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Have some old phones laying around? You can turn them into an intercom system, shows you how – “Talking over the phones is easy. You put DC current through the phone and it transmits and receives audio. So two phones and a current source (about 25mA) all in series will give you a talking circuit. A suitable current source can be as simple as a 9V battery and a series resistor whose value is adjusted (with both phones offhook) till about 25mA flows. You can then bypass the battery and the resistor with a capacitor to couple the audio straight across and get a loud and clear connection.” Thanks kryten007! [via] Link.