Corded Telephone
What Is A Telephone
The word telephone is a combination two Greek words tele, meaning from afar and phone meaning sound/voice. Alexander Graham Bell invented the telephone on March 10, 1876, in Boston, Massachusetts. The telephone is a device for transmission and reception of voice from far off places.
Telephone Components
A basic telephone consists of following items:
Handset consisting of:
Microphone: The microphone converts the voice signals into electrical signals for transmission over a pair of wire
Earphone: The earphone converts the electrical signal into voice signals.
Hybrid coil: It is a device which sits between the microphone, earphone and rest of the telephone & allows transmission and reception of electrical signal on same pair of wire.
Hook Switch: The switch which closes & permits connection of telephone line to telephone when telephone handset is lifted.
Dialer: The dialer allows dialing of called party’s telephone numbers. The dialers in old telephone of rotary type use to cut the line as many number of times as the number being dialer and are called pulse dialer as this action generate electrical pulses for signaling to exchange. In touch pad or push button type of dialer a combination of two tones, as per table1, are generated for each digit and sent.
Table 1 Touch tone matrix
| 1,336 Hz | 1,477 Hz | 1,209 Hz | |
| 697 Hz | 1 | 2 | 3 |
| 770 Hz | 4 | 5 | 6 |
| 852 Hz | 7 | 8 | 9 |
| 941 Hz | * | 0 | # |
Ringer or Alerting device: The ringer unit in telephone receives 90 volt interrupted ac signal (60 Volt in Germany) from exchange to indicate the incoming call. In old telephones it used to drive electric bell with magnet & hammer. In new telephone the ringer signal activates a ringer unit to generate a ringing or beeper sound. Some special telephones, for use in industrial noisy areas, activate flashing light and external hooter to indicate incoming calls.
Working of Telephone
The telephone is supplied with a -48 V (24V in some places) DC voltage from exchange. The phone ringer & other circuits are connected to the telephone line through a capacitor, a device which blocks the flow of DC current but permits AC current. This enable the ringer current from exchange to activate the ringer circuit for incoming call signal.
Receiving Calls
The receipt of ringing signal from exchange activates the ringing generator inside telephone & it signals to the user incoming call by generating ringing tone/beep. When subscriber lifts the handset, the hook switch which connects the telephone to the telephone line from exchange gets closed. This allows the current to flow through the telephone & signals to the telephone exchange that telephone is off hook. Detecting this signal the exchange disconnects the ringing signal & connects the telephone to caller line.
Making Calls
When the telephone is lifted from idle condition for dialing the number, the hook switch gets activated and the subsequent current flow indicates to telephone exchange that the subscriber want to dial number. To indicate its readiness the exchange sends the dial tone to the telephone. The subscriber on receiving dial tone, dials the number using dialer. The exchange connects subscriber to line when caller lifts the handset.
After conversation, the subscriber places hand set on cradle. This makes the hook switch off and DC current in line is cut, indicating to exchange that conversation is over. The exchange then disconnects the connection.
