rajkot

Friday, March 26, 2010

TELEPHONE TRANSMITTER

  1. This is a simple, but very useful circuit that can be used to transmit telephone conversations.
  2. When the telephone receiver is on hook the voltage across the lines will be about 48 volts. 
  3. The preset R7 is so adjusted to obtain a 24.7 V across between the cathode of D2 and ground. 
  4.  At this voltage the Zener diode D2 will be in breakdown and the transistor T1 will conduct. This makes the transistor T2 OFF.
  5.  When the receiver is off hook, the line voltage drops to about 11 volts. This makes the transistor T1 OFF and subsequently the T2 ON.
  6. The T2 in switched ON condition will provide a DC path for the transistor T3 used in the FM transmitter section.
  7.  The transistor T3 is wired as a common emitter radio frequency oscillator. In simple words the transistor T2 serves as an ON/OFF switch for this oscillator.

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