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Fw: phone broadcaster

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  • Nicolae Sfetcu
    Thank you, Bela! Nicolae Sfetcu _/_/_/_/_/_/_/_/ ... From: bela ofori To: Sent: Wednesday, October 31, 2001 7:02 PM
    Message 1 of 1 , Oct 31, 2001
      Thank you, Bela!

      Nicolae Sfetcu

      ----- Original Message -----
      From: "bela ofori" <erau_3000@...>
      To: <nsfetcu@...>
      Sent: Wednesday, October 31, 2001 7:02 PM
      Subject: phone broadcaster

      > Phone Broadcaster Click here for the circuit
      > diagram
      > Here is a simple yet very useful circuit which can be
      > used to eavesdrop on a telephone conversation. The
      > circuit can also be used as a wireless telephone
      > amplifier.
      > One important feature of this circuit is that the
      > circuit derives its power directly from the active
      > telephone lines, and thus avoids use of any external
      > battery or other power supplies. This not only saves a
      > lot of space but also money. It consumes very low
      > current from telephone lines without disturbing its
      > performance. The circuit is very tiny and can be built
      > using a single-IC type veroboard that can be easily
      > fitted inside a telephone connection box of 3.75 cm x
      > 5 cm.
      > The circuit consists of two sections, namely,
      > automatic switching section and FM transmitter
      > section.
      > Automatic switching section comprises resistors R1 to
      > R3, preset VR1, transistors T1 and T2, zener D2, and
      > diode D1. Resistor R1, along with preset VR1, works as
      > a voltage divider. When voltage across the telephone
      > lines is 48V DC, the voltage available at wiper of
      > preset VR1 ranges from 0 to 32V (adjustable). The
      > switching voltage of the circuit depends on zener
      > breakdown voltage (here 24V) and switching voltage of
      > the transistor T1 (0.7V). Thus, if we adjust preset
      > VR1 to get over 24.7 volts, it will cause the zener to
      > breakdown and transistor T1 to conduct. As a result
      > collector of transistor T1 will get pulled towards
      > negative supply, to cut off transistor T2. At this
      > stage, if you lift the handset of the telephone, the
      > line voltage drops to about 11V and transistor T1 is
      > cut off. As a result, transistor T2 gets forward
      > biased through resistor R2, to provide a DC path for
      > transistor T3 used in the following FM transmitter
      > section.
      > The low-power FM transmitter section comprises
      > oscillator transistor T3, coil L1, and a few other
      > components. Transistor T3 works as a common-emitter RF
      > oscillator, with transistor T2 serving as an
      > electronic 'on'/'off' switch. The audio signal
      > available across the telephone lines automatically
      > modulates oscillator frequency via transistor T2 along
      > with its series biasing resistor R3. The modulated RF
      > signal is fed to the antenna. The telephone
      > conversation can be heard on an FM receiver remotely
      > when it is tuned to FM transmitter frequency.
      > Lab Note: During testing of the circuit it was
      > observed that the telephone used was giving an engaged
      > tone
      > when dialed by any subscriber. Addition of resistor R5
      > and capacitor C6 was found necessary for rectification
      > of the fault
      > __________________________________________________
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