Isolate AC doorbell circuit for external standard doorbell chime

Discussion in 'The Projects Forum' started by retiredtech, Dec 28, 2015.

  1. retiredtech

    Thread Starter New Member

    Dec 6, 2015
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    Have an existing Nutone LA-305WL doorbell. The 16 V AC from the doorbell button activates the LA-305 via an electronic circuit, most likely an opto coupler on the board. If I connect an external standard old ding-dong doorbell in parallel with the existing unit it causes the LA-305 to ring constantly. So I need to create a circuit that will isolate the old doorbell away from the LA-305 as every attempt to use relays and or bridge circuit continues to leak AC back into the LA-305 input and run the doorbell constantly.
    My thought would be to use an AC opto coupler like the H11AA1 and its output to drive a 12 volt DC relay which in turn would send the 16 V AC to the external doorbell. I need only the front door for the external doorbell.
    Any thoughts would be appreciated....
     
  2. tranzz4md

    Member

    Apr 10, 2015
    139
    27
    For me the easiest solution would definitely be a relay. Get any relay that will activate with your doorbell transformer that has an isolated normally open contact, and wire your LA305 across it. Just connect your old style doorbell and relay coil in parallel. Radio shack has changed a lot so I don't know about them anymore, but you could get a relay with or without a socket from an electrical supply house, Graingers, Fry's, whatever. Unfortunately old doorbells seemed to always have odd voltages (I think 20v was common, and 16 too), but an AC relay coil can tolerate a pretty good variation for such a short event, or throw a resistor in series with the coil. I make these remarks assuming that this LA305 device is a solid state electronic circuit type device,,, but I have no knowledge of it. In my experience, Nutone devices are weird, cheesy mechanisms that are stylishly enclosed and strike a chime bar with a decent timbre.
     
    Last edited: Dec 29, 2015
  3. sailorjoe

    Member

    Jun 4, 2013
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    I don't understand why your original solution didn't work. Isn't your old style doorbell just a push button switch spring loaded so it's normally open? Can you verify that? Maybe the thing is shorted? Can you verify that your doorbell system is wired essentially like the attached diagram? Thanks.
     
  4. retiredtech

    Thread Starter New Member

    Dec 6, 2015
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    0
    As shown on my "doorbell.jpg" diagram, this is how the electronic door bell is wired. It requires both sides of the 16 V transformer to power the internal DC circuits on the electronic board. When the door bell button is pressed the other side of the 16 V AC is sent to the "F" terminal which, I assume, has an opti coupler circuit in it to sense the AC. Now when I parallel the old solenoid circuit the AC is fed back through the solenoid to the Front Door input circuit causing the LA-305 to run all the time. I need to get the solenoid portion isolated away from the "F" terminal. That is why I was going to create a similar optical circuit.
    Amazon has some solid state relays that are optical input so may start out trying one of them.
     
  5. sailorjoe

    Member

    Jun 4, 2013
    361
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    Why assume that there is an optical coupler for the F terminal, please? Could just be a normal connection to an internal relay coil.
    The solenoid is for a buzzer indicator downstairs, yes? It's wired so that the solenoid circuit is completed when the front doorbell is pushed.
    I'm betting that your new switch is in parallel with the solenoid, not the front door bell button. A button in parallel with the existing button can't change the operation of the circuit.
     
  6. retiredtech

    Thread Starter New Member

    Dec 6, 2015
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    I am having difficulty in following your analysis of this circuit? First, in talking to the tech support at Nutone they assured me that the "F" input terminal was indeed an optical coupler and NOT a relay. Second, the downstairs doorbell is a 16 V AC solenoid that strikes 2 sounding bars when momentarily activated. The circuit that I show above is correct and the ONLY way to isolate the downstairs doorbell is to create a similar optical coupler like on the LA305 doorbell. This way when you press the front doorbell button the 16 V AC will be isolated away from the doorbell solenoid downstairs which will now get its 16 V AC through a separate circuit using a 12 V DC relay and opto coupler.
    Like the tech guy said this LA305 doorbell is designed to only be used with another LA305 door bell and NOT a standard old "ding-dong" doorbell.
     
  7. retiredtech

    Thread Starter New Member

    Dec 6, 2015
    4
    0
    UPDATE
    After talking to the tech again he suggested doing just as "Tranzz4md" suggested above. Just use a relay and isolate the LA305 and put the old doorbell in parallel with the relay so they both get activated together. Forget all this optical stuff! I am building the circuity now.....
     
  8. sailorjoe

    Member

    Jun 4, 2013
    361
    63
    Ok, I get it now. I got confused between "doorbell" and "doorbell button" in the dialog.
    The solenoid is another doorbell system with a separate doorbell button.
    Sorry for my confusion.
     
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