Electronics

Don P. · Joined: 24 Oct 2007 Posts: 1

In the USA, telephones are rung by 20 Hz, 90-200 volts. (I measured; scope shows 20 Hz, 173 RMS Volts.)

I am building a radio sound effect wherein the user pushes a button and a real telephone rings. Success so far for modern phones, but the older ones don't work, and it's important that they do (so many old radio shows used old-style phones ringing).

I've traced the problem to the fact that it's so hard to pass 20 Hz through a transformer intended for 60 Hz. (I use a 120-volt to 9-volt transformer turned backwards.) There's not enough iron in the core, yet I am using a transformer capable of 3 amps in its normal use.

Here's the question: My home phone is hooked to the "net" via a modem that also handles the internet and my cable TV. That modem rings the phones in the house just fine, yet it doesn't weigh as much as my 9-volt transformer, or little more. How do they do it?

neon · Joined: 25 Feb 2006 Posts: 590

you right 20 cycles into a 60 cycle is bad not enough iron for it. a good modem can detect a signal down 60 DB into a 600ohms load. try using a smal cap and a detector like a comparator lm339 that will get you there the transformer is mostly for isolation and F/B to detect a ring just detect the voltage ac increase. besides some old modems transformer can be bought as a modem at one time the baud rate was 1200 600 and even 300 believe it or not.

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