The English Ringing Converter Technical Description

This device can`t be legally hooked up to the PSTN as there is all sorts of red tape you need to go through to get the device registered for that. The basic functional blocks of the English ringing converter are pretty simple. It has a ringing detector, a loop detector, a ringing generator, a power supply, and a microprocessor for control. American ringing is a 2 second on, 4 second off ringing voltage at 20Hz.
The English Ringing Converter Technical Description - schematic

English ringing is 400ms on, 200ms off, 400ms on, and 2 seconds off ( a double ring ). It is also at 25Hz and and a slightly lower voltage ( 80VAC vs 108VAC ). When the converter detects ringing on the line, it disconnects Rosemary`s phone from the line and switches the telephone over to the internal ringing supply. It then generates 25Hz ringing voltage by driving 24 volts into a split bobbin transformer backwards to generate the high voltage ringing. When Rosemary answers her phone, the converter detects the loop current and disconnects the phone from the ringing supply and connects it back up to the network. In the case of Rosemary initiating a call, no action is required on the part of the converter, as the phone is normally connected up to the network by default. The first thing you need to do is detect inbound ringing from the network. In the US, this is 108VAC at 20Hz with a cadence of 2 seconds on and 4 seconds off. This is done with a. 47uF 250V capacitor, a 2200 ohm resistor, and an optocoupler. The extra zener diodes prevent false tripping of the ringing detector for anything less than about 35VAC. This circuit also provides a REN of 1. 0. The next thing you need to detect is an off-hook condition on the phone. This is done with a loop current detector that consists of an 82 ohm resistor and optocoupler. The resistor and optocoupler have about a 1. 5V forward drop when loop current is flowing, but have no AC...

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