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!
----- Original Message -----
From: "bela ofori" <erau_3000@...>
Sent: Wednesday, October 31, 2001 7:02 PM
Subject: phone broadcaster
> Phone Broadcaster Click here for the circuit
> 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
> 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
> 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
> 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
> when dialed by any subscriber. Addition of resistor R5
> and capacitor C6 was found necessary for rectification
> of the fault
> Do You Yahoo!?
> Make a great connection at Yahoo! Personals.
Your message has been successfully submitted and would be delivered to recipients shortly.
Changes have not been saved
Press OK to abandon changes or Cancel to continue editing
Your browser is not supported
Kindly note that Groups does not support 7.0 or earlier versions of Internet Explorer.
We recommend upgrading to the latest Internet Explorer, Google Chrome, or Firefox. If you are using IE 9 or later, make sure you turn off Compatibility View.