Insight - How Telephone Works

“Mr. Watson, I want you here”, Alexander Graham Bell knew so well that he has created a revolution when he said these words after testing his telephone experiment. It was the first of its kind and an invention that revolutionized the way we communicate with people separated by geographic distances and barriers. Years after its invention, telephones have penetrated almost every nook and corner of this world and a large proportion of these connections are landline phones. Once a luxury, telephones aremore of a habit now, indispensible requirement of any 20^th century citizen. These devices have been sitting silently in most homes in modest corners that no one seems to question what goes on inside them that they keep on working without maintenance and battery.In this insight, we intend to look inside them to know for sure, what are they made of.

Fig. 1: Image of a Basic Telephone

Fig. 2: Image Indicating Base, Handset Cord, and Handset of Telephone

The image above showsa basic telephone. The standard parts of any corded phone package are the base, a handset cord and the handset. The shape of the base and the handset can vary as per the manufacturer. Innovative designs often feature an LCD display and for that require a battery, the one in this insight doesn’t require a battery as it doesn’t have any LCD or similar electronic accessory attached to it.

Fig. 3: Close View of Base Indicating its Various Parts

The base contains the keypad, switchhook, ringer and a small LED that glows whenever the handset is picked up. The ringer is a small speaker that produces a sound when the call comes in. The hook switch is a tilted structure over which the handset is placed. Whenever the phone is not in use, the handset rests over the hook switch. This position is called ‘on hook’ and the circuit that brings the tone to the handset is open. When the handset is picked up, the telephone goes’off hook‘and circuit is restored to its closed position, thus giving a dial tone.

Fig. 4: Bottom Side of Telephone

Fig. 5: Jack on Side of Telephone for Connecting Handset Cord

The images above are the bottom and the side views of the phone. On the bottom, an input jack for line-in and switches for the ringer volume and tone-pulse selector are there. On the side, a jack for connecting the handset cord is there.

Internal Circuitry of the Base

Fig. 6: Telephone Internal Circuitry

The base houses the heart of the device and all its functionality. The following can be distinctly identified in the image above are:

1. A PCB housing the dialer circuitry, equalizer circuit and frequency-tone generation circuit.

2. Ringer

3. Hook Switch terminating at the PCB.

4. Jack for attaching handset.

5. Wiring extending to the Line-in input.

The last four parts derive their purpose from the PCB, the core of the base circuit.

PCB

Fig. 8: Keypad and Frequency Generator Portion of PCB

The images above show dialer keypad and the frequency generator, respectively. The keypad of the telephone usually is a membrane type keypad whose membrane and keys can be seen below:

Fig. 9: Membrane-Type Keypad

Fig. 10: Equaliser Circuit Portion of PCB

In order to adjust the voice signal from the user, an equalizer circuit consisting of resistors and capacitors is mounted on the PCB. The objective of this circuit is to normalize the voice of the user irrespective of the length of transmission line used, thus making transmission and reception of voice signal suitable for the user.

There are two ways in which a phone number can be dialed: pulse and tone. The previous is the old method which was designed for rotary dial telephones. In pulse dialing, each number produces a certain number of pulses in form of electrical signals which are to be decoded at the telephone exchange. This whole process is quite slow and with the arrival of key pad phone, it has become obsolete. Key pad phones brought a system of pulse dialing or Dual Tone Multi Frequency. In this type of dialing, pressing each number produces an analog signal wave which is a combination of low and high frequencies. A thorough article on DTMF can be reached here The dial tone that the user hears when the handset is picked up is a signal that indicates that a phone call can be made. Telephone services of various countries have different dial tone frequencies.

Fig. 11: Matrix Circuitry in Reverse Side of PCB

The reverse of the PCB shows the matrix circuitry which is quite similar to that of the earlier insight about scientific calculator. Also, in circuitry section, two ICs are there:

1. 9214 Series IC: A CMOS based oscillator IC which provide a dialing pulse or dual tone multi-frequency dialing.

2. 2418 IC: A tone-ringer IC which produces the ringing sound when a call comes, thus working just like an electromechanical bell.

