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Design and Applied Technology (Secondary 4-6)

Theme 2 - Case Study of the Evolution of Wire Rotary Dial

Telephone to Wireless Mobile Phone

This theme-based learning task enables you to understand

(a) Evolution of craft and design of telephones,

(b) Design culture, and

A. Background

Telephones are a necessity for many people. The modern telephones have evolved from their

past through stage after stage of renovation and revolution over a century. A telephone

processes signals and voice along the same cable.

A fundamental telephone consists of a bell for notifying recipients of incoming calls and a

dial for callers to input numbers for making outgoing calls. The workflow is as follows:

(a) The caller picks up the handset of a telephone;

(b) The switch hook is released by the pick-up;

(d) An electric current flows through the circuit;

(e) The telephone exchange detects the DC;

(f) The telephone exchange sends a dial tone to the calling telephone to indicate the

readiness of making calls;

(g) The caller inputs the numbers required through the dial;

(h) The tone generator behind the dial converts the numbers into Dual-tone multi-frequency

(DTMF) tones;

(i) The telephone exchange connects the caller’s line to the recipient’s line that the

telephone represents;

(j) The telephone exchange sends a signal to the telephone at the recipient side;

(k) If the receiving telephone is on hook, the signal-induced current passes through the

receiving telephone’s alerting/ notifying devices, such as a bell, which are connected to

a different circuit (Circuit 2), notifying the recipient of the call;

(l) When a recipient picks up the handset, the switch hook disconnects the alerting/

notifying devices (Circuit 2) and forms another circuit (Circuit 3, same as Circuit 1 but

at the recipient side); and

(m) The line is thus established and the caller can talk to the person answering the call.

The voice parts of a telephone inside the handset consist of

(i) A transmitter, i.e. microphone; and

(ii) A receiver.

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Vitek VTC-3516 Specifiche Pagina 81