Phone to audio interface (SSI202 input)

(Circuits -> Telephone)
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From: Jim Earl

You have to isolate the chip from the phone line, or you'll have all kinds of problems. Let's see how I can do this with ascii art:

                    .22 uf              10k pot
                     400v    ||(----------->
Phone line tip  o-----)(----)||(           <---o to SSI202 input
                            )||(           >
Phone line ring o-----------)||(-----------o---o ground

The transformer is a 600-ohm to 600-ohm line transformer. I use the circuit as-is, and works fine. Doesn't take the phone off hook, you'll need to add some circuitry for that. To set the pot, turn it down all the way, (for minimum audio into the decoder) then hold down a tone on the phone while you slowly advance the pot up until the VALID DIGIT line changes on the chip. Then advance the pot a little past that point. That should do it.

Also, it might not be a bad idea to put a couple of diodes back-to-back across the secondary of the transformer. I'm not sure if enough voltage will be generated to harm the SSI chip when the phone rings or not. Mine has never had a problem, but it might be worth the cost of the two diodes for good luck.


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