Transistor Circuit Noise

Discussion in 'The Projects Forum' started by eschatt, Oct 6, 2009.

  1. eschatt

    Thread Starter Member

    Dec 12, 2008
    17
    0
    Hi,
    A project that I have been working on is to create a switch controlled by the telephone that I can use to turn on my computer remotely. Right now it is pretty basic, the circuit that I am using allows the parallax basic stamp to count the number of rings of the telephone (just as if I was pressing a button. The basic stamp is hooked up to the relay switch in the circuit). If I hang up after the fourth ring of the phone my computer will be turned on by the basic stamp.


    The circuit I am currently using is similar to the one below (except the constant voltage is about 6 volts instead of 12 because the input from the phone is only about 12 volts):
    [​IMG]

    The project is working now, but it creates a lot of background static on the home phone line. Does anyone know why it might be creating this noise? I have played around with bridge rectifiers and when connected to the input from the phone it seems to help cut down on the static a bit.
    I am also open to other ideas on how to turn on my computer remotely that people have heard of.

    Thanks in advance for any help!
    -Eschatt
     
  2. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    How exactly are you hooking this circuit up to the phone line?

    What is driving the input to R1?
     
  3. eschatt

    Thread Starter Member

    Dec 12, 2008
    17
    0
    Hi StayatHomeElectronics,
    First, I am hooking this circuit to the phone, by connecting the circuit to the ringer; Using the voltage when a call is received and the ringer goes off to trip the relay.

    Second, the voltage from the phone (more precisely the ringer) is driving the input to R1. The positive voltage from the ringer goes to R1 and the negative voltage is grounded into the circuit (I apologize if my technical talk is not totally accurate or correct).

    Thanks!
    Eschatt
     
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    One of the things you may need to consider is some sort of isolation scheme between your detection circuit and the phone line. Here is a link to a ring detector circuit that illustrates one isolation technique.

    hgmjr
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Your circuit is grounding one side of the phone line, thereby unbalancing it. That is what is causing your noise. Second, the Telco will soon be knocking on your door to find out what the <snip> you are doing to their line!
     
    Last edited by a moderator: Oct 7, 2009
  6. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    The situation highlighted by mikeml is the reason you need to isolate your circuit from the phone-line in a manner similar to the one I mentioned in my earlier reply.

    hgmjr
     
  7. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Just currious... Are the Zeners absolutely necessary? I'm guessing that that author used them to insure that a voice signal would not false trip the circuit. I would think that the voice signal (superimposed DC) level would be sufficiently lower than the Ring AC voltage so that the value of R1 could just be increased, if not using the Zeners. This is more of a question rather than a statement. :)
     
  8. eschatt

    Thread Starter Member

    Dec 12, 2008
    17
    0
    Hi,
    hgmjr's circuit seems to be just the answer to my question. Thanks!
    -Eschatt
     
  9. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
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    I build one of these 40years ago. In those days, I used an NE2 neon lamp in series with a ~100K resistor connected across the phone line. The NE2 was mounted next to a CDS photo-resistor, so its resistance changed when the phone rang.
     
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