PHONE BROADCASTER

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.

AM TRANSMITTER
==============The main feature of this trans- mitter is that it is free from the LC (inductor, capacitor) tuned circuit and
operates on a fixed frequency of 12 MHz which is extremely stable. An LC based tuned circuit is
inherently unstable due to drift of resonant frequency on account of temperature and humidity
variations. The circuit is very simple and uses only a few components. The figure shows the complete
circuit diagram of the transmitter. Resistors R1 and R2 are used for DC biasing of transistor T1. The
capacitor C1 provides coupling between the speaker and the base of transistor T1. Similarly, resistors
R3, R4 and R5 provide DC bias to transistor T2. Resistor R5 also provides negative feedback which
results in higher stability. The oscillator section is a combination of transistor T2, crystal Xtal,
capacitor C2 and resistors R3, R4 and R5. The crystal is excited by a portion of energy from the
collector of transistor T2 through the feedback capacitor C2. Thus the oscillator circuit generates the
carrier frequency at its fundamental frequency of 12 MHz. Any crystal having the frequency in short
wave range can be substituted in this circuit, although the operation was tested with a 12MHz crystal.
Transistor T1 serves three functions:
* It provides the DC path for extending +Vcc supply to transistor T2.
* It amplifies the audio signals obtained from speaker.
* It injects the audio signal into the high frequency carrier signal for modulation.
The loudspeaker converts the voice message into the electrical signal which is amplified by transistor
T1. This amplified audio signal modulates the carrier frequency generated by transistor T2. The
amplitude modulated output is obtained at the collector of transistor T2 and is transmitted by a long
wire antenna into space in the form of electromagnetic waves. The transmitted signals can be received
on any short wave receiver without distortion and noise. The range of this transmitter is 25 to 30 metres
and can be extended further if the length of the antenna wire is suitably increased along with proper
matching.