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how to acheive bi-stable impulse control with an SSR?

Discussion in 'Electronic Basics' started by [email protected], Mar 18, 2007.

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  1. Guest

    Hi, I'm re-designing a yet-to-be-implemented home remote control
    lighting system that uses mechanical latching bistable (or impulse)
    relays. These particular relays will change state with a low voltage
    pulse on only 1 input pin.
    I would like to convert the system to use SSR's but I'm uncertain
    which to choose and how to achieve the one-wire bi-stable control. I
    prefer SSR's for the higher operation count. A single relay will
    drive a resistive load at most
    of 2.5A 120V AC. The control voltage can be anything below 24V AC or
    DC. I don't care about the retain-state-even-thru-a-power-outage
    feature offered by the mechanical relays. I'm using simple momentary
    contact pushbuttons for controls with no debouncing.

    questions:

    1) Do any SSR's exist that already contain some or all of the extra
    features I need? (debouncing, bi-stable, one-wire control)? I'd like
    to keep it cheap --at most $25 per relay.

    2) The question #1 is no, can you point me to some simple SSR support
    circuits that would work?

    --zeb
     
  2. Ross Herbert

    Ross Herbert Guest

    I don't recall any SSR"s with all the functions you require.

    The 4013 D flip-flop is a suitable device to use for momentary on-off
    toggle operation.

    Some tuition material is here http://www.doctronics.co.uk/4013.htm

    Here the toggle input is provided by a 4093 astable pulse generator
    but this could be replaced by your momentary action switch. The SSR
    input would replace the output led. You can run it on 12Vdc.

    Since it would be wise to co-locate the 4013 toggle close to your SSR
    you should have some resistive input protection on the D input (eg. 1K
    ohm series resistor). Your momentary switch (which might be some
    distance away) must have some debouncing which could be achieved by
    simply using a 220 - 470nF cap directly across it.
     
  3. Chris

    Chris Guest

    Hi, Zeb. SSRs exist which provide the function you want -- the SSAC
    KRPS ProgramaCube comes to mind:

    http://www.ssac.com/catalog/KRPSGen.pdf

    If you decide to go this route, look at the data sheet to help specify
    your product.

    The hangup, of course, is that you're asking for this functionality at
    about the retail price of a standard SSR. Not gonna happen.

    If I might suggest something, why not just replace your switch with an
    alternating action pushbutton? That will give you the function you
    need for the price of a switch (i.e. much less than $25). You can
    then use a low AC or DC control voltage and replace with SSRs at your
    convenience.

    Cheers
    Chris
     
  4. Guest

    The one nice thing about the momentary switch control is that a pulse
    from one switch turns it on, a pulse from a different switch across
    the room
    can turn it off --no complicated 3-way switch wiring. I could have a
    10-way
    switch if I wanted.
     
  5. Chris

    Chris Guest

    OK, Zeb. If you want to use multiple inputs, an alternating
    pushbutton alone won't do the job.

    If the SSAC device seems a little pricey, or if you just would like to
    try your hand at some digital logic, you might want to go with the
    suggestion of Mr. Herbert and use a 4013 flip-flop. I'm assuming you
    have a ground-referenced power source at each pushbutton here to
    utilize the "one wire" concept. Each switch will have to have a
    following diode to prevent two power supplies from being shorted to
    each other when two switches are simultaneously pressed (view in fixed
    font or M$ Notepad):

    |
    | _/ To logic/ssr
    | .----o/ o-->|-- - - - --o----->
    | | |
    | --- |
    | - |
    | | |
    | === |
    | GND |
    | _/ |
    | .----o/ o-->|-- - - - --o
    | | |
    | --- |
    | - |
    | | |
    | === |
    | GND |
    | _/ |
    | .----o/ o->|--- --- - --'
    | |
    | ---
    | -
    | |
    | ===
    | GND
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

    At the logic side, you're going to have to provide input protection
    for your logic as well as debouncing. To supplement the 12VDC power
    supply and the CD4013, you should also add a 74C14 hex inverter with
    shmitt trigger input. This will provide hysteresis to allow "snap
    action" for the CD4013 clock, like this:


