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Not enough current to trigger relay.

Discussion in 'General Electronics Discussion' started by COffin, Sep 27, 2015.

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  1. COffin

    COffin

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    Sep 27, 2015
    Hi All,

    I'm currently having trouble with a phone system and I just don't have enough electronics background to solve this problem.

    I have a phone that is supposed to activate an external horn. The horn is part of a separate system and can be energized by a 24Vdc source. Inside the phone I managed to find a 12vdc source that was used to run the buzzer. I thought that this was perfect, I would just use a 12V relay to act as a switch for a 24vdc source that I have available and the horn would sound when the phone is ringing - easy. The problem is that the 12Vdc source inside the phone lacks the current to trigger the relay - not easy. I basically need any scenario that makes the horn sound when the phone rings.

    Note: The original phone for this system that used to make all this happen is no longer available so I got to work with what I have.

    Thanks for any help!
     
    Last edited: Sep 27, 2015
  2. duke37

    duke37

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    Jan 9, 2011
    You could use a 24V relay and switch it with a FET or BJT, controlled from the 12V supply.
    Put a catch diode across the relay to prevent high voltages ocurring.
    You may be able to run without a relay, if ground isolation is not needed.
     
  3. Arouse1973

    Arouse1973 Adam

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    1,087
    Dec 18, 2013
    Hello
    All you need is a transistor switch and an external power supply(wall wart). The wall wart powers the relay and the output from the phone triggers the transistor.
    Adam
     
  4. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    LOL, Trevor, you beat me!
     
  5. hevans1944

    hevans1944 Hop - AC8NS

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    Hey, guys, I am schematic challenged at the moment. Would someone please post a simple wiring diagram for the OP?
     
  6. COffin

    COffin

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    Sep 27, 2015
    Please excuse the crude drawing but would this work?

    Also these are the transistors I have on hand:
    TIP120
    IRL520N
    LM7915CT
    2N3904
    D882-Y
    LM317T
     

    Attached Files:

  7. duke37

    duke37

    5,364
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    Jan 9, 2011
    That circuit will not work and I have a stiff neck so it is difficult to see.:)

    Measure the current drawn by your hooter when fed direct from your power supply.

    I cannot draw a circuit at the present, I will try to get the equipment in the next week or so.

    The components you list are not all transistors. Look up the data and list the primary characteristics of the transistors.

    Data is always good, you cannot design an optimum circuit when you do not know what is needed.
     
    Arouse1973 likes this.
  8. COffin

    COffin

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    Sep 27, 2015
    Thanks for all the help everyone but I don't think any of this will work anyway. When I was measuring the current for the horn circuit, I noticed that ringing wasn't continuous, it has its own circuit that creates a 2 second on and off sound. So if I want this to work, I would have to integrate a holding circuit for the 24V while the phone was ringing. Judging by how all this is going, It would all be way too far over my head.

    All the best and thanks again for your time!
     
  9. Martaine2005

    Martaine2005

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    May 12, 2015
    Hey COffin,
    Don't give up so quickly. This may be a lot simpler than you think.
    You may be able to connect a sound bomb directly to the phones ringer.
    Here in the UK, the land line has 75vac when the phone is ringing and only about 4 to 7vac when not.

    What phone is it? Model number?

    EDIT: Your 'horn' can be removed from the circuit board and used separately too.
    Martin
     
    Last edited: Sep 28, 2015
  10. AnalogKid

    AnalogKid

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    Jun 10, 2015
    Restating the problem:
    1. Input: Pulsing +12 VDC signal, 2 seconds on, 2 seconds off.
    2. Output: Steady +24 VDC switched to a horn.

    Important question: Is the circuit ground for the phone the same different from the circuit ground for the horn? In other words, does the phone have to be kept fully isolated from the horn? If not, a corrected version of your schematic probably can be made to work. But my guess is yes, the two circuits must be electrically isolated from each other.. ebay to the rescue.

    There are many 555 timer modules on ebay that have a trigger input and a relay output. Some are retriggerable. That is, if a new trigger pulse comes in before the timer is done responding to the previous trigger, the timer restarts without the relay changing state. A pulse that is 2 sec. on and 2 sec. off is a trigger once every 4 seconds. If the timer is set to 5 seconds, a string of input trigger pulses will look like one continuous signal at the relay contacts. The only possible drawback is that when the trigger pulses stop, the horn might keep going for another 4.99 seconds. The overall circuit function is called a missing pulse detector, and all implementations have the same characteristic.

    For this to work you will need a small wall wart as noted above to power the timer circuit. But it's all modules and wires, no soldering.

    ak
     
  11. COffin

    COffin

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    Sep 27, 2015
    Hi Martin,

    The phone is a Vingtor DT-700 Phone Series that is manufactured by Dial-Tone Industries Private Limited. I couldn't find anything online resembling it when I searched for it. The phone system here operates via a phone exchange(Zenitel SMP 200) that has 48Vdc output constantly whether it is ringing or not. It is a 2-wire setup so I'm not sure what prompts the phone to actually ring, maybe a frequency sent down the lines?

    To add, the horn circuit draws ~17.5mA
     
  12. Martaine2005

    Martaine2005

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    May 12, 2015
    I must have missed that bit.

    Martin
     
  13. AnalogKid

    AnalogKid

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    Jun 10, 2015
    You said there is not enough current from the phone to drive a relay coil directly. How much current is available? There are other switching devices that can be used.

    ak
     
  14. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Why can't the other system (horn) follow the timings of the phone ringing?
    Adam
     
  15. Tha fios agaibh

    Tha fios agaibh

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    Aug 11, 2014
    In the US the ringer signal is 90vac 20hz.

    I wish the country of origin was listed in the header of each post for clarification.
     
    Arouse1973 likes this.
  16. Minder

    Minder

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    Apr 24, 2015
    Canadian flag shown?
    M.
     
  17. Tha fios agaibh

    Tha fios agaibh

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    Aug 11, 2014
    I did not see a flag, nor a comment like "Not enough current eh?"
     
  18. davenn

    davenn Moderator

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    Sep 5, 2009
    forum.JPG


    ;) ;) ;) :)


    dave
     
    Arouse1973 likes this.
  19. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    :D

    80a4ded1d4e3738adbd32661dc181ac2_600x600x1.jpg
     
    Tha fios agaibh likes this.
  20. davenn

    davenn Moderator

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    Sep 5, 2009
    haha but , hell, I have missed comments an OP or other contributor has made and gone off on a tangent
    Its easy to do, specially when the computer screen is just a blur for me :(
     
    Last edited: Sep 28, 2015
    Arouse1973 likes this.
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