Help with 110v controlled 12v circuit....switch? relay?

Discussion in 'The Projects Forum' started by matt7738, Feb 6, 2015.

  1. matt7738

    Thread Starter New Member

    Feb 6, 2015
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    I think I'm asking this the right way......What kind of switch works on 110v and switches a 12v circuit?

    My project involves using a security light with a photocell. My plan is to use the on/off status of the light bulb. The goal is for the presence of 110v to close the 12v circuit. The wires going to the bulb socket will be the 110v circuit. I'll use that to open or close a 12v circuit. The 12v circuit is independently energized.

    Basically, if 110v is present, switch is closed and 12v flows. If 110v is absent, the 12v circuit is open. The 12v circuit is 12v 2a, but may run up to 5a. I'm still working on that. Also working on another scenario where I may need to reverse the trigger....110v present will open the 12v circuit.

    Is a switch like this reasonable? Or would it be a relay?

    I don't think I can use the photocell directly on the 12v circuit since the bulb is 110v.

    The alternative I've come up with is to use the bulb socket connection to energize a power strip(or socket) with a 12v power supply plugged into it. I'd rather keep the number of transitions to a minimum and keep it neat. I'd also rather not solder resisters, capacitors, and what have you.

    Any help is appreciated.
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Small relay with a 120Vac coil and a screw terminal plug-in base is the safest. Other approaches conflict with this forum's TOS.
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    A 120v(ac?) relay is the basic item you could use to switch a 12v circuit.
    The relay is fitted with N.C. & N.O. contacts which will give you the inversion.
    Max.
     
  4. matt7738

    Thread Starter New Member

    Feb 6, 2015
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    Thanks. When searching for the relay, do I just look for '120v AC 12v DC' relay? Are those the correct terms?
     
  5. wayneh

    Expert

    Sep 9, 2010
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    Personally, I'd probably follow your plan B; tap the socket with an adapter from the hardware store (one that adds a socket to the base of a bulb) and plug a 12V DC wall wart (or smaller, more modern SMPS) into that. The problem with the relay approach is that you'll need to wire it and then enclose it safely, and you'll still need your 12V power supply. I'm not sure it will be smaller and neater. I guess it depends how you handle it.
     
  6. matt7738

    Thread Starter New Member

    Feb 6, 2015
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    My searches for a relay is turning up relays using low voltage to operate high voltage. I'm trying to go in the other direction.
     
  7. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Make sure your relay contact rating is sufficient for 12V @ 5A..
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You require a relay with a 120vac or dc coil, as the case may be, these are quite common.
    Max.
     
  9. matt7738

    Thread Starter New Member

    Feb 6, 2015
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    Thanks for the suggestions. I will look for a relay.
     
  10. Reloadron

    Active Member

    Jan 15, 2015
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    Not knowing your location or what is available to you makes it difficult to give you a part number and supplier. Here in the US for example McMaster Carr Supply. Enter Relays in the search box and you come up with dozens that will work for you. You see 120 VAC control (relay coil voltage) and contact ratings. You get one that plugs into an 8 pin octal base with the screw terminals Mike mentioned way back in post #2.

    Ron
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If you use ebay here is 351307643059 one you would need an octal base for it.
    Max.
     
  12. matt7738

    Thread Starter New Member

    Feb 6, 2015
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    Thanks for all the help.
     
  13. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    I recently needed an isolated low voltage when 240Vac power was present or not. I could have used a relay with a 240Vac coil to switch a low voltage dc circuit on/off. Instead I did it another way (mostly because I am "thrifty").

    Our local thrift shops have boxes and boxes of wall-warts (plug-in-power AC supplies) with any combination of AC out, DC out without a filter, with a filter, regulated, unregulated, etc. They sell them for a $1 or less.

    Go find one that puts out ~12Vdc and connect it to the same place you were going to connect the relay...
     
  14. ebeowulf17

    Active Member

    Aug 12, 2014
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    The thrift store method is great until you need the reverse function. If high voltage OFF needs to turn low voltage ON, then the DPDT relay will make life way easier.
     
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