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when the telephone rings certains voltages are send from the telephone lines. Once i messured them but it varies from country to country, i suggest you set up an osciloscope and start messuring the signals, then you will have a better idea of how to detect them.
Measuring ring voltage with an ordinary scope is not an easy task, as the
telephone line is balanced and most scopes grounded. You will need an
isolation transformer that can pass 16 to 20Hz.
For simple experiments, you can assume the voltage will be higher than
something like 50V. Detecting can be done with a bridge rectifier, say
four 1N4004 diodes, in series with a resistor and use an opto coupler to pass the ring status information to other logic circuit.
Isolation of telephone line with other logic or active circuit is important in many
cases unless you use a floating circuit.
use one transformer and then rectify the output of the transformer this will give you certain voltage at the output. But remember that before the transformer you have to use capacitor to block dc voltage.