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how to do a ring detector???


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Hi Lethenl,
If I wanted to make a telephone ring detector I would simply use a ring detector IC. It will even make a "ringing" sound in a speaker.

If I wanted to do it the hard way and use many discrete parts, I would first study the theory about how a telephone and a telephone system works.

In my country, the telco uses a ringing signal of 90VAC RMS at 20Hz. If I am far from the central office and have up to 5 telephones ringing, the ringing signal's voltage may drop to 60VAC RMS. My telephone line also has 48VDC on it to detect DC current which would signal "off-hook" which would answer the ringing. or give me a dial tone without having any ringing.

So to detect ringing, you must block the 48VDC and detect the 60 to 90VAC. Use a coupling capacitor to block the DC and feed the coupled signal to a full-wave bridge rectifier. Filter the rectified signal with a small capacitor and connect it to a 60V zener diode. Connect the zener diode through a current-limiting resistor to the detector (LED) of an opto-isolator. The opto's output will conduct when the telephone line is ringing.

That is all. It has been many years since I worked with phone systems so maybe I missed something. That's the way I remember Motorola did it in their now-obsolete MC34012 and MC34017 ICs.
I have attached Motorola's datasheet for you to see how their IC detected ringing and "rang" a speaker.

MC34012.pdf

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