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Your own 8-line telephone exchange

Your own 8-line telephone exchange

This miniature, 8-line home telephone exchange (PABX) works with standard telephones and features high-quality, user-friendly progress tones. Connection to telephones is via a two-wire unbalanced loop with a common ground. Only pulse dialling is supported although the exchange could be adapted to use a DTMF tone decoder.

Overview
The analogue circuitry comprises the following parts:

  • Call sensor
  • Originating loop
  • Destination loop
  • Transmission bridge
  • Tone generator
  • Ring Trip

Call sensor
When the exchange is idle, power is applied to the call sensor via a TLP598G photo-relay. The sensor comprises 8 CNX35U opto-isolators – one for each line. When an extension is lifted "off-hook", its associated opto-isolator asserts a sense line. If only one sense line is asserted, the controller latches the binary code of the originating extension and initiates a call. The call sensor is powered-down until the end of the call.

Originating Loop
The calling line is connected to the originating loop by an opto-triac. Loop current flows through transformer T1 secondary, and through the CNX35U loop status opto-isolator which detects dialling pulses and, ultimately, hang-up.

A small time delay is allowed for things to settle down before dial tone is delivered to the caller via the primary of transformer T1. If the loop is not closed at the end of the time delay, the exchange reverts to the idle state.

Destination loop
The called extension is connected to the destination loop by an opto-triac. There are two paths through the loop: for DC only via transformer T2 and a TLP598G photo-relay; and for AC via the AC ringing generator and a TLP3043 opto-triac. The paths are switched alternately producing the required ring-ring effect interspersed with silence. The 598 and 3043 light emitting diodes are connected back-to-back as a safety interlock. The TLP3043 contains a zero-crossing circuit to ensure a clean switch.

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