Intercom

The requirement for a talk switch at each end on a two-wire system complicated the design slightly. Capacitor C17 is connected in series with the remote speaker, and is short-circuited when the talk switch is pressed (R22 limits the discharge current). The electronics at the local end detects the DC path when the switch is pressed.

IFB Intercom

Intercom

Intercom

Low cost intercom using transistors

comprises a 3-stage resistor-capacitor coupled amplifier. The complete circuit, along with microphone and earpiece etc, can be housed inside the plastic body of a cellphone toy

Simple Intercom

Perhaps you need an intercom. Maybe you want to be able to communicate with your house from your shed or garage. Or perhaps as in my case some years ago, you are upstairs in your bedroom doing your schoolwork.

Two line intercom plus a telephone changeover switch

The circuit presented here can be used for connecting two telephones in parallel and also as a 2-line intercom. Usually a single telephone is connected to a telephone line.

Use old telephones as intercom

You may wish to use a 12 to 24V battery, like a Gel Cell, since the car power or cheap power wall wart style supply is probably too noisy. You may have to adjust the resistor value (1.2K for 24V). Whatever gets the phones working and does not use more than 30mA total loop current.

Intercom Preamp Schematic

Intercom Preamp Schematic Diagram and Components