ring generator circuit

Posted on Feb 6, 2014

Restore the older style of telephones which used a pair of gongs to annunciate an incoming call, AND long to hear the nostalgic peel of those bells Presented here is a design for a `Telephone Ring Generator` which when connected to a suitable telephone will cause the bells to ring in the correct `cadence`- for what I might call the `British Post Office` type format.

ring generator circuit
Click here to download the full size of the above Circuit.

(in common use in many parts of the world) 400mS ring, 200mS silence, 400mS ring followed by 2 second silence (3 second total - then cycle repeats) The ringing frequency of the bells is reckoned to be about 25Hz The ringing sequence used in the USA is 4 secs ring, 8 secs silent, 30Hz ringing frequency, then cycle repeated. The design could in fact be "re-hashed" to deliver this format, a later update may follow. From my own recollection of telephone exchanges c1960/70`s the ringing (and also busy and disconnected ) signals were generated by micro switches actuated by sectored discs all rotating on the shaft of a constant speed motor ! Whilst there are designs and ideas on the internet that try to achieve the above; they range from feeding raw 110v mains AC into the phone (crude and dangerous!), oscillator into audio amplifier feeding the phone (may work but somewhat messy), to electronic devices using free running RC oscillators and a lot of associated components (high parts count). A plug pack or similar is used to obtain a low DC voltage (15v) to power the electronics - by conventional means; bridge rectifier, filter capacitor, 3 terminal regulator etc. Nothing new here. Additionally low voltage AC from the transformer secondary is sampled through a diode to provide a unidirectional 50Hz signal which can be used as a "clock" This positive going half-wave sinusoid is fed to a 4093 Schmitt trigger, its action being to...

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