Momentary switches actuating latching relay

Discussion in 'The Projects Forum' started by Docktor, Feb 19, 2015.

  1. Docktor

    Thread Starter New Member

    Dec 31, 2013
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    Hi. I've spent the past six months slowly searching the AAC forums for a simple solution for a circuit design on board my motorboat.

    There is a searchlight on the boat that I want to be able to switch on and off from two separate locations, an inside steering location and an outside steering location.

    I want to use momentary switches in both locations so that the switches remain in a uniform position - and don’t look actuated in one location and not in the other. I’d like to keep it really simple and utilise a latching relay because the cable run to one of the steering positions is a long distance and I want to use a smaller gauge wire (tinned marine grade cable is expensive where I live!). Most all circuit designs that look like they might work in my application use a push to make, and a push to break, button - whereas I want to use the same button in each location to make and break - regardless of which switch location was used to initially switch on the searchlight.

    I’d be grateful for any suggestions.

    Many thanks - from Ireland!
     
  2. MikeML

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  3. MaxHeadRoom

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    Jul 18, 2013
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    How about relay flip flop same button, N.O. momentary for each function, just run a couple in parallel,
    Max.
     
  4. Docktor

    Thread Starter New Member

    Dec 31, 2013
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    MikeML - thanks, three-way circuit definitely ticks the box for keeping it simple on boar the boat - but I outruled it for my application because the searchlight is running 100W on a 12V system - allowing for voltage drop it meant considering larger cables than I wanted to use - that's why I am considering a relay setup.
     
  5. Docktor

    Thread Starter New Member

    Dec 31, 2013
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    Max - could you expand on the relay flip-flop a little. I am American Boat and Yacht Council Electrical certified - and a ham radio operator - but my logic experience is somewhat limited.
     
  6. MaxHeadRoom

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    Jul 18, 2013
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    I have used the circuit shown in the past for a simple PB flip flop, IOW one push ON, second push OFF, one PB.
    There are 3 spare sets of N.O./N.C. contacts on the relays shown, if you use different relays or voltages the R/C values, may have to be modified to suit by experiment.
    In your case two N.O. P.B.'s in parallel one for each station.
    Max.
     
  7. crutschow

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    Mar 14, 2008
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    You could use an impulse relay.
    They alternate contact position with each pulse.
    These can be mechanical or electronic.
     
  8. Docktor

    Thread Starter New Member

    Dec 31, 2013
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    Thanks Max - this might be beyond my ability to modify - but I'm going to look at this further.
    Crutschow - an impulse relay definitely sounds like what I had thought would work in my application - can an impulse relay have two actuators - in parallel I presume?
     
  9. MikeML

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    You can use small wire in the 3-way switching circuit to power the 12Vdc relay coil, which requires wire sized only for ~200mA. The wire between the battery, relay contacts, and the searchlight can be sized for 10A...
     
  10. MaxHeadRoom

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    A magnetic latching relay usually requires two pulses, one polarity to attract, another to repel.
    This usually precludes one P.B.
    The relay flip flop will revert to the off state when powered off, the magnetic latch has memory and remains in the last state.
    Max.
     
  11. Docktor

    Thread Starter New Member

    Dec 31, 2013
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    Thanks MikeML - that's neat and really does keep it clean and simple - which is what I need on the boat.
    Max - ultimately, the relay flip-flop would be more advantageous when the battery isolator is turned to the "off" position - next time the battery isolator is turned "on" I would rather that the searchlight is not powered!

    I have found some sealed, automotive, impulse relays online that might fit my application specs. I need advice on the circuit design as there is one other factor to my switches that I want to incorporate......

    Each momentary switch has a LED indicator "telltale" that could be used to indicate the state of the circuit if energised. Would the best way to do this be to run a current limited parallel cable off the high amperage searchlight circuit after the relay - or is there a more direct (less cable run, modularity) option to achieve this?
     
  12. Reloadron

    Active Member

    Jan 15, 2015
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    You can do this using push buttons and a relay latching circuit all low current to power an automotive 12 volt relay to feed the higher current lamp. You can also have an On indicator as well as an Off indicator. All things considered while not as fancy I would run with the three way switch design Mike suggested to simply turn On or Off an automotive relay feeding your light. Simple and reliable. You want an On lamp you can place one beside each switch. You really can have it anyway you want including with some fancy switches but all you want is a reliable search light on a boat. Personally, I will go the simple route given the choices.

    Ron
     
  13. MaxHeadRoom

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    Wouldn't the visual of a search light being ON be a suitable tell tale? ;)

    P.B.'s with indicator lamp?
    (The relay circuit has three sets of spare contacts)
    Max.
     
