THE SPECIALITIES OF PROJECT
This project keeps eyeon the telephone conversation. Some people use telephone on telephone lines and try to listen secret conversation. This causes loss to the owner. This project is a good for use in house and in office.
TECHNICAL DESCRIPTION AND WORKING OF THE PROJECT
This project has been developed and tested by our readers and sent to the “T.V.Engineers Technical lab” for publication in the book. The spare parts and PCB of the projects published in the series of our book publication or available in the market. The information of project code is given in the first and last page of the books.
This circuit of the project is using IC LM339 and LM7805. The IC2 LM7805 is a regulator IC and IC1 LM339 is a voltage comparator. The working system of the project is very simple . When a call from telephone line is received then the line voltage drops from 48V to 12V. As soon as another telephone is connected to the line, it drops to 8V. The IC LM339 detects the voltage difference and makes LED to glow. When first telephone is lifted then LED1 glows. In case the another telephone in parallel is lifted then LED2 glows.
In case circuit does not work properly then adjust VR1 (5kOhm) so that LED1 glows. Holding the first telephone lift the second telephone and now adjust VR2 (5K ohm) so that LED2 glows. The circuit is run on 9V battery.
PRECAUTIONS WHILE MAKING THE PROJECT
- While connecting diode in the circuit observe the polarity of the diode.
- Take care of polarity of electrolytic capacitor.
- Take care of base emitter and collector of a transistor while connecting a transistor.
- Connect all components after getting checked with multimeter.
Use IC base for the IC. The IC may get damage while connecting it in the circuit with soldering iron.