Telephone SystemSchematic DiagramRevision 1.5, April 21, 2002Copyright 2002 Michael M. Doddmike@mdodd.com

+C3470F

+9V Bus In

Mic. Bus In

Com. Bus Out

C40.1F

C50.1FCo

Mi

Co

+9VD1

1N4001 U278L05

In Out

Com

-In4

+In3

Bypass2

C21F

+

R13.3K

U1LM4864

Gnd7

5

8

Vo1

Vo2

R220K

C11F

+

Shutdown16

Vdd

+C1470F

+9V Bus

Com. Bus

Mic. Bus

C20.1F

C30.1F

Co

+9VD1

1N4001 U178L05

In Out

Com

R12.2K

Telephone Power Supply Circuit

Telephone Station Circuit

Mic

Mic

Co

+9V

Mic

+9V Bus Out

Mic. Bus Out

Hook SwitchShown in

off-hook (talk)position

Com. Bus In

J1

Ear

Ear

J2

J4

Tlk

Sw

Hup

R4 10K(Optional)

To mute speaker:1. Install R4.2. Cut trace betweensolder pads, at arrownear R3.3. Connect muteswitch to solder pads.Switch open: Mute.Switch closed: Normal.

Earphone orSpeaker

+

-

Carbon orElectret

Microphone

CW

ComNC

NO

R31K

VolumeControl

+9V

Com.

Mic. Bus

PowerSupply

PhoneStation

PhoneStation

PhoneStation

Telephone System Wiring

Brief Circuit DescriptionA three-wire bus connects multiple telephone stations together as a party line. Connect a 9VDC wall-plug AC/DC converter to the +9V and Common wires. The Telephone Power Supply (only one is required) produces +5V through a resistor on the Microphone Bus wire.A hook switch (or push-to-talk switch) connects the microphone to the Mic. Bus when the handset is lifted from the cradle, and connects it to Common (to mute the local microphone) when the handset is not in use.

When someone talks, the microphone modulates the Microphone Bus. Each Telephone Station has an audio amplifier that amplifies the signal on the Mic. Bus and sends it to the earphone or loudspeaker. Carbon microphones work best, but electret can also be used. It is best not to mix microphone types on the same system because of differing audio levels.

Permission is granted to use this circuit for personal, non-commercial applications only.