How to trigger a relay on a 1-2 VAC voltage drop?

Discussion in 'Analog & Mixed-Signal Design' started by jwrothwell, Oct 2, 2018.

  1. jwrothwell

    Thread Starter New Member

    Aug 9, 2018
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    I'm trying to interface a Ring Doorbell Pro with an electronic chime in my whole-house intercom. The wires coming from the Ring Doorbell (Pro) have an AC voltage across them of 17.1 to 17.2 (bounces between those values on my digital meter), and when the doorbell button is pressed, the voltage drops to 17.0 volts. Is there any reliable way to detect this small voltage drop and then momentarily turn on a relay to pulse the electronic chime in my intercom system? This chime requires just a momentary (fraction of a second) circuit closure on the intercom board -- and these terminals also have an AC voltage (22V) across them, presumably because it's designed to work with the standard 24VAC doorbell transformer.

    If at all possible I'd like to do this without the complexity of an MCU. Possible? TIA.
     
  2. ScottWang

    Moderator

    Aug 23, 2012
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    Could you draw a block diagram and label their details.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    5,717
    Detecting that small AC voltage change could be problematic.

    What is energized when you push the button?

    Can you post a reference to the Ring Doorbell Pro?
     
  4. dendad

    Well-Known Member

    Feb 20, 2016
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    It will be easier to detect the door bell current and use that to operate your intercom.
     
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  5. jwrothwell

    Thread Starter New Member

    Aug 9, 2018
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    This is what I'm referring to:
    https://shop.ring.com/products/video-doorbell-pro

    Previous versions of this were battery-powered, but this one (the one I have) get their power from the 24V transformer that powers the typical chime or bell, with a Ring-provided "black box" that connects across the chime's striker solenoid coil. I have no idea what device does, but it somehow allows the Ring Doorbell to draw power for its circuitry without activating the chime until you actually press the button.

    Since I didn't have a mechanical chime, I inserted into the circuit, just the coil of one. That allows the Ring Pro to operate but I don't get a chime inside the house -- that's why I'm trying to interface that circuit to my electronic chime in the intercom.

    I'm going to attempt a block diagram as someone else requested. I've never done that on this forum but I'll try to muddle my way thru it...
     
    Last edited by a moderator: Oct 2, 2018
  6. jwrothwell

    Thread Starter New Member

    Aug 9, 2018
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    Thanks for that suggestion. I'll measure that now. The only load is a small solenoid coil, so I'm not expecting much current. We'll see...
     
    Last edited by a moderator: Oct 2, 2018
  7. crutschow

    Expert

    Mar 14, 2008
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    According to the manual, the voltage across the doorbell solenoid coil (two center wires below) from the Pro Power Kit (labeled Ring) should go from 0V to 28Vac when you push the button.
    You should be able to use that to control a 28Vac coil relay and trigger your inside chime.

    upload_2018-10-2_12-17-7.png
     
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  8. jwrothwell

    Thread Starter New Member

    Aug 9, 2018
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  9. jwrothwell

    Thread Starter New Member

    Aug 9, 2018
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    Yeah, I should have taken a step back and remembered that the solenoid coil would have a voltage applied when the button is pressed. I think i just got lost in the weeds of my measurements -- which read nothing across the coil. So it appears that the Ring power unit is defective -- I get nothing there when the button is pressed. I'll be contacting Ring Support now. Thanks for pointing out the obvious to me. ;-)
     
  10. jwrothwell

    Thread Starter New Member

    Aug 9, 2018
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    Thanks for responding. I prepared the diagram, but I don't think it's needed now -- crutschow has pointed me in the right direction.
     
    Last edited by a moderator: Oct 2, 2018
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