Having multiple extension telephones at home is very convenient. You can make or receive phone calls practically anywhere in the house. This circuit disables other telephones connected to the phone line whenever a telephone (either the master or any extension phone) is in use. The circuit is inexpensive and is guaranteed to keep the phone conversation private. The circuit does not need an external power supply. It gets its power from the telephone line.
The no-load voltage at the telephone line, when the telephone handset is ‘on-hook,’ is around 48 volts. However, when the handset is off-hook, terminal voltage drops to between 5 volts and 15 volts. This is due to the impedance of telephone line and the telephone set. The voltage of the telephone line is the key factor that controls the operation of this circuit. diodes D1, D2, D3 and D4 are connected as bridge rectifier to make the circuit non-polarized. Lifting the handset causes the terminal voltage to drop from 48V to about 10V.
