Extension Phone Switcher

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-polarised. Lifting the handset causes the terminal voltage to drop from 48V to about 10V. The drop in voltage does not, however, occur rapidly. therefore while the terminal voltage is still high (above the threshold voltage level), both zener diodes D5 and D6 are turned on. Current flows through resistor R3, triggering SCR1 and providing a link to the telephone set connected to lines L1(a) and L2(a). When the terminal voltage drops below the threshold voltage of the zener diode, diode D5 reverts to its nonconducting state, cutting off the gate drive to SCR1. However, once the SCR is on, it will remain in that state as long as the current flowing through it does not fall to near zero level. Thus the link continues. Zener D6 maintains the voltage across resistor R2 and LED1/LED2 indicates as to which telephone is in use. The low off-hook voltage of the line will disable the other extension phones. The line voltage will not turn on zener diodes D11 and D12, even if the handsets of the other extension phones are lifted. Use the following procedure to check up the system after wiring:

1. Lift the handset of each telephone to see whether the corresponding LED lights up. Return the handset back in its cradle; the LED should turn off. Use the same procedure to check the other phones.

2. Lift the handset of phone ‘A;’ its corresponding LED should light up. The other phones should be cut-off (no dial tone).

3. Lift the handset of phone ‘B’, then return the handset of phone ‘A’ to its cradle. Now ‘B’ telephone’s LED should light up and the dial tone should be heard through the ear-piece.