    VCC
    + 1uF D = 1N4001 or equiv.
    | +||
    '---||--o---------. VCC
    || | | +
    .-. | |
    10K| | | |
    | | | | +
    '-'.-------)--------. '-----o
    VCC VCC | | | | To SSR
    + + ===| |1/2 4013| .-----o
    | | GND| .---o---. | | -
    D - D - | | R | | ___ |/
    ^ ^ '---oD Qo----)---|___|-o-| 2N3904
    | ___ | |\ | | | 22K | |>
    o-|___|--o-|H>O----oCLK | | .-. |
    | 10K +| |/1/6 | | | 22K| | |
    D - 10uF--- 74C14 | Q'o----' | | ===
    ^ --- | S | '-' GND
    | | '---o---' |
    === === | ===
    GND GND === GND
    GND
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

    Be sure to apply power to the appropriate pins of the two ICs, and
    also to connect all unused CMOS inputs to GND.

    As always, be sure not to use line conduits for your low voltage
    wiring, and be careful to limit your experimental stuff to areas where
    safety isn't an issue. Build in haste, repent at leisure.

    Good luck
    Chris
     
  6. Guest

    Chris, thanks for that response. Your circuit has some features
    that I'm just beginning to understand. I have some developments, some
    clarifications and some questions.

    I decided to try and use a 5VDC power supply and the 74HC or 74AC
    families
    because I hear the 4000 series parts are being phased out --or at
    least
    not recommended for new designs.

    My 5VDC power supply will be in a central location in the house sent
    out
    the each switch with two wires thru metallic conduit. Up to 100 feet.

    Q1: Given that I'm now using 74xx, couldn't the Q output drive the SSR
    directly?

    Q2: I've figured out near the top of the schematic with the 1uF cap
    and
    the 10K resistor: You are providing a reset to make sure the FF
    comes
    up in a consistent state, correct?

    Q3: I don't understand anything to the left of the Schmitt trigger.
    Where's
    my switch supposed to go? Couldn't my
    switch feed directly into the trigger?

    Q4: I've noticed that all the Schmitt triggers have inverted outputs.
    Does
    this matter? Maybe the FF is only looking for an edge,
    regardless of
    direction?
     
  7. Chris

    Chris Guest

    Good to hear back from you, Zeb. First off, 4000-series CMOS is what
    you'd call mature -- not obsolete. I'd bet you'll still be able to
    find it in 7 years -- twenty, probably not. The good thing about
    using 12V for a power supply is that it's very common, and
    automatically gives you 2.4 times the noise rejection capability of
    the same logic at 5V for no cost. As for the rest, one question at a
    time:

    1) You'll have to look at the SSR -- most of them are capable of being
    driven by 5V logic, but the current requirements differ. Some require
    only 3mA at 5V. Those should be directly driveable by your 74HC
    output (typically can source or sink at least 4mA).
    If your SSR requires a higher input voltage or draws more than 4mA,
    you might have to add the transistor again.

    2) You got it -- the R-C provides a reset pulse at power on. If it
    truly makes no difference at all, you can omit it, and just GND the
    reset input like the set input.

    3) Sorry -- I should have given that. The switch input should go like
    this (view in fixed font or M$ Notepad):

    VCC VCC VCC
    + + +
    | | |
    .-. | |
    1K| | - D | D
    | | ^ -
    '-' | ^
    _/ | | ___ | |\
    .---o/ o->|-----o-----o---o-|___|-o-|H>O-
    | | | 10K +| |/
    | | | ---
    | _/ | - D ---
    o---o/ o->|-----o ^ |10uF
    | | | |
    | | === ===
    | _/ | GND GND
    o---o/ o->|-----'
    |
    ===
    GND
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

    Thanks for noticing the issue -- I split the circuit up and missed the
    1K pullup -- my bad. The circuit wouldn't have worked without it.

