither comparator is pulling down, so
    an NPN or N channel driver will be on in th window.
     
  18. Guest

    John Fields wrote:
    (snip)
    (snip)

    I think you will get the largest possible voltage difference between 1
    switch closed and 2 switches closed if R1=1/(sqrt(2))*R2. That is why
    I recommended 10k at each switch and 6.8K as the common series
    resistor. I chose higher values than you did, because I was imagining
    (for no particular reason) the whole thing running from a 9 volt
    battery.
     
  19. Rich Grise

    Rich Grise Guest

    You don't have to reveal the end purpose of a design, just to
    tell us what's expected to be switched. You want a one-and-only-
    one gate. I seem to remember this from the Don Lancaster TTL
    cookbook.

    <wheels turn, smoke comes from ears>

    Ew. A one-and-only-one gate. All inputs low, output = low. One
    input high, all others low = output high. Two or more inputs
    high, output = low.

    Sounds like a _really_ exclusive-or gate! ;-P

    I think I'll use that as an exercise on these new tools that
    this latest client turned me on to. ;-) With a micro, it's
    trivial - count bits. With combinatorial logic - heck, in
    Verilog or VHDL it's the equivalent of a "switch" statement.

    Oh My God. We've come full-circle.

    Thanks!
    Rich
     
  20. Rich Grise

    Rich Grise Guest

    You've just designed a Rube Goldberg doorbell. ;-P

    Cheers!
    Rich
     
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