Lighting Control

Discussion in 'The Projects Forum' started by Nexxus, Mar 4, 2014.

  1. Nexxus

    Thread Starter New Member

    Mar 4, 2014
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    I am trying to build a very basic lighting control module that will turn on and off a common household light (120VAC 60 Hz) with a signal from a micro controller.

    I have a standard 3 wire connection at the light switch where I want to put this module consisting of a hot, neutral, and ground wire. I want to use a SPST relay to connect the hot and neutral wires and hopefully turn on the light, but my question is where should I connect the ground wire?

    My understanding is that the hot and neutral wires carry current and the ground wire acts as a safety measure connecting to the metal housing of the light switch, microwave, etc... so that you don't shock yourself if the hot or neutral wires come into contact with the housing of the project.

    I found this SPST relay on Digikey that is rated to handle 120 VAC but I am not sure where I should connect the ground wire on this relay or if I should connect it to the ground of the protoboard that I am building my circuit on.
     
  2. inwo

    Well-Known Member

    Nov 7, 2013
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    There should not be (only) a hot, neutral and ground in a light switch.:confused:

    How was the existing switch connected?

    I'll let the moderators decide if you are qualified to do this.
     
  3. sheldons

    Active Member

    Oct 26, 2011
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    no what you will have at the switch connection is a live feed-a switch wire-and your earth connection- the neutral will be in the ceiling rose directly connected to one side of your light
     
  4. Nexxus

    Thread Starter New Member

    Mar 4, 2014
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    With my current switch all I have are three wires that connect to it, a white, a black and a copper (no insulation) wire. I looked up the color indications and found that white is neutral, black is hot, and the copper wire is ground (for USA standard wiring). What connections should I see?
     
  5. inwo

    Well-Known Member

    Nov 7, 2013
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    That's different!

    A white, black, and ground is fine.
    That is not a hot, neutral, and ground.

    The white is hot, if connected correctly. And it should be marked with tape or such.
     
    Last edited: Mar 4, 2014
  6. #12

    Expert

    Nov 30, 2010
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    Sheldons has it right. It is common practice for an electrician to run a standard cable to the switch, 12-2-g. It displays as black, white, and green (or naked) but it is not, "to code" in the way you might think. The white is not neutral or the black is not power. One of them is the switched wire to the load. If there are only 3 wires in a switch box, one of them is not normal!

    ---------------------SAFETY CONCERN!-----------------
     
  7. Nexxus

    Thread Starter New Member

    Mar 4, 2014
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    I have enough experience with high voltage AC not to get myself into serious trouble, just never done anything inside of a house so I want to be careful and learn more about it.

    Ok great, so my white is hot. This makes me think (correct me if i'm wrong) that if I were to put the white wire and the black wire across the switch the light should light up just fine. The third wire should be connected to ground on the board where I am soldering the circuitry to.
     
  8. #12

    Expert

    Nov 30, 2010
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    That's why I kept correcting after you saw the post. I forgot which one is not what you expect.
     
  9. inwo

    Well-Known Member

    Nov 7, 2013
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    Correct.

    How will the connections be done?
    A separate enclosure?

    If yes.
    I would use an in-line fuse on the wires in the switch box. Run all three wires to your project. The ground only being connected to any exposed metal or fasteners.

    I would not ground to circuit board. Others may disagree. And maybe even convince me to ground circuit.:D

    See what #12 thinks. Could go either way!
     
    Last edited: Mar 4, 2014
  10. inwo

    Well-Known Member

    Nov 7, 2013
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    Sorry.:p I'm to quick.
    I hate those guys that quote. No chance to edit.:D:D

    I'll delete.
     
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  11. #12

    Expert

    Nov 30, 2010
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    You can do all the switching you want without the circuit board being connected to "bond" as long as it is entirely enclosed in an approved box. If the box is metal, bond that. If it isn't, there is nothing to connect the bond wire to.

    Darn electricians, keep changing the rules.:mad:
     
  12. Nexxus

    Thread Starter New Member

    Mar 4, 2014
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    Yes I will definitely use in line fuses.

    My original plan was to have a plexiglass box to house the SPST switch as well as the wiring in. My MCU would be separated from this box and only have two wires going to the coil of the SPST switch inside the box and use a different ground.

    What are your concerns for grounding directly to the circuit board?
     
  13. inwo

    Well-Known Member

    Nov 7, 2013
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    I think the reason OP stumped me on this, is that there is really no safe way to do this with CB mounted relay.

    What is needed is an approved relay which keeps the mains wires and low voltage wires separate.

    Such as these.
    One is HVAC type fan control.
    Other is occupancy relay control.
    http://www.ebay.com/itm/like/350814936976?lpid=82
     
  14. Nexxus

    Thread Starter New Member

    Mar 4, 2014
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    Ok so if I get an approved box to house the switch in, there is no need to connect the bond wire to anything at all? I'm assuming a homemade plexiglass box would not go under the 'approved' section.
     
  15. inwo

    Well-Known Member

    Nov 7, 2013
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    None, I guess.:)

    It seems unnecessary, and is a connection to mains that could only cause confusion.:confused:
     
  16. Nexxus

    Thread Starter New Member

    Mar 4, 2014
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    Thanks inwo, are there any approved relays you recommend? I'm looking for something that has a low DC coil voltage (6 - 10 V)
     
  17. #12

    Expert

    Nov 30, 2010
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    Expanding: I do lots of control circuits. Relays with 24 VAC coils switching one side of a 240 VAC power line most of the time, other 24 VAC circuits on the load side part of the time. Half a dozen relays and a circuit board or two in one big, metal box and none of them are "bonded". The box is bonded. Any power cables entering or leaving the box have their bonds connected together. That covers it.
     
    inwo likes this.
  18. inwo

    Well-Known Member

    Nov 7, 2013
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    The second picture is a universal control relay that has a coil voltage of 10-30vdc.

    If wall box isn't too full, it may fit inside with only low voltage wires coming out.
     
  19. #12

    Expert

    Nov 30, 2010
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    Worse. Connecting bond to neutral at any place besides the breaker box is both a code and a safety violation because it intentionally connects bond to a current carrying conductor.
     
  20. inwo

    Well-Known Member

    Nov 7, 2013
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    I was talking (and OP) about the ground wire, not the neutral.
     
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