    4) Schmitt trigger ICs all hapen to be inverters. Not sure why.
    Actually, you can make a non-inverting schmitt trigger with two CMOS
    inverters and a couple of resistors, like this:

    | ___
    | .----|___|-----.
    | | 220K |
    | | |
    | ___ | |\ |\ |
    | Vin -|___|--o--| >O--| >O--o--- Vout
    | 33K |/ |/
    |
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

    You have to look at whether the flip-flop clocks on the positive-going
    transition or the negative-going transition. The above circuit uses a
    flip flop that clocks on the positive transition. That will occur
    about 100mS after you press the button in the circuit above. If the
    FF were NGT-triggered, it would be 100ms. after you released the
    button. Personal preference, except that people are accustomed to
    things happening when they push the button, not when they let it go.

    Have fun with your project. Good luck.

    Cheers
    Chris
     
  8. Chris

    Chris Guest

    That looks terrible -- try this:


    VCC VCC VCC
    + + +
    | | |
    .-. | |
    1K| | - D | D
    | | ^ -
    '-' | ^
    _/ | | ___ | |\
    .---o/ o-|<---o---o---o-|___|-o--|H>O-
    | | | 10K | |/
    | | | ---
    | _/ | - ---
    o---o/ o-|<---o ^ |
    | | | |
    | | === ===
    | _/ | GND GND
    o---o/ o-|<----
    |
    ===
    GND
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

    Cheers
    Chris
     
  9. Guest

    Some of this circuit baffles me. I think I know 1, 2 & 3 below. But
    what are 4, 5, & 6 for?

    1) The input to the schmitt trigger (inverter) needs to be held high
    because we want a high (inverted) output when the switch is pressed.
    2) The 1K resistor is there because we don't want to dump the entire
    current of the power supply to ground when the switch is pressed.
    3) When the switch is pressed the capacitor dumps its current thru the
    10K resistor and to ground making the input to the trigger low. If
    the 10K resistor was not there the cap discharge could fry the switch
    contacts.

    4) The diodes next to the switches?
    5) The "extra" connections to VCC with the diodes?
    6) The "extra" ground connection with the diode?

    --zeb
     
  10. Chris

    Chris Guest

    Hi, Zeb. 1), 2), and 3) are good. The extra diodes provide circuit
    protection for the logic. The extra diodes for the switches aren't
    necessary if you're using switches rather than another logic gate for
    the circuit inputs. Now the diodes from the inputs to Vcc and GND at
    the input to your flip flop circuit are there in case there's noise at
    the input which exceeds those limits. You don't want the input to the
    inverter to exceed the power supply evn for a microsecond -- it will
    fry the logic gate. The diode above the cap is there to prevent the
    cap from discharging through the logic gate when you turn power off.

    Try building the circuit as shown, and forget the diodes at the
    switches if you are using switches. It should work.


    VCC VCC VCC
    + + +
    | | |
    .-. | |
    1K| | - D | D
    | | ^ -
    '-' | ^
    _/ | | ___ | |\
    .---o/ o------o---o---o-|___|-o--|H>O- To 4013
    | | | 10K | |/
    | | | ---
    | _/ | - ---
    o---o/ o------o ^ |
    | | | |
    | | === ===
    | _/ | GND GND
    o---o/ o-------
    |
    ===
    GND

    |
    | VCC
    | + 1uF D = 1N4001 or equiv.
    | | +||
    | '---||--o---------. VCC
    | || | | +
    | .-. | |
    | 10K| | | |
    | | | | | +
    | '-'.-------)--------. '-----o
    | | | | | To SSR
    | ===| |1/2 4013| .-----o
    | GND| .---o---. | | -
    | | | R | | ___ |/
    | '---oD Qo----)---|___|-o-| 2N3904
    | From Shmitt | | | 22K | |>
    | Trigger ----oCLK | | .-. |
    | Inverter | | | 22K| | |
    | | Q'o----' | | ===
    | | S | '-' GND
    | '---o---' |
    | | ===
    | === GND
    | GND
    |
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

    Good luck
    Chris
     
  11. Guest

    Thanks for keeping with me. Off to the breadboard I go.
    --zeb
     
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