need help about phone line

Thread Starter


Joined Mar 5, 2007
hi all
i make project that can detect DTMF/SCK by sm8223 IC,but i want when i recieve call open phone line to send some command by AVR controller (mega16),
is there method that i can open phone line?
thanks alot


Joined Nov 6, 2005
Use a 56K Modem transformer, which is a little 600 Ohm one-to-one ratio audio transformer. These are available from many suppliers and quite cheap, or just desolder one from a scrap PC modem board.

Connect one winding across the line using a small relay, that will take the phone line 'off hook' and allow audio to be sent or received via the transformer.

Put some diodes across the 'local' side of the transformer to limit the audio level so line switching & voltage spikes don't damage your audio circuit...