Adding a relay to a driveway alert product

Discussion in 'The Projects Forum' started by chrisring, Sep 1, 2013.

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  1. chrisring

    Thread Starter New Member

    Sep 1, 2013
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    I could really use some help with this. Previously, I owned a Chamberlain brand driveway alert system, not only did it have its built in chime on the base unit, it had a relay on board that could connect to doorbells, a dialer, or whatever.
    Recently I ordered a replacement. The previous model being discontinued, I ordered the new model (Chamberlain CWA2000) that has no relay, only a small speaker, and blinking LED. I was confident I could find a place on the board to tap a relay into, and I did find one spot that reads 3.5 volts when alarming, but it does not have the amperage (?) to power a 3.3 volt relay. Other spots on the board read 7 volts when idle, and 11 volts momentarily when alarming. These will close a 9 volt relay even when in the 7 volt idle stage but I do not have the know-how to use that difference. Can anyone help?
     
  2. wayneh

    Expert

    Sep 9, 2010
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    How is this thing powered? Does it have a wall-wart AC/DC adapter?

    And what would you like to control, does it require DC or AC?
     
  3. chrisring

    Thread Starter New Member

    Sep 1, 2013
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    AC to DC wall -wart, with 4 AA batteries for backup. The relay closes an AC doorbell circuit that rings a buzzer in my shop and a bell in the house, and energizes another timed relay that turns off a green led at the front of the house and turns on a blinking red LED at the front gate.
    Anyway, I was hoping to find a relay that can be triggered by the speaker on the driveway alert directly, or even by its sound if necessary. The unit beeps a number of times depending upon which sensor has detected motion, so it is helpful for the relay to do the same.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    That spot you found that shows 3.5V when alarming is a place to start. It might be that you could use a small transistor to react to that voltage and power your relay from the DC power supply and battery.

    Connect the relay to power supply positive and the opposite side to the collector pin of an NPN transistor (or darlington). The emitter pin goes to negative. Connect the 3.5V signal to the base pin using a 470Ω resistor. That value is a guess - you may need to try smaller values. If 470 works, try 1kΩ and use that if it works, to limit the load on that point in the existing circuit.

    The relay should have a reversed-biased (normally non conducting) diode across its pins. This handles an emf spike that could damage the transistor when the relay coil is de-energized.
     
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  5. chrisring

    Thread Starter New Member

    Sep 1, 2013
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    Thanks so much, that's exactly the kind of solution I was looking for. I bet it works. I'll post the results when I get it done.
     
  6. chrisring

    Thread Starter New Member

    Sep 1, 2013
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    Wayneh-
    Thanks for your help. It was a nice practical little project for a beginner like me, and I learned a lot.
    It works fine too.
     
  7. shezza

    Member

    Mar 19, 2014
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    Had to reawaken this thread...
    The beep is just too quiet and too short, so I wanted to connect up a piezo buzzer, but also wanted to make it last longer than a beep. Is there in way to go besides using a Pulse timer relay?
     
  8. wayneh

    Expert

    Sep 9, 2010
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    You should probably start your own thread for this.

    It's a little hard to understand what you are asking. Are you following the OP's approach of driving a relay, or do you just want to extend the tone of the existing alarm?
     
  9. shezza

    Member

    Mar 19, 2014
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    I am terrible with forum etiquette! I shall start a new thread, mods... feel free to edit me out of here.
     
  10. George Sackinger

    New Member

    Mar 25, 2015
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    I'm trying the same mod. I haven't found the spot that changes voltage when triggered. Can you help me out?
     
    asipes likes this.
  11. ScottWang

    Moderator

    Aug 23, 2012
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    Hijack.
    You could start a new thread, and this thread will be closed.
     
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