3/5/2023 0 Comments Vintage telephoneHaving understood our telephone, how might we simulate a telephone line to connect it to? I sat down for an evening in the hackerspace with my GPO 746 to find out. (I remember some students at my university getting into trouble for doing this.) Who needs a 2600 Hz tone to scam the telco! The Other Side Of The Socket On some payphones this offered a means to circumvent the coinbox circuit which disabled the dial. The effect is to interrupt the line current and create a series of pulses set by the number dialed, which were originally used in a mechanical exchange to advance the switchgear a step with each pulse.Īn interesting side-effect of this circuit is that with some telephones it was possible to simulate dialing by rapidly depressing the cradle in sequence. It returns at a constant rate set by a governor, and as it does so it closes a set of contacts here labelled SW2 to switch in a low resistance across the handset, and it passes its other contacts, SW1, over a toothed wheel. A spring loaded disk is rotated to a point representing the desired number by the user before being let go. SW1 and SW2 are the contacts in the dial, and since it has become a viral meme that people below a certain age may be unfamiliar with dial phones it’s worth explaining the operation of the dial for them. Thus the exchange can ring the bell by sending an AC voltage on top of the DC standing voltage. Daderot When the handset is put on its cradle SW3 is opened and there is no DC current path, but through the capacitor C1 there is an AC path to the bell B1. The speed governor is diagonally across the mechanism, and the toothed wheel that drives it also serves to operate the left hand contact. The two sets of spring contacts can be seen, on the left the one for pulse dialing and on the right the one that switches in the low resistance across the handset. Any audio on the line is heard in the earpiece, and any speech into the carbon microphone varies that current and creates an audio signal in return.Ī telephone dial, front and rear. When the handset is off the cradle, SW3 is closed, and because the line has a standing DC voltage on it a current flows through the handset. On the left is the handset, with a carbon microphone and an earpiece in series. But its most basic operation can be explained with something like the much-simplified schematic we’ve pictured. A phone like my GPO 746 contains a lot of clever circuitry to give it better audio, and to protect it from line faults, over-voltage, and other mishaps. The great thing about a passive device whose roots lie around the turn of the twentieth century is that its principles are relatively easy to understand. In this diagram SW2 is open and SW3 is closed, so the handset is off the cradle but the dial is not in use. Just How Does A Telephone Work? A grotesquely simplified schematic of a dial phone. It’ll undoubtedly work with my digital answerphone, but will it still work with a 50-year old model with an old-fashioned dial designed for use with a mechanical Strowger exchange? If I’m going to have to hack on this old phone, I’d better figure it out now in case I have to hook it up to something else to keep it communicating. Along a sliver of glass will come faster Internet that I can currently dream of, and I’m expecting that the box that our telephone provider puts on the end of it will have a telephone socket that is simply the front-end of a dedicated VOIP client. If all goes well, I’ll be receiving a new fibre optic connection in a few weeks. If you like old phones, you can still have one, and picture yourself in a 1950s movie as you twirl the handset cord round your finger while you speak.Īs we move inexorably forward, it’s worth considering for how much longer we’ll be able to do this. In a manner of speaking though, your telephone wall socket hasn’t forgotten. The iconic British telephone of the 1960s and 1970s, the GPO model 746. As we move inexorably towards a wireless world in which the telephone line serves only as a vehicle for broadband Internet, it’s easy to forget the last hundred years or more of telephone technology that led up to the present. When we make a telephone call in 2020 it is most likely to be made using a smartphone over a cellular or IP-based connection rather than a traditional instrument on a pair of copper wires to an exchange.
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