LED voltage to trigger a relay

Discussion in 'The Projects Forum' started by BillyShope, Sep 29, 2014.

  1. BillyShope

    Thread Starter New Member

    Sep 29, 2014
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    I wish to trigger a relay by exposing the sensor from a nightlight to an area on my computer monitor which I can change from black to white. The tiny board on the nightlight, containing the LED, has been removed and wires soldered to the connections at the light's larger board. To this point, everything works beautifully. Trouble is, the up voltage is only a little over 4 volts and it never drops below about 0.75 volts. I don't want to control the Hoover Dam generators, but I would like to control, say, a garage door opener, though it might take a chain of 3 or 4 relays to get there.
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Look into a logic level MOSFET; they are often used to control DC voltages that are greater than the supply voltage. If you can post the voltage and current of the device you want to control, you will get more specific responses.
     
  3. BillyShope

    Thread Starter New Member

    Sep 29, 2014
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    Thanks, but don't wish to control a DC voltage. I wish to control 110V AC at 8 amps. My trigger is a little over 4V of DC.
     
  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Then your best bet is an optocoupler controlling a triac circuit or a solid state relay. Or if you are set on relays, there are relays with low voltage DC coils that can be used to control a larger relay. I am not aware of a low voltage relay that will directly switch 110 vac at 8 amps.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If it the normal residential Garage door opener then you rarely have to switch the 120vac to control it?
    But if 120vac is the aim, then I agree with the opto and triac circuit, it will work out just about as cheap anyway.
    Max.
     
  6. BillyShope

    Thread Starter New Member

    Sep 29, 2014
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    Thanks, guys. I need a source for the relays. Many years ago, I helped a single mom with a bed-wetting child by using a chain of relays to enable the Sonalert voltage of a bed-wetting sheet to a point where it would operate a 110V AC solenoid attached to a fog horn. Never had to use it. When the kid heard that fog horn, that was the end of the bed-wetting. Anyway, that was when Radio Shack was a real hobby store. Now, I don't know where to turn. I'll look into the optocoupler. I'm a retired engineer, but it's ME, not EE.
     
  7. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Check the specifications of this. http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_162342_-1
    I think it would do what you want. There are cheaper ways, but they are more difficult.
     
  8. BillyShope

    Thread Starter New Member

    Sep 29, 2014
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    Sounds great! But, to an ME, it sounds like magic (like a thermos bottle to an EE, as the old joke goes). So, this thing triggers with a DC of anywhere between 3 and 32 volts? And drops out if the voltage drops below 3 volts? And switches household voltage? Sounds crazy, but, if that's what it does, I'll take it! And, thanks a lot. Wish I had found this when I was working with that bedsheet.
     
  9. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    That's the way I read the specifications. A solid state relay (SSR) is really a more complex circuit that usually consists of an optocoupler and a triac along with a few other bits packaged together. I have bought a few SSRs, but it's cheaper to make my own. Good luck.

    ETA: I am neither an EE nor a ME; in fact, I am not an E of any kind. I am a retired MBA who likes to tinker with electronics.
     
  10. BillyShope

    Thread Starter New Member

    Sep 29, 2014
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    Thanks, tracecom. My masters is in automotive engineering, which also is of little help. I'll be ordering one of those SSRs.

    Please consider my original post answered satisfactorily.
     
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