Now with a whole electronic circuitry working inside a telephone circuit and not needing any batteries to run on would be a ludicrous statement. Well, iIt does have power, not from our mains supply but from the telephone exchange. A simple telephone derives its power from the line-in connection that it receives from the telephone exchange. Known as the “talk battery”, a line voltage of -48V is provided by the line-in coming from the telephone exchange. This voltage is reduced down and optimized to the phone’s requirement by the internal circuitry on the phone.This serves as sealing current and also reduces noise in the connection and improves signal transmission and reception quality for long distances.

You might be wondering about that negative sign on the voltage. The minus sign is not a typo. Why not +48V or -50V? First of all, using a negative voltage puts the line at cathode position with respect to earth which, being at 0V, acts as anode. If any positive value of voltage was taken, the line-in wiring would have depleted with the material going to the earth due to electrolysis as the earth is one big container for numerous naturally occurring electrolytes. Also, generating voltage of magnitude of 48 would is easier to generate. Standard 6V or 12V batteries can combine up to provide the voltage. Also, the weather impacts of lightening that might induce voltages into the telephone line are less likely to disturb a -48V line than a positive voltage line.

When someone is calling to a customer premises phone, an additional line voltagein the range of 70 to 120volt is superimposed upon -48V. Termed as ringing voltage, the value of this voltage varies from country to country but 90V is most common.This voltage might even give the user a mild shock.

Hook Switch

Fig. 12: Hook Switch for In-Line Telephone Connection

Shown in the image above is the hook switch which connects or disconnects the telephone from line-in. It is a simple switch having a contact and spring assembly. The spring holds to two metal contact platforms out of which one is stationary and the other is movable. In the pressed state, the movable platform is pressed downwards where it doesn’t make any contact with the stationary ones and no dial tone is released, as soon as the handset is picked up or is “off hooked”, the metal contacts on the platforms form a circuit and dial tone can be heard. The image below shows the internal structure of the hook switch.

Whenever handset is picked up, the hook switch gets closed; the resistance across the pair reduces to almost a short circuit, completing the circuit of the telephone-exchange loop. The connection conveys the off-hook message to the telephone exchange. This whole process has been improvised from time to time and users have been provided with more lines and quicker connectivity. Currently, telephone PCB employs an integrated circuit (IC 91214) in this case to carry out the process of conveying the message as well as generating DTMF frequencies for dialing a number.When the handset is off hook for a long time and no signal is generated, telephone line gets disabled and user has to reset the hook switch to use the phone.

Fig. 13: Hook Switch Configuration

Handset

Fig. 14: Handset

Handset is very simple assembly comprising of a speaker and a microphone section. Enclosed in a handy plastic casing, it is a simple connection of a condenser microphone and a simple speaker to the jack that connects it to the base of the telephone.

Fig. 15: Handset Internal Features: Spearker and Microphone

Fig. 16: Wiring Pattern of Handset

Fig. 17: 4-Pin Wire Connector of Handset

The image above describes the manner in which the wires coming from the speaker and the microphone are inserted into the handset-base connector. It is a four pin connector corresponding to the number of wires that are inserted and is connected to the base with the help of the handset cord.

Fig.18: Curled-Wire Cord

The handset cord is circular curledspiraling wire connecting the base and thehandset. It is coiled so that the wires don’t create clutter near the phone and can be stretched to manifolds the coiled length so that “multitasking” can be carried out while on phone. The jacket of the coil is made from flexible PVC and is resistant to ambient adversities such external noise and heat.

Telephone Connector

Fig. 19: Modular Electrical Connector Plug

The connector plug of the telephone are of different types such as 8P8C, RJ61X, RJ48s etc but the one mentioned in the image above is RJ11 plug that has two inputs. It is a modular electrical connector which has only two connector pins while the number of wires coming out of speaker and microphone require four pins. To understand this, one must know about full duplex communication. In full duplex communications, a single channel serves the purpose of receiving as well transmitting information. In a two wired telephone connection, both the wires are used for sending as well as receiving voice.The user hears his voice already so the conversation is not affected. Using a two line system reduces the cost of wiring to half without making any compromises on voice quality. A clip type latch is there which aids in nicely attaching the plug to the latch.

Corded telephones have come a long way from their invention and features such as caller identification, alarm, hands free speaker phone are also coming embedded in these phones. Cordless phones are also an enhanced version of these phones that provide more mobility to the handset for a limited range around the base. Landline corded phones have placed themselves in a permanent in every household and are indispensible in large offices.