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auto dialer output to trip relay

Discussion in 'General Electronics Discussion' started by lk2554, Nov 6, 2013.

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  1. lk2554

    lk2554

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    Nov 6, 2013
    I am trying to use the phone output of the sensaphone 1104 (auto dialer when it detects dry contact input) as a dry input to a DIY cellular alarm. I would like the 1104 to manage my door sensors and then become an input to the cellular unit after it has done its delaying. The 1104 has no other outputs other than it will call a phone number when an alert condition exists after the programmed delay. Any thoughts on using the phone wire output? I had considered wiring a low voltage relay into the sensaphone, but, I don't have schematics for it, so I would be guessing. Thanks in advance for any words of wisdom
     
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    The alarm autodialler will probably have a line grab relay that activates to connect the coupling transformer to the phone line. This will be a small electromechanical relay or a reed relay. Probably an electromechanical relay, since the autodialler has a cascade output for other phones.

    When the autodialler needs to dial out, it will activate this relay and probably listen for a dial tone. Since you won't be connecting it to a line, it won't hear a dial tone, so it will release the relay within a few seconds.

    But you can monitor the contact closure of the relay with your circuit.

    In fact you don't even need to open up the autodialler. You can just detect the line current that the autodialler draws from the phone line when the relay contact is closed. You just have to provide a voltage to simulate the voltage from the actual phone line, since the autodialler won't be connected to a real phone line.

    To do this, connect a positive supply rail (ideally around 12V; 5V may not be enough) from your circuitry to one side of the phone line going into the main socket of the autodialler. Connect the other wire of the autodialler phone line through a 1k resistor to your 0V rail, and also through a 10k resistor to the base of an NPN transistor such as BC547 or 2N3904, whose emitter is connected to 0V.

    When the autodialler detects an alarm and tries to dial out, it grabs the line. This causes significant current to flow in the autodialler's phone line loop, and this current causes voltage to be dropped across the 1k resistor. This voltage forward-biases the transistor via the 10k base current limiting resistor, and the transistor pulls its collector down to 0V. The collector can be fed to a digital input of your cellular alarm circuit, and a pullup resistor (e.g. 10k) to the VCC rail. The circuitry in your cellular alarm system will see the transistor's collector go low for a few seconds each time the autodialler tries to dial out.

    You may want to change the dialling retries on the autodialler so it doesn't keep trying to dial out.

    If that's not clear please let me know.
     
    Last edited: Nov 8, 2013
  3. lk2554

    lk2554

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    Nov 6, 2013
    Thank you! Great answer and very clear. Thanks for taking the time to educate me. What you described is exactly what I was looking for.
     
  4. lk2554

    lk2554

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    Nov 6, 2013
    Kris, after looking at the circuit board, it looks like reed relay SD1A05AWJ is the one you are talking about. Its output looks to be tied to the transformer. Can I avoid any circuit construction by just isolating the relay contacts from the rest of the circuit and using it as the dry contact for my cellular alarm? The cellular alarm basically is looking for a short on its sensor inputs. It specifies dry contacts (no power of any kind for use as sensor input. I realize your way allows just using the phone wire output and non destructive hookup. I don't ever plan on using this with a phone line again, so I don't mind cutting the runs to the transformer and wiring in two contact wires if you think it will work. Thanks again
     
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Yes you can do it that way. But the line circuitry is already isolated; that's why they use the relay and the transformer.

    There's a chance that you can just connect the telephone line connections from the autodialler to the dry contact input on the cellular alarm and it will work, but it's possible that the circuitry between the relay contact and the line connection (especially the bridge rectifier next to the line connector) will prevent the autodialler from pulling the cellular alarm input low enough to activate it. There's also a chance that operation will be borderline, and it may work when you test it but fail when a real alarm occurs, because of differences in temperature, phase of the moon, etc, and you may not want to take that risk.

    In that case, you can disconnect the relay contacts from the line circuitry (you only need to disconnect one side) and bring the relay contacts out on a separate pair of wires, or you could also disconnect the line circuitry from the line connector and bring the relay contacts out on the line connector, so you don't need to add any new wires.

    Edit: You could even try leaving the circuit intact and bringing the relay contacts out on the two unused pins of the line connector, so the autodialler would remain compatible with a telephone line. You would need a line plug and cable with four connections though.
     
    Last edited: Nov 9, 2013
  6. lk2554

    lk2554

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    Nov 6, 2013
    Thank you, I will give it a try. I am going to cut one side of the relay and bring it out to the phone plug since the external phone wire can plug in the wall and the other side of the wall connection runs near the cellular alarm. I am cutting the run and not preserving the wiring for peace of mind that the remaining circuitry doesn't dump some unwanted signal or power into the cellular input. There appears to be an opto-isolator and some pull up resistors and caps tied to the relay output. Thanks again for your sage advice.

    Bob
     
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