Phone Broadcaster Circuit

An interesting circuit design that is also used as a wireless telephone amplifier, is demonstrated below which has been proven very useful in prying on a telephone conversation and therefore is termed a phone broadcaster circuit.

The more amazing or we can say a comforting fact about the phone broadcaster circuit design is that it draws the power essential to operate the circuit directly from the active telephone lines. Therefore, it avoids additional expenses in external battery and prevents the circuit from the extra bulkiness of other power supplies.

Circuit Description of Phone Broadcaster Circuit

To add more, though the circuit phone broadcaster derives its power from the telephone line, it does not harm the overall performance of the telephone. This tiny circuit can be easily fitted inside a telephone connection box of 3.5 cm x 5 cm, using a single-IC type Veroboard.

Two sections: the automatic switching section and the FM transmitter section form the entire circuit of the telephone broadcaster.

Automatic Switching Section | Phone Broadcaster Circuit

The resistors R₁ to R₃, transistors T₁ and T₂, zener D₂ and diode D_1, and preset VR₁ comprise the automatic switching section. As shown in the circuit diagram of the phone broadcaster, resistor R₁, along with the preset VR₁, works as a voltage divider. If the voltage across the telephone wire is 48V DC, the voltage at the wiper of preset VR₁ varies from 0 to 32V which can be adjusted.

The switching voltage of the circuit phone broadcaster depends on the Zener breakdown voltage and the switching voltage of the transistor. In this circuit below, the voltage values are 24V and 0.7V respectively. So that the transistor T₁ may conduct, the preset voltage of VR₁ must be adjusted above 24.7 volts and Zener breakdown occurs, resulting in conduction.

Consequently, the transistor T₁ cuts off T₂, as its (of T₁) collector gets attracted towards the negative supply. At this stage of the circuit, if we pick the handset of the telephone, the transistor T₁ also cuts off as the line voltage drops to about 11V. As a result, transistor T₂ gets forward biased through resistor R₂ and offers transistor T₃ with a DC path in the FM transmitter section.

FM Transmitter Section | Phone Broadcaster Circuit

The coil L₁, oscillator transistor T_1, and a few other components comprise the low-power FM transmitter section. In this circuit, transistor T₂ acts as an electronic ‘on’/’off’ switch, and transistor T₃ works as a common-emitter RF oscillator. The oscillator frequency is automatically adjusted by the audio signal available across the telephone line via transistor T₂ along with its series biasing resistor R₃. The RF signal after modulation is fed to the antenna. Finally, the entire phone conversation can be heard on the other side of the FM receiver remotely when the frequency matches the FM transmitter frequency.

Check out other phone/telephone related circuits posted on bestengineeringprojects.com

1. Cell Phone Jammer Circuit
2. Telephone Amplifier Circuit
3. Add on Device For Telephone
4. Telephone Converter Circuit
5. Telephone Spy

NOTE:

The addition of resistor R₅ and capacitor C₅ was recommended for error-free service to avoid the engaged tone of telephone service during practical application.

PARTS LIST FOR PHONE BROADCASTER CIRCUIT