Looking for Help with a Voltage Controlled Switch

Discussion in 'The Projects Forum' started by eschatt, Dec 12, 2008.

  1. eschatt

    Thread Starter Member

    Dec 12, 2008
    17
    0
    Hi,
    I am working on a switch that would release a relay after I hang up after the third ring of the phone. I have worked out the code and tested it succesfully. My problem that I have run into though is creating a circuit so that the micro controller could "see" when the phone rings (voltage changes). I have included a diagram of the circuit that I am currently working with. When all goes well the ground voltage from the phone ringing will trip the transistor and in turn the relay; alternating the voltage from positive to negatvie so that the input for the BS2 can "hear" the rings. When the code hears the proper number of rings, it will trip a seperate transitor and relay combo, that part is working well. It works properly, code and all, when I connect it to a 5 volt power supply, simulating the phone ringing. But I have discovered that for one reason or another I am getting a voltage of about 4 Volts from the Base of the Transistor, which prevents the phone from ringing properly when a call comes in (it doesn't ring at all, it instead "picks up" immediately - the caller hears static).


    I have tried adding a diode and resistor to the base of the transistor but both attempts cause the circuit to stop working. And when I remove the 100 ohm. resistor the relay no longer works properly (it no longer switches), but the phone works properly then. I have looked at possibly using an SRC, but it looks like I might have the same problem of voltage interfering with the phone; would an SRC work in my case? Any other tips/alternatives/help are all much appreciated! Thanks for looking!
    -Eschatt
    [​IMG]
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Try replacing the 100 Ohm resistor with a 100k Ohm resistor, and putting a 10k Ohm resistor between the base of the transistor and the signal in.

    Right now, when the signal in tries to go below about 4.3v, it can't. It's held high by the base-emitter junction. (Think of it as a forward-biased diode.)
     
  3. eschatt

    Thread Starter Member

    Dec 12, 2008
    17
    0
    Hi,
    So know I am really scratching my head. :D. I wired the circuit with the modifications that Sgt Wookie recommended. Though now the relay will only be tripped when the ground is connected to the emitter. Then here's the part that really gets me, when I try hooking up the 12V ground from the phone (the ringer/speaker) it doesn't trip the relay. I am going to continue to experiment/play around with the circuit. Any ideas why this might be happening? or any alternative circuit set ups? I have been reading up maybe using CSRs or comparators. Thanks always!
    -Eschatt
     
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