Telephone Exchange

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High voltage

Analogue schematic

Without the 10F decoupling capacitor at the junction of T2 and the 1K current limiting resistor, a 50Hz buzzing was audible beneath the tones during ringing. Also, the primary of T2 could not be used for an outside line - or other audio I/O connection - without considerable signal power being lost in the 1K resistor.

Each line has a 10K pull-up. These keep the AC bell coupling capacitors in the telephones charged to the DC supply voltage. This is vital for 2 reasons: If the bell capacitors are discharged, the telephones can emit a single ding when the TLP598G in the call sensor is re-enabled; secondly, it prevents a loud click being heard by the caller when the other TLP598G closes the DC path to the called line. After the AC ringing current is interrupted, a short delay is allowed before the DC path closes for the pull-up to bring the line voltage back to the DC supply.

The 10F capacitor in parallel with the 1K current limiting resistor in the originating loop ensures that signal power from the tone generator is not lost in the resistor. The capacitor cannot be returned to ground since this would interfere with detection of dialling pulses.

Controller

Controller schematic

The 1 timer capacitors have 15-ohm series resistors (not shown) to limit the discharge current.

LF OUT is a synchronised version of LF IN to prevent glitches.

The 18 KHz clock is (just) super sonic to avoid any risk of pickup.

Tone generator

Tone generator schematic

US frequencies are shown. I did not have the UK frequencies when this was drawn.