Types of Telephone Dials

Types of Telephone Dials Points : Types of Telephone Dials, Rotary or Mechanical dial, Push button dials There are the following two types of telephone dials:
(a) Rotary or Mechanical dial
(b) Push button dials (a) Rotary or Mechanical dial Dial is an impulse conveyance device. while subscriber lifts his handset, D.C. loop is complete and stable current flows through line provide by exchange. The impulsing cam of dial breaks circuit as lots of times as number dialled, therefore producing pulses of current and sends it over subscriber’s line to exchange.

Dial consists of finger plate by ten holes in it. These holes are equally spaced around 2/3rd of the outer ring of the finger plate. The numbers 1 to 0 are written on a number ring below the finger plate. These numbers can he seen through the ten holes of the finger plate. There is a finger stop adjacent to the digit “0”. The rotation of the fingerplate is c1ockwie against the tension of a spring that restores it to its normal position, after dialling a digit. During the anti clockwise or reverse movement of the finger plate, the speed of the plate is kept constant with the help of a “Governor”. The governor is a mechanical device and consists of a number of weights on the spring. It moves along with the dial through gear assembly.

Dial also consists of an impulsing contact. This contact is normally closed. When the dial operates, a cam opens and closes this contact, thus producing pulses of current on the line. The number of pulses produced corresponds to the number of times the contact closes. The adjustment of the dial is such that it produced 10 pulses per second, whereas ± 10% variation is acceptable. The number of pulses generated by the dial corresponds to the digit dialled. For example;

By dialling digit 1, one pulse will be produced.
By dialling digit 2, two pulses will be produced.
By dialling digit 3, three pulses will be produced.
By dialling digit 0, ten pulses will be produced.

Dial also consists of off-normal contacts. These contacts are normally open or off. These contacts remain close, when the dial move forward and then come back to rest position. There are two advantages of these contacts, (i) when dialling, due to the interruption of current clicks are produced and are not acceptable. The off-normal contacts are used to bypass the speech circuit when current from the exchange flows through them and (ii) when the contacts closes, the impedance of the line reduces, hence the current increases. The constructional detail of the rotary or mechanical dial is shown in the figure. The mechanism of the dial is such that it has to provide the necessary pause between two successive digits. This pause is known as inter-digit-pause and is necessary for the switching circuits in the exchange to distinguish between the digits dialled by the subscriber. This inter-digit- pause is provided by the space between the first hole of the finger plate marked “1 and the finger stop. This extra distance produce two extra pulses with every digit dialled. In one type of dial, the two extra pulses were suppressed by short circuiting the impulsing contacts for the last two pulses. While in the other type, the first two pulses are not generated by the mechanical design of the cam, which open the contacts.

Opening and closing of electrical contacts by a cam that revolves with the dial and engages with a joint are shown in the figure from (a) to (h), for dialling the digit 5.

(b) Push button dials In tone dialling type of telephones, pressing the digit keys causes the dial network to produce specific tones that the central office recognizes and decodes. This type of telephone is also called touch-tone service telephone. It contains push buttons and tone generating circuits, instead of a rotary dial. When a button is pressed, a signal of two frequencies is generated and is transmitted to the central switching office to identify the key. In these frequencies, one is in the low frequency band and the other is in the high frequency band. Thus by pressing the key 5 transmits two signals, one at the frequency 770 Hz and the other at the frequency 1336 Hz. These frequencies are translated into a series of pulses similar to those made by the rotary dial. Basis for the selection of these frequencies is such that there is no possibility that the harmonics of one set of frequencies is mistaken for another set of frequencies. If a fourth hand of high frequency, 1633 Hz, is provided then 16 keys can be accommodated which are used for special applications. These frequencies are indicated by the intersection of the frequency lines in the tone matrix as shown in the figure.

The use of tones speeds up the dialling operation. The buttons marked “*“ and ‘#“ are for special services.

Modern telephones, controlled by microprocessor, provide more opportunities for personal communications such as;
(i). Putting calls on hold.
(ii). Group listening or conferencing between more than two subscribers.
(iii). Redialling a previously dialled number with one touch.
(iv). Memorising and automatically alphabetising names and numbers for one touch calling, etc.

For computer with modem, a telephone is a link to Internet and electronic mail services. The modern enables the computer to send printed messages over the telephone hues to other computers.