Build an Intercom using two old Telephones 

You're welcome.

Try to use a similar voltage DC power supply from any unused small electronics instead.

Yes .. it can be done. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-zwUS1_Hs6p4.html

Yes .. you can sense loop current and turn on a LED. Others use a buzzer. Check out this video. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-3l5se0gGM18.html

@@0033mer Thats a great answer and a great video you linked to. I just wanted to ask here just to be certain. Will using a bigger battery really fix the problem of carrying the voice signal over long distances? My friends had an intercom but you couldn't hear anything after 700m~half a mile, and in the video you linked to, he says using a bigger battery will fix that problem if you want a mile or so.

Put an Ammeter in series with the loop and adjust resistance to get 15 to 20 mA of loop current when both phones are off hook.

The phones will not ring. If you ran multi-conductor wire between the sets you could wire up a buzzer system.

Interesting. Does the one phone make a clicking sound if you rapidly make and brake the connection on the other? I mean rapidly tap the switch that the handset breaks when you hang up. I wish I had 2 old telephones to try it out.

Yes ... You can hear clicking in the earpiece if you tap the other phone's hook switch. The clicking would be caused by the voltage change in the line.

@@0033mer would it be oissbkie ti wire up 4 phones if more batterirs were added

@@Flapjackbatter Morse code .. Glad I stopped by this video ! Lots of good ideas coming

Without the resistor, the 9V battery would essentially be shorted through the phones, and would rapidly drain itself (and get rather hot in the process). The resistor establishes a constant, reasonable DC current through the phones (about 30 mA) that can be modulated by the two phones to communicate.

Your're welcome!

Yes ... a rotary phone will also work.

The resistor is needed to limit the current. Check that all your components are in series.

Are you using them close together?

No .. the circuit is designed to connect two telephone sets together which contain hybrid transformers. The current loop goes through both transformers which enable the phone sets to communicate in full duplex. You would need to simulate that with your setup.