In this intercom schematic, the 8 ohm speakers is used as microphone and listening speaker. The 10K potentiometer controls volume and the total gain can be set with de 5k R. Use a double switch with 2 ...

It doesn’t have to be that complicated, though, as this simple two-wire power-and-data ... the twisted pair is a power supply and decoder circuit, which sends 9 volts up the line to power ...