  14. Docktor

    Thread Starter New Member

    Dec 31, 2013
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    Thanks Ron and Max - apologies for the delay in updating........work commitments.

    I think the three-way switch is the most practical solution too - simple and little to go wrong. My only problem is that the switches that I am using - Carling Contura 3 - are currently a mixture of OFF-(MOMENTARY) ON and OFF-ON. All switches are "bottom half depressed" on the open position and "top half depressed" on the closed position - this looks uniform on the steering location. Using a three way switch would mean (MOMENTARY) ON - OFF - (MOMENTARY) ON - which would change the overall appearance of the helm.

    The Contra switches that I am using have two leds each - one "locator" led which illuminates the upper window of the switch (this led is connected to the instrument backlight circuit) - and, the other "indicator" led which is in the lower half of the switch which I planned to use to indicate when the consumer is energised - this is where I am asking for help......is my only option limited to running a long, low amperage, cable run from the relay back to the switch to indicate when the relay output is energised?

    Many thanks.

    Phil.
     
  15. Docktor

    Thread Starter New Member

    Dec 31, 2013
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    Here is the newly constructed outside helm position - sorry the image quality is not great - but you get the idea of the Contra switches looking uniform in the off positions..... Outside Helm.JPG
     
  16. Reloadron

    Active Member

    Jan 15, 2015
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    If you want to use a single momentary switch it can be done. A circuit could be put together using a flip flop driving a transistor that would turn a relay on and off. Bumping the momentary once would turn the light on and bumping it again would turn the light off and the LED in the switch could be worked into the scheme. Something like that maybe? The relay could just be a standard automotive type 12 volt relay.

    Ron
     
  17. MaxHeadRoom

    Expert

    Jul 18, 2013
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    IMO if you intend building an electronic circuit, you may as well use the relay flip flop (#3) and use the single momentary switches you wanted.
    Max.
     
  18. crutschow

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    Mar 14, 2008
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    After some thought on the matter I generated a circuit that will light the PB switch based upon the state of the relay using only one wire to the switch (and the chassis ground, of course) with no added circuitry at the switch.

    An LTspice simulation of the circuit is shown below.

    The key to the circuit is transistor Q1 which detects the current from the relay driver to the switch LED when the relay is energized. The transistor action for this state causes the voltage at Node 3 to be high.
    (D2 prevents reverse-bias base emitter breakdown).

    When the PB is pressed, the current from the relay driver to the LED stops (since the LED current is now supplied by the PB voltage), causing the transistor to turn off and dropping Node 3 voltage to 0V.
    This causes the XOR gate to generate a clock pulse to toggle the flip-flop U2 to the OFF relay/LED state.
    (C1, R3, and D3 are for switch debounce.)
    (Note that the LED doesn't go off until the PB is released.)

    Pushing the PB momentarily again, with the relay off, raises the Node 1 voltage which causes the XOR gate to generate another clock pulse, triggering U2 back to the relay ON state.

    The circuit can be controlled by more than one PB in parallel.
    All the PB LEDs will indicate the state of the relay.

    Note that the power and ground pins for the FF and XOR gate are not shown and must be connected.
    Also all the unused inputs for the other XOR gates and FF in the packages must be tied to ground.

    Edit: The note by the switch should say "Simulated PB Switch".
    Here's my write-up of the circuit with an added power-on reset circuit.

    PB Relay LED.gif
     
    Last edited: Feb 24, 2015
  19. crutschow

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    Mar 14, 2008
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    Docktor, did you ever decide on a solution to your problem?

    Edit: Here's my latching relay circuit design with an update for an alternate circuit that can be operated by a push-button to ground, thus eliminating the need for any power at the push-button switch.
     
    Last edited: Mar 2, 2015
  20. Docktor

    Thread Starter New Member

    Dec 31, 2013
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    Crutschow - sorry for the delay in replying - I seriously appreciate the consideration you have given my project - thank you. I really am a doctor - orthopaedic surgeon - and it's been a busy trauma schedule for me over the past two weeks.

    The boat is made of steel and therefore I need to use an isolated/insulated return to ground - and can't rely on the hull for common ground as you would consider doing with the chassis of an automobile.

    Rona & Max got me thinking a bit more about using a flip-flop and I eventually came across a CANBUS-esque system which might work in my application - there's a link to it here http://www.navelitaly.com/public/info/f/ad116.pdf

    This module would allow me control, with momentary push-button switches, from at least two locations for each circuit I want to energise. In addition, it is rated at 10A. Finally, the control wires are small gauge - probably CAT5 cable - which would help my long cable runs.

    The only thing is that it does not solve my "circuit active" light dilemma.

    Anyone got experience with this type of control actuator - this one might be expensive as it is marketed at the marine industry - and will therefore have a premium cost!
     
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