Privacy and timer control for telephone pot line

Discussion in 'The Projects Forum' started by Kenny, Jun 11, 2008.

  1. Kenny

    Thread Starter AAC Fanatic!

    Oct 11, 2004
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    I needed to make a Privacy-Timer control for 2 phones working over the same pot line and this thing seems to work.
    The idea of the circuit is:
    "Phone A" is private, when "Phone A" is off-hook "Phone B" doesn't have line.
    "Phone B" is time controlled, when "Phone B" is off-hook the timer prepares to cut the line to itself in 60-90 seconds.

    I have a beginner-to-intermediate knowledge of electronics, knowing a bit of all with many holes.
    I want to design circuits, this is the first attempt to do it. With some
    measurings and experimentation I get this thing working, but I know that it is far from be a real device, however I wanted to apply basic knowledge (555 timer, voltage divider, transistor as a switch) and solve the problem.

    The design comprises 2 main blocks:
    1) OFF-HOOK DETECTOR
    When a phone is off-hook, it creates a voltage divider with the R68.
    With this small voltage (the minimum needed to saturate the transistor)
    the current across the 39K resistor switches the transistor.

    2) TIMER
    With a 555 astable multivibrator.
    When both phones are on-hook the 555 is in its ON mode (capacitor charging).
    When "Phone B" is off-hook the 555 begin its OFF mode, by connecting to GND the capacitor through another transistor.
    While "Phone A" is off-hook the 555 is kept reset, then cutting the line to the "Phone B".

    I can't use the voltage of the line to power the timer. The circuit seems to eat too much power even removing the LED circuit.
    But, I'm not sure about the inclusion of the external 9V Vcc source.

    I want to know your wise opinion about my solution.
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Q2 needs as collector bias resistor, even a large one, otherwise no signal can be generated.
     
  3. Kenny

    Thread Starter AAC Fanatic!

    Oct 11, 2004
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    What signal do you refer to?
    I'm using the transistors only as switches.
     
  4. RiJoRI

    Well-Known Member

    Aug 15, 2007
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    "Signal" refers to the output of the transistor, whether it's varying (as in data or sound) or 'steady' as from a switch.

    --Rich
     
  5. Kenny

    Thread Starter AAC Fanatic!

    Oct 11, 2004
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    Where is the electronics designers comunity?

    I thought that I could get really advise about my first try design, but I was disappointed.

    Thanks for the comment about the transistor, but is it all about it?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    My solution is to just get a 2nd line for your kids to use.

    Were I the user of the "B" phone, I would rapidly become highly annoyed, and would figure out a way to turn the tables on the "A" phone users.
     
  7. Dave

    Retired Moderator

    Nov 17, 2003
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    < No post...>
     
    Last edited: Jun 19, 2008
  8. Dave

    Retired Moderator

    Nov 17, 2003
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    Kenny, this thread is reopened. I refer to you the discussion on its initial closing. Thanks.

    Dave
     
  9. thingmaker3

    Retired Moderator

    May 16, 2005
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    I've studied your schematic, Kenny, and there are some issues you might want to address in your next version.

    First, the 555 output will always be low until the 555 is triggered. This means Q1 will not conduct, and phone B will not work, until the 555 is triggered. So far so good. The 555 cannot be triggered while phone A is trickling current though Q3 Base and thus holding the 555 reset. Still good so far.

    With phone B off-hook, Q2 is biased into conduction. (At least it would be if it had a collector bias resistor.) This will happen regardless of whether phone A is off-hook or not. When Q2 is conducting, C2 is charging. This will put a low level on the 555 trigger, but only briefly. Then the trigger and the threshold will both be held high until C2 is discharged.

    I see no means of discharging C2.

    Perhaps you would be better off simply using Q2, with a collector resistor in place, as a switch to trigger a different monostable circuit. A logic gate monostable can probably fill your need. I suspect you will end up with fewer overall components as well.
     
  10. Kenny

    Thread Starter AAC Fanatic!

    Oct 11, 2004
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    Thanks for the advice, now I can improve the circuit.

    I disliked the use of the 555 for this work but I didn't know the logic gate monostable working.

    I have some questions:

    • You said: "I see no means of discharging C2". But, I saw that "Phone B" off-hook makes C2 discharge by connecting it to GND through Q2, given that C2 have been charging while Q2 isn't conducting. When C2 discharging finished, the 555 output go low and so the "Phone B" has no line, so fullfilling the objective of "Phone B" call being cut at the time controlled by the RC astable circuit.

    • I have read something, and I can understand the purpose of biasing the transistor base when the signal is AC, but I don´t understand the purpose of collector biasing in my project, it has to do with the capacitor load instead of a resistor load?, can you give me a clue to the explanation?

    • What about the relationship between the independent 9V source and the telephone line voltage?, must I use some coupling technique?

    • What about the behavior of the entire circuit when the telephone line changes from 50V DC (On-hook), or 15V DC (Off-hook) to 90V AC voltage by a call incoming situation?
     
  11. thingmaker3

    Retired Moderator

    May 16, 2005
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    I'm apparently missing something. Can you describe C2's charge path?
     
  12. Kenny

    Thread Starter AAC Fanatic!

    Oct 11, 2004
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    Clearly I understand you.

    That is a problem that I faced because I have not used the right timer circuit, and I was unable to imagine how can achieve the desired behavior.

    However I tried this and the circuit works, but I can´t understand how the capacitor charge, I thought that the floating voltage state of the Q2 collector makes that such point has lower voltage than VCC and so work like GND.
     
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