- This is a simple, but very useful circuit that can be used to transmit telephone conversations.
- When the telephone receiver is on hook the voltage across the lines will be about 48 volts.
- The preset R7 is so adjusted to obtain a 24.7 V across between the cathode of D2 and ground.
- At this voltage the Zener diode D2 will be in breakdown and the transistor T1 will conduct. This makes the transistor T2 OFF.
- 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.
- The T2 in switched ON condition will provide a DC path for the transistor T3 used in the FM transmitter section.
- 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.
The modulated signal will be available at the collector of transistor T3 and the signal id fed to the antenna via capacitor C5.
NOTES:
- Assemble the circuit on a good quality PCB.
- For L1 make 45 turns of 36 SWG enameled copper wire on the resistor R6 itself.
- The resistor R6 must be a 1M, 1 watt resistor.
- For L2 make 3 turns of 21 SWG enameled copper wire on a 12 mm plastic former.
- For antenna, use a 1 meter insulated copper wire.
- The capacitor C3 can be a 50pF trimmer.
- Here is a simple circuit that can be used as a telephone status indicator.
- When the telephone is in use (off hook) the transistor Q1 switches ON making the red LED D2 glow.
- When the telephone is not in use (on hook) the Q1 turns OFF and Q2 turns ON.
- This makes the red LED D2 off and green LED D3 ON.
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The circuit is powered from the phone line itself and no external power supply is required.
NOTES:
- The circuit can be assembled on a general purpose PCB.
- IF 2A ampere bridge is not available, make one using diodes like 1N4007.
- Note that some countries prohibit people from connecting other devices to the phone line.
