Telephone - Wiring up old phone ringer to arduino - Electrical Engineering Stack Exchange - Wire voltage for vintage telephone
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Phone lines normally run on v DC (referenced to ground) when the line is idle. During the ringing cycle (in the US, 2 seconds on, 4 seconds off), a ringing voltage of Look at page 2 and 3 of the nocensor.info from this page. Sparkfun have done it using an H-bridge and voltage booster circuit. Quite nifty.